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The wording of the labels is really misleading and distracting so I standardized it, with below normal meaning lower than normal (the way below is used later in the page) and above normal meaning higher and removing the normative "smart,"
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[[File:Sailing-yachts.Tuiga.Lulworth.Cambria.Cannes.2006-09-26.jpg|thumb|300px|Example of wind force on different sail types on different points of sail. Regattas in [[Cannes]], 2006.]]
 
'''Forces on sails''' are primarily due to movement of air near and relative to the sails.
 
Understanding the [[force]]s on sails is important for the design and operation of the [[sail]]s and whatever they are moving, [[sailboat]]s, [[ice boat]]s, [[sailboard]]s, [[land sailing|land sailing vehicles]] or [[windmill sail]] rotors.
 
[[Aerodynamic force]]s from air [[pressure]] differences and air [[viscosity]] acting near the sail occur along the entire surface of the sails, but can be summed into one [[net force]] vector.  
Net aerodynamic force may be [[Vector decomposition|decomposed]] with respect to a boat's course over water into components acting in six [[degrees of freedom (mechanics)|degrees of freedom]]. Two components with respect to wind direction can also be resolved: [[Drag (physics)|drag]], which is the component directed down wind, and [[Lift (force)|lift]], which is the component [[Surface normal|normal]] to the wind and perpendicular to drag. This analysis is important to boat design, operation, balance, [[Metacentric height|stability]], seakindliness and seaworthiness.<ref name=Marchaj_decompose>{{cite book|last=Marchaj|first=C. A.|title=Sail performance : techniques to maximise sail power|year=2003|publisher=Adlard Coles Nautical|location=London|isbn=978-0-7136-6407-2|pages=Part 1 ch 5 p20 fig 16 "Seakindliness and Seaworthiness". Part 2 Ch. 4 "The effects of Aerodynamic Forces" p76 fig 58|edition=Rev. ed.}}</ref>
 
Briefly, when the sail is oriented at a right angle to the wind, as in a boat sailing downwind, the aerodynamic force is almost entirely derived from the [[Parasitic_drag#Form_drag|form drag]] component - the wind "pushes" the sail along in the direction of the wind.
 
When the sail is arranged across or into the wind the sail acts as an [[airfoils|airfoil]]. The resulting [[surface force]] includes a [[lift (force)|lift]] or pressure component normal to the sail surface as well as a tangential [[Parasitic_drag#Skin_friction|drag]] stress component. Various mathematical models explain the extent and direction of the aerodynamic force on the sail(s). By the law of [[conservation of momentum]], the wind [[Newton's laws of motion#Newton.27s third law|moves]] the sail as the sail [[Reaction (physics)|redirects]] the air backwards .<ref>"When air flows over and under an aerofoil inclined at a small angle to its direction, the air is turned from its course. Now, when a body is moving at a uniform speed in a straight line, it requires a force to alter either its direction or speed. Therefore, the sails exert a force on the wind and, since action and reaction are equal and opposite, the wind exerts a force on the sails." ''Sailing Aerodynamics'' New Revised Edition 1962 by John Morwood Adlard Coles Limited page 17</ref><ref name=Gilbert1>{{cite web|last=Gilbert|first=Lester|title=Momentum Theory of Lift|url=http://www.onemetre.net/design/downwash/Momentum/Momentum.htm|accessdate=20 June 2011|quote=errata should read F=mw/unit time}}</ref><ref name=UNSW1>{{cite web|title=The physics of sailing|url=http://www.phys.unsw.edu.au/~jw/sailing.html|accessdate=21 June 2011}}</ref>
 
==Overview==
 
The analysis of the forces on sails takes into account the theoretical location of the propulsive force or [[center of pressure (fluid mechanics)|centre of effort]], the direction of the force, and the intensity and distribution of the [[Aerodynamic force|aerodynamic]] [[surface force]].
 
The [[fluid mechanics]] and [[aerodynamics]] airflow calculations for a boat are more complex than for a rigid [[wing]]ed aircraft. [[Structural analysis]] also is involved in modern optimal sail design and manufacture. [[Aeroelasticity]] models, combining [[computational fluid dynamics]] and structural analysis, are at the frontiers of sail study and design.<ref name=Fossati1>{{cite book|last=Fossati|first=Fabio|title=Aero-hydrodynamics and the performance of sailing yachts : the science behind sailing yachts and their design|year=2009|publisher=International Marine /McGraw-Hill|location=Camden, Maine|isbn=978-0-07-162910-2|coauthors=translated by Martyn Drayton|page=307|chapter=10.3 The frontiers of numerical methods: aeroelastic investigation}}</ref> However, [[turbulence]] and detachment of the [[boundary layer]] are not yet fully understood.<ref name=Clay_Navier-Stokes>{{cite web|title=Millennium Prize Navier Stokes equation|url=http://www.claymath.org/millennium/Navier-Stokes_Equations/}}</ref> Computational limitations persist.<ref name=LEFD1>{{cite web|title=Pressure PIV and Open Cavity Shear Layer Flow|url=http://www.me.jhu.edu/lefd/PPIV/index.html|publisher=Johns Hopkins U. Laboratory for Experimental Fluid Dynamics|accessdate=22 October 2011}}</ref> The theoretical results are corrected by reality. So, [[wind tunnel]] scale model and full scale testing of sails are required for optimum sail design, function and trim.
 
Some complexities of boat sails:
* The wind is not constant.
* The boat is not traveling in uniform velocity.
* There may be a mast in front of the sail, disturbing the airflow, although this may be mitigated by profiling it.
* A mast is not infinitely stiff.
* The boat profile and position influence the airflow.
* A sail is usually made of thin and deformable fabric.
* The air is viscous, causing losses by friction.
* The flow of the air varies from slow to fast and turbulent to [[laminar flow|laminar]].
Though some software algorithms attempt to model these complexities,<ref>[http://www.mh-aerotools.de/airfoils/javafoil.htm JavaFoil<!-- Bot generated title -->]</ref><ref>[http://www.mecaflux.com/voile.htm Logiciel Calcul Voile Bateau Aile Portance<!-- Bot generated title -->]</ref><ref>For example, see ''[[XFOIL]]'' and ''AVL'' programmed by Mark Drela</ref> the following assumptions make the analysis much simpler:
* the water more or less flat
* the wind more or less constant
* the sail is set and is not adjusted
 
==Centre of effort==
 
The point of origin of net aerodynamic force on sails is the [[Center of pressure (fluid mechanics)|centre of effort]] (or also centre of pressure).
In a first approximate approach, the location of the centre of effort is the geometric centre of the sail. Filled with wind, the sail has a roughly spherical polygon shape and if the shape is stable, then the location of centre of effort is stable. The position of centre of effort will vary with [[Sail-plan|sail plan]], sail trim or [[airfoil]] profile, boat [[Sailing#Trim|trim]] and [[Points of sail|point of sail]].<ref>http://www.adeps.be/pdf/Theorie2005.pdf</ref>
 
==Direction of force on sails==
 
The net aerodynamic force on the sail is located quasi at the maximum [[Draft (sail)|draught]] intersecting
the [[Camber (aerodynamics)|camber]] of the sail and passing through a plane intersecting the centre of effort, normal to the mast, quasi perpendicular to the [[Airfoil|chord]] of the sail (a straight line between the leading edge (luff)
and the trailing edge (leech)).
 
Net aerodynamic force may be [[Vector decomposition|decomposed]] into the three [[translation (physics)|translation]] directions with respect to a boat's course in a seaway: surge (forward/astern); sway (starboard/port, relevant to [[leeway]]); heave (up/down). The force [[Term (mathematics)|terms]] of [[torque]] in the three [[Euler angle|rotation]] directions, roll (rotation about surge axis, relevant to heeling). pitch (rotation about sway axis), yaw (rotation about heave axis, relevant to [[Broaching (sailing)|broaching]]) may be also derived. The scalar values and direction of these components may be very dynamic and dependent on many variables on a boat and in a seaway including the [[point of sail]].<ref name=Marchaj_decompose/>
 
The net force vector, <math>\ F_T</math>, is resolved into components in relation to course in a seaway with:
 
* <math>\ F_R </math> = the driving force directed along the course sailed
 
and
 
* <math>\ F_H </math> = the heeling force perpendicular to the course and the mast.
 
The heeling force can be resolved as a function of heel angle, <math>\ \theta </math>, to:
 
* <math>\ {F_{lat}} = {F_H} \times cos(\theta) </math>, the lateral or leeway force
 
and
 
* <math>\ {F_{vert}} = {F_H} \times sin(\theta) </math>, the vertical or heave force.
 
Net aerodynamic sail force can also be resolved into two components with respect to wind direction: [[Drag (physics)|drag]], which is the component directed down wind, and [[Lift (force)|lift]], which is the component [[Surface normal|normal]] to the [[freestream]] wind and
perpendicular to drag. The generation of lift and drag, components of <math>\ F_T</math>, and their contribution to boat motion are discussed below.
 
==Pressure on the sail==
 
For the purposes of modern [[Sail#Sail construction|sail making]] and study, pressure distribution measurements are done in wind tunnel and full scale experiments as well as in computer models.<ref name=Marchaj1>
Marchaj, Czeslaw A. ''Sail Performance, Techniques to Maximize Sail Power, Revised Edition''. London: [[Adlard Coles]] Nautical, 2003. Part 2 Aerodynamics of sails, Chapter 2 "How and Why an Aerodynamic Force is Produced", page 49 "Pressure differences - the right way to explain sail forces"</ref><ref name=Lafforgue1>http://www.finot.com/ecrits/Damien%20Lafforgue/article_voiles_english.html Damien Laforge Sails: from experimental to numerical</ref><ref>{{cite book|last=Fossati|first=Fabio|title=Aero-hydrodynamics and the performance of sailing yachts : the science behind sailing yachts and their design|year=2009|publisher=International Marine /McGraw-Hill|location=Camden, Maine|isbn=978-0-07-162910-2|pages=ch 8.12 Wind tunnel tests; ch 10.2 numerical methods|coauthors=translated by Martyn Drayton}}</ref><ref name="AFT">http://appliedfluidtech.com Applied Fluid Tech, Maryland USA</ref><ref name="WBSails">http://www.wb-sails.fi/news/index.html WB-Sails Finland</ref> 
According  to  [[kinetic theory]], at the microscopic level, air [[pressure]] is the result of [[collision]]s between perpetually moving air particles. Their energy, measured  by [[temperature]], determines  their velocity. In  still  air, the average particle of air randomly moves around an imaginary fixed point in space, colliding  with  other particles without too much average movement away from this point. Wind is the particles moving in large numbers in the same direction. So, air  pressure on a  sail has two origins: temperature  and the mechanical influence of wind.
 
[[Pitot tube]]s<ref name=EngToolBox1>{{cite web|title=The Engineering toolbox. Pitot tubes|url=http://www.engineeringtoolbox.com/pitot-tubes-d_612.html|accessdate=25 October 2011}}</ref>  and other types of [[manometer]]s are used in wind tunnel and full scale testing to measure the differences between local [[static pressure]]s at various points on the sail and [[atmospheric pressure]] (static pressure in  undisturbed flow). Results are graphed as [[pressure coefficient]]s (static pressure difference over wind induced [[dynamic pressure]]) to obtain windward "pressure" to leeward "suction" distribution curves along the mast/sail's chord.<ref>Marchaj p 57 Part 2 Ch 3 Distribution of pressures over sails figs 39 and 41</ref><ref>Fossati 8.12.2 p229 Test apparatus and measurement set-up</ref><ref name=Crook2>{{cite web|last=Crook|first=A|title=An experimental investigation of high aspect-ratio rectangular sails|url=http://ctr.stanford.edu/ResBriefs02/crook2.pdf|work=see Figure 2|format=PDF|publisher=Center for Turbulence Research Annual Research Briefs|accessdate=22 October 2011}}</ref><ref name=StanfordYachtResearch1>{{cite web|title=An explanation of sail flow analysis|url=http://syr.stanford.edu/SAILFLOW.HTM|accessdate=22 October 2011}}</ref><ref name=Viola1>{{cite journal|last=Viola|first=Ignazio|coauthors=Pilate, J,  Flay, R.|title=UPWIND SAIL AERODYNAMICS: A PRESSURE DISTRIBUTION DATABASE FOR THE VALIDATION OF NUMERICAL CODES|journal=Intl J Small Craft Tech, 2011|year=2011|volume=153|issue=Part B1|format=PDF|url=http://www.ignazioviola.com/ignazio_maria_viola/home_files/Viola_IJSCT2011.pdf|accessdate=22 October 2011}}</ref>
 
===Role of atmospheric pressure===
 
There are fewer air particles at high altitude. Collisions between slower, colder particles are less violent and less frequent.  Thus, there is less pressure. At sea level there are more particles with more energy, resulting in more frequent and violent  collisions, or higher pressure.
 
Close to the sail, collisions occur between sail and air particles. These collisions generate a force on the sail at sea level of about 10 [[kilogram-force|tonnes-force]] per square meter of sail (101325 [[pascal (unit)|Pa]]).  If the pressures on each side of a sail are perfectly balanced, the sail does not move.
 
===Value of force===
 
Forces of  air on each side of  the sail are due to:
* On the windward side, atmospheric pressure, wind pressure, and virtually no depression due to wind.
* On the leeward side, atmospheric pressure, a bit of depression and almost no wind pressure.
 
To simplify the manipulation of these forces, the forces are summed into a single force for the entire surface of the profile in a simple formula valid for airplane wings, rudders, sails or keels (see [[Lift (force)#Methods to determine lift on an airfoil|lift]]):
 
<math>F = C \times E </math>
 
with
 
* <math>E </math>: force obtained with maximum wind (see [[Max Q]]);
 
*<math> C </math>: Aerodynamic [[coefficient]]
 
::{| class="toccolours collapsible collapsed" width="80%" style="text-align:left"
!Explanation
|-
|
Briefly, each [[Fluid parcel|parcel]] of air crashing onto a small surface element of the sail, dS, generates a force dF. The force exerted on that part of the sail equals the [[pressure]] of the air, p,  on the sail times the small surface area in  a  direction opposite  to  the [[Vector area|normal]]  [[unit vector]], n: <math>\vec dF  = -p \, dS\, \vec n</math>.<ref>{{cite web|last=Roussel|first=J|title=MÉCANIQUE DES FLUIDES|url=http://perso.ensc-rennes.fr/jimmy.roussel/apprendre/fluides_parfaits.pdf |deadurl=yes |accessdate=23 October 2011}}{{dead link|date=September 2013}}</ref><ref>{{cite web |title=Intégrales en physique : Intégrales multiples |url=http://fr.wikiversity.org/wiki/Int%C3%A9grales_en_physique/Int%C3%A9grales_multiples |publisher=Wikiversite |deadurl=no |accessdate=2 September 2013}}</ref>
 
The airflow formula is derived from [[Bernoulli's principle]]. In steady state, along a [[Streamlines, streaklines, and pathlines|pathline]], and if [[heat transfer]] is neglected, with V, the speed of the wind relative to the sail; <math> \rho </math> (rho), the air [[density]]; g, the acceleration of gravity; z the elevation of a point on  the pathline; and p, the pressure at  the point:
:<math> \frac{V^2}{2.g} + z +\frac{p}{\rho.g}  = \mathrm{constant}</math>
 
As the elevation changes are small and are negligible compared to other terms, then :
:<math> \frac{V^2}{2.g} +\frac{p}{\rho.g}  = \mathrm{constant}</math>
 
The fluid is considered as incompressible or has little density change. (At Mach = 0.4, the error remains below 2%). At constant speed, consider that the sail which moves in the air at speed <math>V_0 </math> or that the air reaches the speed <math> V_0 </math> on the sail are exactly equivalent. Suppose that the air is fixed and the sail moves. Applying the formula to the parcel of air over the sail and then the same parcel of air before its arrival on the sail:
:<math>\frac{p_0}{\rho.g} = \frac{V^2}{2.g} + \frac{p}{\rho.g}</math> so <math>p_0 = \frac{\rho . V^2}{2} + p</math>
 
The [[pressure]] on the sail, <math> p </math>, is the difference between total pressure([[stagnation pressure]]), <math> p_0 </math>,  and the [[dynamic pressure]], <math> q </math>. The total pressure <math> p_0 </math> is constant on the pathline. So overall it vanishes when integrating the formula dF over the entire surface of sail, as pressure <math> p_0</math>  from one side of the sail is exactly balanced by the total pressure <math> p_0 </math> on the other side of sail, and is therefore eliminated. This leaves the dynamic pressure remaining: <math> \ p = q </math>
 
The [[dynamic pressure]] is equal to <math> q = \frac12 \times \rho V ^ 2 </math>. The [[dynamic pressure]] is the volume density of the kinetic energy of the air parcel <math> q = \frac12 \times \rho V ^ 2 = dE </math>.
 
where <math> \vec dF  = -dE \, dS \, \vec n</math>.
 
In this formula is dE is unknown but bounded. Indeed V is between 0 and V_0 as if the speed exceeds V_0, so that means the surplus energy which come from another energy than the Bernoulli principle has been neglected, i.e. sailing generate aerodynamic phenomena not negligible ever seen in reality (shock wave ...).
 
Its maximum is [[Max Q]] <math> = \frac{\rho . V_0^2}{2}</math>
 
<math> dE = c \times MAXQ </math>
 
With <math> \, c </math> a percentage of the kinetic energy density ranging from 0-100%. The percentage <math> \, c </math> is unknown, it must be determined by other means (additional equation or testing).
 
Hence integrating over the whole surface : <math> F = C \times E </math>
 
with
 
* <math>E </math>: force obtained with maximum wind <math> = Max Q \times S =\frac12 \times \rho V_0^2 \times S</math>;
 
* <math>C </math>: aerodynamic [[coefficient]]. The  portion of dynamic pressure (or the energy density) transmitted to  the sail.
 
Note that the total surface area, <math>\ S </math>, is equal to the surface of the windward inner surface, <math>\ S_i </math>, plus the leeward exterior surface, <math>\ S_e </math> : <math>\ S = S_i + S_e </math>.
 
with
* <math>F = C \times Max Q \times S = C_i \times Max Q \times S_i + C_e \times Max Q \times S_e</math>
 
But for practical reasons of comparisons of  airfoils, the surface,<math>\ S </math>, used in the tables is not the total surface of the object (wing, rudder or sail), but a characteristic surface. The virtual  surface intersecting the chord, <math>\ S_c </math>, is often used.
 
The form factors, <math>\ \alpha_i </math> and <math>\ \alpha_e </math> relate inner and outer to  chord surfaces: <math>\ S_e = \alpha_e \times S_c  </math>,  <math>\ S_i = \alpha_i \times S_c </math>.<ref name=piano1>{{cite web|title=Chapter: 05. Aerodynamic Characteristics|url=http://www.lissys.demon.co.uk/pug/c05.html|work=Piano software|accessdate=26 October 2011}}</ref>
 
So <math> F = Max Q \times S_c \times (C_i \times \alpha_i + C_e \times \alpha_e)= Max Q \times S_c \times C_c </math>
 
: <math>\ C_c</math> = coefficient of lift found in tables.
 
As the tables are based on the characteristic surface <math>\ S_c </math>, it follows that the coefficient <math>\ C_c </math> in the tables depends on two factors:
 
* a percentage factor of transmission of the dynamic pressure (or energy) <math>\ C_i </math> and <math>\ C_e </math>
* form factors. <math>\ \alpha_i </math> and <math>\ \alpha_e </math>.
 
In a slim sail airfoil profile, the surface intersecting the chord is close to the other surfaces, that is to say <math>\ \alpha_e \approx \alpha_i \approx 1</math>.
 
Usually, when it is  said that sail is 10m ², it actually means that the surface of the upper surface of the airfoil wing is 10m ². The actual surface of the sail including both sides is 20m ², but it is the value of 10m ² which must be used in the formula <math>F = C_c \times E = Max Q \times S_c \times C_c</math> for lift tables.
 
Although this calculation is an aid to understanding, the exact calculation of C is complex and uses the fundamental principles of dynamics. The calculation is discussed in the section: The case of multiple sails. In the following section, to lighten the notation,<math>\ C_c</math> will be noted <math>\ C</math> and <math>\ S_c</math> will be noted <math>\ S</math>.
|}
 
According to the [[Bernoulli's principle|Bernoulli equation]], the maximum stress of wind or maximum density of [[kinetic energy]] for the entire surface of the sail is:
 
<math>E  = e_c \times S = Max Q  \times S = \frac12 \times \rho \times S  \times V^2 </math>
 
The full expression of the force is:
 
<math> F = \frac12 \times \rho \times S \times C \times V ^ 2 </math>
 
with
 
* <math> F </math> : [[aerodynamic force]], expressed in [[newton (unit)|newton]]
 
* <math> \rho </math> (rho) : air [[density]] (<math> \rho </math> varies with the [[temperature]] and the [[pressure]]) ;
 
* <math>S </math> : typical surface. For the sail, it is the sail area in m²
 
* <math>C </math> : aerodynamic [[coefficient]], which is [[Dimensionless quantity|dimensionless]].  It is the sum of two percentages: the percentage of recovered energy on the leeward side + the percentage of the recovered energy on the windward side.<ref>coefficients of shape are neglected, because these are to close to 1.  Usually sails have insignificant thickness relative to their other dimensions.</ref> For this reason, the aerodynamic [[coefficient]]  can be greater than 1, depending on the angle of upwind sailing.
* <math>V </math>: Speed is the speed of the wind relative to the sail ([[Apparent wind]]) in m/s.
 
The sail is deformed by the wind, taking an [[airfoil]] form. When the flow of air around the profile is [[laminar flow|laminar]] the telltales of the sail (tufts of yarn or  ribbon attached to it) are stable, and the wind induced depression factor becomes crucial.  Based on studies and theories of sail design:<ref name=Larsson_pressure_dist>{{cite book|last=Eliasson|first=Lars Larsson & Rolf E.|title=Principles of yacht design|year=2007|publisher=International Marine|location=Camden, Me|isbn=978-0-07-148769-6|pages=Ch 7 Sail and Rig Design pp 142, 143 Fig 7.1|edition=3rd ed.}}</ref> 
* Depression on the upper (leeward side) represents two thirds of the aerodynamic force,
* The pressure on the lower surface (facing the wind) represents one third of the aerodynamic force.
 
===Breakdown of force: introduction to the concepts of lift and drag===
 
The general form of the force <math>F = \frac12 \times \rho \times S \times C \times V^2</math> is calculated or measured in an air stream, with speed as uniform as possible, arriving on the sail. The force is decomposed along three dimensions with  respect to wind direction. The [[Viscosity#Gases|viscous air]] rubs on the airfoil, and  creates resistance to  movement. More importantly, this viscosity disrupts the air flow around the airfoil. This disturbance causes a considerable force perpendicular to the airfoil. Because the airfoil is not infinite in  length, the ends  also generate a force in the remaining dimension.
 
[[File:FAA Lift Drag.JPG|thumb|500px|Airfoil diagram showing the relation of drag <math>\ C_D</math> and lift <math>\ C_L</math> to angle of attack. Lateral lift is almost never shown because the airfoil is treated as having infinite aspect ratio and measured values are low.]]
 
The breakdown according to three dimensions is:<ref>{{fr icon}} [http://www.aerodrome-ecuvillens.ch/pilote%20guide/aerodynamique.pdf texte very educational in french]</ref><ref>{{fr icon}} http://www.onera.fr/conferences/mesures-aerodynamique/cours-aerodynamique-mesures-efforts.pdf</ref><ref>{{fr icon}} http://docinsa.insa-lyon.fr/polycop/download.php?id=157288&id2=1</ref>
:<math>\vec{F} = \vec{F_x} + \vec{F_y} + \vec{F_z}</math>.
 
with:
*<math>\vec{x}</math> : The axis parallel to the direction of particles' movement <math>\ V</math> not yet disrupted by the sail, that is, well before the particles arrive on the sail. Force projected on this axis <math>\vec{x}</math> is called drag. For convenience, the force has the same equation <math>F_x = \frac12 \times \rho \times S \times C_x \times V^2</math>. The aerodynamic coefficient is replaced by a coefficient, <math>\ C_x</math>, adapted to this axis. By nature this force is resistive, i.e. the profile takes energy from the air. In the literature <math>\ C_x</math> is also noted <math>\ C_D</math>, with D for drag.
 
*<math>\vec{z}</math> :  The axis perpendicular to the direction of movement of particles <math>\ V</math> not yet disrupted by the sail, and perpendicular to the [[wingspan]].<ref>The reference for wingspan is often based on the line of all the first quarter of the chord. This first quarter chord is chosen because this is at the [[aerodynamic center]] where the pitching moment, M, does not vary with angle of attack <math> \ C_M(1/4c) = - \pi /4 (A_1 - A_2) </math> (see [[Airfoil]] and [[Aerodynamic center]]). The line is often a straight line.</ref> Force projected on this axis <math>\vec{z}</math> is lift. For convenience the force has the same equation <math>F_z = \frac12 \times \rho \times S \times C_z \times V^2</math>, where the aerodynamic coefficient is replaced by a coefficient, <math>\ C_z</math>, adapted to this axis. The direction of this force with respect to the sail varies with the value of the incidence. In the literature <math>\ C_z</math> is also <math>\ C_L</math> with L for lift.
 
*<math>\vec{y}</math> : The last axis. The force of the last axis is called lateral lift. The lateral lift's equation <math>F_y = \frac12 \times \rho \times S \times C_y \times V^2</math>. It is zero for an infinitely long airfoil.  For a sail, the profile has two ends and thus the lateral air pressures are balanced perfectly.  The airfoil shape is usually straight, long and thin, (bent airfoils such  as the [[gull wing]] are rare) which creates a low lateral lift compared to the first two axes. In our case of a sailing boat, usually the side lift is negligible. The airfoil model is then reduced to a simpler two-dimensional system. ( Note, 3D effects are taken into account as they  may have some influence.  For example, the induced drag is a purely 3D but the modeling is done in 2D.) The case of a spinnaker is a perfect counter example. The spinnaker has a low aspect  ratio and a high camber draught and it is difficult to determine clearly the axis of lift. The spinnaker generates forces along the three axes, and the vertical force has a great importance for pitching.
 
== Lift's effect on the sail ==
 
To study the effect of lift we can compare cases with and without lift.<ref>[http://www.francelaser.org/lettre/mf-jvp/jvp1.htm Vent réel - vent apparent - forces aéro et hdrodynamiques<!-- Bot generated title -->]</ref> As an approximation of a [[gaff rig|gaff sail]], take a sail that is rectangular and approximately vertical, with an area of 10 m² - 2.5 m of foot by 4 m of leech. The apparent wind is 8.3&nbsp;m/s (about 30&nbsp;km/h). The boat is presumed to have uniform velocity, no heel and no pitch and there are no waves. The density of air is set at: ρ = 1.2&nbsp;kg/m³.
 
===Sailing in stalled flow===
 
The boat is [[Points of sail|running downwind]]. The shape of the sail is approximated by a plane perpendicular to the apparent wind.
 
The depression effect on the sail is second order, and therefore negligible. The remaining pressures are:
* on the windward side atmospheric pressure and wind pressure
* on the leeward side only the atmospheric pressure
 
Forces of atmospheric pressure cancel out. There remains only pressure generated by the wind.
 
Roughly speaking, collisions of particles on the sail forward all their energy from wind to 90% of the surface of the sail. This means that the Cz or aerodynamic lift coefficient is equal to 0.9.
 
<math>F = \frac12 \times 1.2 \times 10 \times 0.9 \times 8.3^2 = 372 \ newton</math>
 
Wind on sail could be modelled as a jet of air with the sail a deflector. In this case the  [[Momentum|theorem of momentum]] is applied. Effort on sail varies as a sinusoid of angle of attack, <math>\alpha</math>, with wind.<ref>[http://mpsn.free.fr/fuidique/CoursTd/TdC_Fluide_ch4_1-3_noprint.pdf]</ref><ref>[https://prof.hti.bfh.ch/uploads/media/F-HY-Tf_02.pdf]</ref><ref name="sinus">[http://www.meca.u-psud.fr/cours/L3_TP_Jet.pdf see équation (10)]</ref><ref>[http://fly.in.free.fr/MecaFlotte/chap4%20bach.pdf page 12]</ref> Force is <math>F = \rho \times V^2 \times S_{air} \times \cos ( \frac{\pi}{2} - \alpha) </math> <ref name="sinus" /><ref>[http://hmf.enseeiht.fr/travaux/CD0102/travaux/2h/tpld/moulins/sommaire/theorie/jetplat.htm]</ref><ref>[http://books.google.fr/books?id=ZUbxy_1xWDEC&pg=PA211&lpg=PA211&dq=plaque+jet+d%27eau+force&source=bl&ots=HZHvcs18Wn&sig=KBtpku9lDuVAv-kVp3DTNmNGm-Q&hl=fr&sa=X&ei=__UgUP-1Aaek0QWKroHQBw&ved=0CHIQ6AEwBTgK#v=onepage&q=plaque%20jet%20d%27eau%20force&f=false name of book: Mécanique des fluides 2e année PC-PC*/PSI-PSI*: Cours avec exercices corrigés by Régine Noel,Bruno Noël,Marc Ménétrier,Alain Favier,Thierry Desmarais,Jean-Marie Brébec,Claude Orsini,Jean-Marc Vanhaecke see page 211]</ref>
 
At 90° and running downwind, force is maximal and <math>S = S_{air} </math> then  <math>F = \rho \times V^2 \times S </math>, so <math>C_D = 2 = C_{D_{max}}  </math>.
 
In reality, <math>C_{D_{max}}</math> depends on the profile.  The coefficient is set between around 1 to 2.<ref>[ftp://nrg-nl.com/pub/www/library/report/1995/c95061.pdf] Cd of plate</ref> Two is also a good number for many rigid profiles <ref>[http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA377080 exemple naca0012]</ref> and around one is a good number for a sail.<ref>[http://www.yru-kiel.de/fileadmin/media/pdf/RANSE_Investigations_of_Downwind_Sails_and_Integration_into_Sail_Yacht_Design_Processes.pdf]</ref>
 
===Sailing in attached flow===
 
The boat is close hauled, with the sail set at, for example, 15° relative to the apparent wind. The camber of the sail creates a lift. In other words, the effect of depression on the leeward side comes into play. As air pressure forces cancel out, significant resulting forces are:
* on the windward side wind pressure
* on the leeward side wind depression
 
The only unknown  to be determined is the drag coefficient. In a well trimmed sail the curve profile is close to optimal airfoil shape [[NACA airfoil|NACA]] 0012.<ref>http://www.lmm.jussieu.fr/~lagree/TEXTES/RAPPORTS/rapportsX/voilesNorvezPernot.pdf</ref><ref>indeed if the airfoil is symmetrical and sail shape not symmetrical</ref> A less well trimmed sail, perhaps  of  older  technology, will have  greater  draft with more camber. The coefficient of aerodynamic lift will be higher but the sail will be less efficient with  a lower lift/drag ratio (L/D). The sail profile may be similar to  NACA 0015, NACA 0018.<ref>[http://www.ae.metu.edu.tr/tuncer/ae443/docs/NACA-All-Re.pdf hnjb324.tmp<!-- Bot generated title -->]</ref>
 
For a given profile, there are tables which give the lift coefficient (Cz), which depends on several variables:
* [[Angle of attack|Incidence angle]] of apparent wind to sail profile,
* The slope of lift of the sail, which depends on its [[Aspect ratio]],
* The surface roughness and [[Reynolds number]], which affect the flow of fluid (laminar, turbulent).
The coefficient is determined for a stable and uniform fluid, and a profile of infinite extension.
 
The Reynolds number is: <math> \mathrm{Re} = {{\rho {\bold \mathrm U} L} \over {\mu}} = {{{\bold \mathrm U} L} \over {\nu}}</math>
 
with
 
*<math> U </math> - fluid velocity or apparent wind [m/s]
*<math> L </math>- characteristic length - since this is a rectangular sail the length at any height will do, e.g. the foot of the sail [m]
* <math> \nu </math> - fluid [[kinematic viscosity]]: <math> \nu = \eta / \rho </math> [m/s]
* <math> \rho </math> - air [[density]] [kg/m³]
* <math> \mu </math> - air [[dynamic viscosity]] [Pa] or Poiseuille [pl]
 
so for this sail about  <math> \mathrm{Re} =  10^6</math>
 
With an incidence angle of 15° and a [[Reynolds number]] of one million a NACA0012 profile reached a Cz of 1.5 (as opposed to 0.9 for 90° incidence).
 
<math>F = \frac12 \times 1.2 \times 10 \times 1.5 \times 8.3^2 = 620 \ newton</math>
 
The lift has increased by 50%. The force on the [[Sheet (sailing)|sheets]] and rig also increases by 50% for the same apparent wind.
 
===Contribution of lift to the progress of the vessel===
[[File:DiagramApparentWind.gif|thumb|150px|Detailed diagram outlining the boat velocity vectors (V), wind (W) and apparent wind (A) for a sailing boat.]]
 
When running downwind the direction of the [[apparent wind]] is equal to that of the true wind and most of the  sail force contributes to the advancement of the ship. There is no sail lift, so the boat can not go faster than the wind, and propulsive force decreases gradually. When the ship approaches the speed of the true wind, the apparent wind speed and the force drop to zero.
 
In the cases with lift, the sail has an angle of incidence with the apparent wind. The apparent wind also forms an angle with the true wind. Similarly, wind creates an angle to the direction taken by the ship. Forces on the sail do not contribute fully to the advancement of ship. With a ship pointing close hauled, an example scenario is:
* <math> \beta </math>,  angle between apparent wind and ship's course, is 40°.
* <math> \alpha </math>,  incidence angle between apparent wind and sail chord, is 20°.
* <math> \lambda </math>, leeway, is nil.
(See  diagram under section [[#Lift/drag. Upwind sail cut and trim|Lift/Drag. Upwind sail cut and trim]]  for  illustration  and  definition of relevant angles on  upwind  sailing.)
 
The lift force  vector, perpendicular to the  apparent wind,  does not participate fully in the progress of the vessel. It forms an angle of 40° to  the  course sailed. The propulsive force vector  is more than 76% of the total value. The remaining 36%<ref>It pushes the sail on the major axis, Fprin, of the vessel and its perpendicular, Fper. F is the forward thrust of the sail. Fprin = F * cos 40° = 76% * F. Fper = F * sin  40° = 36% * F</ref> is perpendicular to the vessel, and generates [[leeway]] angle and  [[Heeling (sailing)|heeling]] [[Torque|moment]].
 
For the same sail with the same apparent wind speed, lift coefficient is 1.5 close hauled and 1 downwind. The vector  of force towards advancement of the vessel remains 15% above cases without lift.
 
[[File:Point of sail wind.png|thumb|500px|center|Velocity vectors boat, wind and apparent wind at different points of sail. For the same boat, apparent wind speed close hauled is much higher than downwind. Consequently close hauled speed is well over 15% of sailing downwind]]
 
The more the boat accelerates the more the apparent wind increases. So the force on the sail increases. At each speed increase apparent wind direction moves.  So, re-trimming the sail is needed for optimum effect (maximum lift). The more the boat accelerates, the smaller the angle of the apparent wind to the direction of the ship. So sail force angle is less oriented towards the course of the boat, requiring bearing down a bit to gain maximum power sailing conditions. The ship can go faster than the true wind. The ship to wind angle can be quite small. Consequently the point of sail may approach  the  dead  zone requiring the boat to back away from the wind.
 
=== Influence of apparent wind ===
When a ship is moving, its velocity creates a relative wind. The sum of the true wind and  the relative wind is called the apparent wind. If the ship moves upwind, the two winds are cumulative, and the apparent wind is larger than the actual wind. Downwind, the effect is reversed, winds are subtracted and the apparent wind is lower than the true wind.
 
=== Influence of rigging tension on lift performance ===
 
Trimming a sail involves two parameters:
* [[Angle of attack]], or  incidence. i.e. the angle of apparent wind to sail chord to  create maximum lift, or a maximum L/D. This angle varies with sail height, which is [[Sail twist|aerodynamic twist]].
* [[Airfoil]] profile which is composed of:  1. [[Camber (aerodynamics)|Camber]] of the sail as defined as the ratio of maximum [[Draft (sail)|draft]] depth to chord length and 2. Draft position.
 
Sail set and shape is  generally flexible.<ref>not the case for hydroptere, wind surf...</ref>  When the sail is operating in lift, if a sail is not properly inflated and stretched, there are wrinkles on the sail. These folds form a break in the profile. The air does not slip along the sail. The air streams come off the airfoil profile. Areas of recirculation or turbulent separation bubbles appear. These areas considerably diminish the performance of the sail. The assumption of a  non  wrinkled profile  will  simplify sail analysis.
 
A sail may be rigid where the canopy is composed of non stretchy fiber. Tightening a flat piece of such cloth inflated by the wind results in folds at the attachment points. To avoid wrinkles, the sail could be tightened harder. The tension can be considerable to eliminate all wrinkles. So, in the case of a taut rigid sail, the inflated shape is static, hollow and with its draft position immobile.
 
The more elastic sail deforms slightly to its locations of high stress on the material, thereby eliminating wrinkles. The sail is no longer flat. Consequently, the sail can take several forms. By varying the tension of the sail, it is more or less empty. It is possible to vary the shape of the sail without folds. The potential sail shapes are intrinsically linked to the cut of the sail. So in the elastic case, there is a family of possible forms and draft depths and positions the sail may take.
 
Sailmakers try to build rigidity into sails for a predictable working shape with a degree of advantageous resilience depending on  the sail's type, application and range:  racing, cruising, high, moderate or variable  wind, etc.
 
The airfoil profile of the sail changes depending on the sail trim. At a given incidence, the sail can take different forms. The shape depends on the rigging tensions such as on [[Parts of a sail#The corners|clew]] corner of the sail,  the [[tack (sailing)|tack]] with [[Cunningham (sailing)|Cunningham]]  adjustment, the [[backstay]],  the  outhaul,  the  [[halyard]]s or  the [[Boom vang|boom vang (kicking strap)]]. These elements help determine the shape of the sail. More exactly, they can decide position of maximum [[Draft (sail)|draft]] along the camber of the sail.<ref>[http://books.google.fr/books?id=3xUwXPoJ1loC&pg=PA150&lpg=PA150&dq=voilier+vitesse+longueur+de+flottaison&source=bl&ots=LshK-LIcxN&sig=wo3DmLfFdrdZpI-Qr_6ikYJ9RV0&hl=fr&ei=rNbKS5uhBcGBOI3T0O0F&sa=X&oi=book_result&ct=result&resnum=10&ved=0CCwQ6AEwCQ#v=onepage&q=voilier%20vitesse%20longueur%20de%20flottaison&f=false book partially scanned  ''Bien naviguer et mieux connaître son voilier'' by Gilles Barbanson,Jean Besson sheet 72-73]</ref>
 
Each profile represents an appropriate value of Cz (lift coefficient). The position of the [[Draft (sail)|draft]] along the chord with the most lift is about 40% of the foot from luff. The leeward side of a sail is close to the [[NACA airfoil|NACA]] series 0012 (NACA 0015, NACA 0018, etc.) within the possibilities of trimming.
 
The position of the [[Draft (sail)|draft]] is not independent of the camber setting. These parameters are linked by the shape of sail.  Modifying the camber modifies the position of the [[Draft (sail)|draft]].
 
====Camber====
The curves of propulsive component of lift and heel versus the angle of attack vary with the camber of the sail, that is to say, the biggest draft depth relative to the chord of the sail. A sail with high camber has a higher aerodynamic coefficient and, potentially, a greater propulsive force. Though the heeling coefficient varies with draft depth in the same direction.  So finding the optimal camber will be a compromise between achieving a large propulsive force and an acceptable list.<ref>Principles of yacht design, by Lars Larsson et Rolf E Eliasson ISBN 0-7136-5181-4 or 9 780713 651812 page 140 figure 7.9 and 7.10</ref>{{,}}<ref>[http://www.hiswasymposium.com/assets/files/pdf/previous/17th%20-%202002/17th%20-3-%20Optimization%20of%20yard%20sectional%20shap%20and%20configurati.pdf figure 5]</ref>
 
[[File:Sail Camber Aerodynamic coef.png|thumb|500px|center| Propulsive and heel aerodynamic coefficients and sail camber depth.]]
 
Note that with a  small camber (1 / 20), performance degrades significantly. The propulsion coefficient plateaus around a ceiling of 1.0.
 
====Draft position====
The curves of propulsive lift and heel as a function of the angle of attack also depend on the position of the draft's proximity to the luff.<ref>Principles of yacht design, by Lars Larsson and Rolf E Eliasson ISBN 0-7136-5181-4 or 9 780713 651812 page 140 figure 7.11</ref>{{,}}.<ref>[http://www.iawe.org/Proceedings/5EACWE/142.pdf figure 5]</ref>
 
[[File:Draft position on sail forces.png|thumb|500px|center|Propulsive and Heel aerodynamic coefficients, point of sail for varied masted mainsail draft positions. After Larsson and Eliasson wind tunnel data. Note a more forward position would likely suit the jib.]]
 
===Influence of Aspect ratio and Sail Planform on Induced Drag===
 
Sails are not infinitely long. They have ends.  For the mainsail:
* [[Boom (sailing)|Boom]]
* [[Sails#Parts of the sail|Head]].
 
The transfer of air molecules from the windward pressured side  to the leeward depressed side around the edge the thin sail is very violent. This creates significant turbulence, loss  of pressure difference and  loss of  propulsion. On the end of a  wing  this  is  manifest  as  [[wingtip vortex]]. On a Bermuda sail, foot and leech are two areas where this phenomenon exists. The drag of leech is included in drag in the usual lift curves.  The sail airfoil profile is considered as infinite (i.e. no ends). But foot drag  is calculated separately. This loss of efficiency of the sail at the foot is called [[Lift-induced drag]].
 
====Influence on coefficients====
 
[[File:Influence of aspect ratio on sail forces.png|500px|thumb|Aspect ratio influences on driving and side forces at different points of sail. From wind tunnel data of C A Marchaj.]]
 
Lift-induced drag is directly related to the narrowness of the extremities due  to  premature  stall over the  heavily  loaded short  chord profile. The longer is the narrow head, the higher is induced drag. Conversely, the sail can be reefed, i.e. reduce surface of the sail without reducing the length of the head. This means that value of the lift-induced drag will be substantially the same. For a given length of head, the more sail area, the lower is the ratio of lift-induced drag on lift. The more elongated the sail, the less lift-induced drag alters value of the lift coefficient.
 
[[File:Windsurf.600pix.jpg|thumb|The curved shape of the mast and the battens to maintain the curved profile of the leech are clearly visible in this picture of a windsurfing sail.]]
 
Lift-induced drag on  the  sail also depends on aspect ratio, λ. The equation is defined:<ref>http://www.dedale-planeur.org/horten/Horten%20critique%20par%20Deszo.pdf</ref> 
:<math>\lambda = {b^2 \over S}</math>
 
with
*<math>b</math> the length of luff
*<math>S</math> the surface area of the sail.
 
Lift-induced drag is:
 
:<math>Ci = {{Cz^2}  \over {\pi \times \lambda \times e}} </math>
 
with
* <math>Cz</math> : [[Lift coefficient]] of airfoil
*<math> \pi </math>  : <math> pi \approx 3.1416 </math>
*<math> \lambda </math> : [[Aspect ratio (wing)]] (dimensionless)
* <math> e </math> : [[Oswald efficiency number]] (less than 1) which depends on the distribution of lift over the sail span. "e" could be equal to 1 for an "ideal" distribution of lift (elliptical). Elliptically shaped ends help reduce induced drag. In practice "e" is the order of 0.75 to 0.85. Only a three-dimensional model and tests can determine the value of "e".
 
Optimal distribution for maximum reduction of lift-induced drag is elliptical in shape.<ref>http://air-et-terre.info/aerodyn_theorique/ligne_portante_3D.pdf</ref><ref>http://j.haertig.free.fr/aerodyn_theorique/ligne_portante_3D.pdf</ref> Accordingly, the luff will be elliptical. So, the mast is not straight as on a classic boat, rather designed with the closest possible form to an ellipse. An elliptically configured mast is possible with modern materials. This is very pronounced on surfboards. On modern sailboats the mast is curved thanks to [[Shroud (sailing)|shrouds]] and  backstays. Similarly, the leech will be elliptical.<ref>This ideal elliptical shape is result of calculus for a stable and uniform flow of wind, as wind is not uniform (see :Influence of altitude: aerodynamic twist and sail twist), the ideal shape must be mitigated.</ref> This profile is not natural for a flexible sail. So, mainsails have battens to maintain this roach curve.
 
An ideal lift-induced drag distribution creates an elliptical sail. But current sails are rather a half-ellipse, as if the second half part of the ellipse was completely immersed in the sea. This is logical because, as wind speed is nil at the sea level (0 m), the sea is equivalent to a mirror from an aerodynamic point of view.<ref>[http://hal.archives-ouvertes.fr/docs/00/45/37/91/PDF/these_Roncin.pdf in French see page 51 ] in this thesis, the author explained that due to proximity of the deck, the deck can be used as mirror surface instead of sea level.</ref>  So only half an ellipse in air is necessary.
 
====Influence on efforts====
 
Formulae are :
:<math>F_L = \frac12 \times \rho \times S \times C_L \times V^2</math>
:<math>F_i = \frac12 \times \rho \times S \times C_i \times V^2</math>
:<math>Ci = {{C_L^2} \over {\pi \times \lambda \times e}} </math>
:<math>\lambda = {b^2 \over S}</math>
 
Then :
:<math>F_i= F_L^2/ (0.5 \times\rho \times V^2 \times e \times b^2 )</math>
 
This result is important(cf. ''Lift/Drag ratio and Power'' paragraph). The induced drag force  (not coefficient) is independent of aspect  ratio (<math>\lambda </math>). In sailing, lift is quite often limited by the maximum righting moment.  Since induced drag force doesn't depend on  <math>\lambda </math>, but does depend on the lift coefficient (<math>C_L </math>) which depends on sail area, for optimal performance <math>\lambda </math> may be changed while keeping span the same. This [[Wing loading|concept]] is often used by airplane designers.
 
<ref>Also see: Marchaj, Sail Performance..., Part 2, Ch 10, "The importance of sail planiform"; Larsson and Eliasson, Principles of  Yacht  design 3rd  ed, Ch  7,  "Sail and  Rig design" ; and Fossati, Aero-Hydrodynamics..., Ch 5.5, "Sail Drag"</ref>
 
===Influence of the height of the foot relative to sea level===
 
The gap between the edge of the sail and the sea surface has a significant influence on performance of  a  Bermudan  type  sail. In effect it creates an additional [[Wingtip vortices|trailing edge vortex]]. The vortex would be nonexistent if the border were in contact with the sea. This vortex consumes extra energy and thus modifies the coefficients of lift and drag. The hole is not completely empty, as the sail is partially filled by the freeboard and superstructure of any sailboat.
 
For a height between the edge of the sail and the deck of the sailboat of 6% of the length of the mast, changes are:
* a 20% increase in the drag coefficient
* a 10% loss in the lift coefficient.<ref name=Larsson1>{{cite book|last=Larsson|first=Lars|title=Principles of yacht design|year=1999|publisher=Adlard Coles Nautical|location=London|isbn=978-0-7136-5181-2|pages=139 figure 7.8|edition=2nd ed.|coauthors=Eliasson, Rolf E.}}</ref>
 
The [[crab claw sail]] may partially circumvent this  problem by harnessing  the [[delta-wing]]'s [[vortex lift]].
 
===Shape of luff, leech, and foot===
 
A sail hauled up has a three-dimensional shape. This form is chosen by the sailmaker. The 3D shape is different for the hauled up form compared to when empty of wind. This must be taken into account when cutting the sail.
 
The general shape of a sail is a deformed polygon. The polygon is slightly distorted in the case of a [[Bermuda rig|Bermuda sail]] and heavily distorted in the case of a [[spinnaker]]. The shape of edges empty is different from shape of edges once the sail is hauled up. Convex empty can go to straight edge when the sail is hauled up.
 
Edges can be:
* convex
* concave
* straight
 
When the convex shape is not natural (except for a free edge in a spinnaker), the sail is equipped with battens to maintain this pronounced convex shape. Except for the spinnaker with a balloon shape, the variation of edge empty compared to straight line remains low, a few centimeters.
 
Once hauled up, an elliptical sail would be ideal. But as the sail is not rigid:
* You need a mast, which for reasons of technical feasibility, needs to be quite straight.
* Flexibility of the sail can bring other problems, which are better to fix at the expense of an ideal convex elliptic shape.
 
====Leech====
 
On a  Bermudan type  sail the oval is the ideal (convex), but a concave shaped leech improves the twist at the top of the sail and prevents overpowering the top of the sail in the gusts, thereby improving the boat's stability. The concave leech makes sailing more tolerant and more neutral. A convex shape is an easy way to increase the sail area (roach). Marchaj<ref name=Marchaj3>{{cite book|last=Marchaj|first=C. A.|title=Sail performance : techniques to maximise sail power|year=2003|publisher=Adlard Coles Nautical|location=London|isbn=978-0-7136-6407-2|pages=208–211|edition=Rev. ed.}}</ref>  discusses crescent  shaped foils like a raked [[wing tip device]] as seen on various  fish  fins, Brazilian  [[jangada|jungada]]  sails, [[crab claw sail]]s,  and America's  cup boat ''Stars and  Stripes''  to  reduce  lift  induced  drag.
 
====Luff====
 
Once hauled up, the edge must be parallel to the forestay or mast.  Masts and spars are very often, except in  windsurfing, [[jangada]] boats, and  [[proas]], straight.  So, a straight luff is usually needed.
 
But the draft of the sail is normally closer to luff than foot. So to facilitate the implementation of draft of the sail  when hauled up, the empty form of luff is convex.<ref>[http://chazard.org/emmanuel/cours-de-catamaran-reglage-de-la-grand-voile-gv www.emmanuel.chazard.org<!-- Bot generated title -->]</ref> This convexity is called the luff curve. Sometimes rigging is complex and  the mast is not straight.<ref>[http://www.finn-france.fr/TECHNIQUE%20VOILE/michaud1.pdf Microsoft PowerPoint - analyse des forces.ppt<!-- Bot generated title -->]</ref> In this case,  the shape of luff empty can be convex at bottom and concave at the top.
 
====Foot====
 
Foot form has little importance, particularly on sails with a loose  foot or free edge. Its shape is more motivated by aesthetic reasons. Often it is convex empty to be straight once hauled up. When the border is attached to a spar or boom a convex shape is preferred to facilitate formation of draft of the sail. On retractable booms, the shape of the edge of the border is chosen based on technical constraints associated with the reel than consideration of aerodynamics. A [[winglet]]<ref name=Al_Atabi /> as used on airplanes to minimise lift induced drag is so far not practically seen on  sails.
 
===Relationship of lift coefficient to angle of incidence: polar diagram===
 
[[File:Polar sail AR.jpg|thumb|upright=2.5|Polar curves showing the relationship between lift and drag for sails of aspect ratios 6, 3, 1 and 1/3 over varied incidence angles.]]
 
The [[Flight dynamics (fixed wing aircraft)#Aerodynamic coefficients|aerodynamic coefficients]] of the sail vary with [[angle of attack]] (incidence of chord to apparent wind). The analysis on a polar diagram correlates to the respective lift and  drag components of aerodynamic force:
* The component perpendicular to the apparent wind is called the lift coefficient;
* The component parallel to the apparent wind is called the drag coefficient.
 
Each incidence angle corresponds with a single lift-drag pair.
 
Summarising the behaviour of the sail at varying incidence:<ref>[http://membres.multimania.fr/tpevoile/faero.htm Etude de la force aérodynamique<!-- Bot generated title -->]</ref>
* When the sail is loose, this is the equivalent to having no sail.  Lift and drag from the sail are effectively null.<ref>If the sail is loose, the sail shakes, thus providing some resistance. The sailing ship is slightly back, in this case there is a slight drag. It is also noted that under these conditions the mast, the rigging, superstructure and topsides will provide much more aerodynamic force than the sail itself.</ref>
* When the sail is perpendicular to the wind, the air movement is turbulent.<ref>telltales are unstable</ref> This is the case of no lift and maximum drag.
* These  are the intermediate cases:
** Sail loose to maximum lift: the flow is attached, i.e. there is an airfoil. There are no eddies (dead zones) created on the sail. It is noted in the case of a good well trimmed sail, maximum lift is greater than maximum drag.
** Maximum lift to maximum dead zone: the wind does not stick properly to profile of the sail. Flow is less stable. Air becomes gradually lifted or taken off. This creates an area on leeward side, a dead zone where depressions form on the  sail. At typical angle, dead zone has invaded the leeward side.
** The dead zone to maximum drag: Dead zone has invaded the whole face on the leeward side, only on the windward side is there an effect. Air in these high angles, is somewhat deviated from its trajectory. Air particles are just crashing on all surfaces of windward side. Force is almost constant, so the polar sailing describes an arc of a circle.
 
As the lift is more effective than drag in contributing to the advancement of ship, sail makers trying to increase the zone of lift, i.e. increase force of lift and angle of incidence. The  task of a knowledgeable sailmaker is to decrease the size of the dead zone at high angles of incidence, i.e. to control the boundary layer.<ref>[http://www.onera.fr/mecao/aerodynamique/phototheque/video/naca12.htm Naca 12<!-- Bot generated title -->]</ref>
 
===Influence of altitude: aerodynamic twist and sail twist===
[[File:Bluenose Sails Away - 1921.jpg|thumb|The gaff [[schooner]] ''Bluenose''.]]
 
The rapid increase of the wind speed with altitude will increase both apparent wind speed and its angle of incidence to course sailed, (β).<ref>[http://syr.stanford.edu/JWEIA557.pdf figure 5]</ref><ref>[http://www.finot.com/ecrits/Damien%20Lafforgue/article_voiles_english.html#Titre_2 Section 2.2 Apparent wind-true wind]</ref> When using sails with lift, the sail must be [[Sail twist|twisted]] to have a consistent angle of incidence of sail with apparent wind, (α), along the leading edge (luff). This  results in  the  lower sail chords being at  smaller angles  to  the  course sailed,  (β - α),  ( see decomposition of forces diagram  below) than the upper chords to compensate for  the  smaller β angle closer to  the deck.
 
The air moves primarily in slices parallel to the ground or sea. While air density can be regarded as constant for our calculations of force, this is not the case for wind speed distribution. Wind speed will increase with altitude.  At the sea surface, the difference of speed between air particles and water is zero. The wind speed increases strongly in the first ten meters.<ref>http://hal.archives-ouvertes.fr/docs/00/16/72/71/PDF/B104.pdf</ref><ref>http://www.ignazioviola.com/ignazio_maria_viola/publications_files/Viola_EACWE2005.pdf Zasso A, Fossati F, Viola I. Twisted flow wind tunnel design for yacht aerodynamic studies. EACWE4 — The Fourth European & African Conference on Wind Engineering
J. N´ prstek & C. Fischer (eds); ITAM AS CR, Prague, 11–15 July 2005, Paper #153</ref><ref name="Keikki"/><ref>http://techniques.avancees.free.fr/tipe/techniquesAvanceesGeneral.pdf</ref>
KW Ruggles gives a generally accepted formula for the relation of the wind speed with altitude:
 
<math>U = \frac {\mu'} {k}  \ ln ( \frac {z + z0} {z0}) </math><ref>[http://syr.stanford.edu/JWEIA557.pdf sheet 2]</ref><ref>[http://airsea.ucsd.edu/papers/MELVILLE%20WK%20-%20JOURNAL%20OF%20PHYSICAL%20OCEANOGRAPHY%207%20-%201977.pdf formula is given  in introduction]</ref><ref>http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD734670&Location=U2&doc=GetTRDoc.pdf</ref>
 
With data collected by Rod Carr<ref>[http://www.onemetre.net/Design/Gradient/Gradient.htm Wind Gradient<!-- Bot generated title -->]</ref> the parameters are:
* k = 0.42,
* z altitude in meters;
* z0 is an altitude that reflects the state of the sea, i.e. the wave height and speed:
** 0.01 for 0-1 [[Beaufort scale|Beaufort]];
** 0.5 2-3 Beaufort
** 5.0 to 4 Beaufort;
**  20 5-6 Beaufort;
* <math>\mu' </math>= 0335 related to viscosity of air;
* U m / s.
 
In practice, the twist must be adjusted to optimize the performance of the sail. The primary means of control is the boom for a Bermuda  mainsail. The more the boom is pulled down, the less twist. For  the  foresail, depending  on  the  rig,  twist is controlled by adjusting jib leech tension  through sheet tension adjustments of:  sheet angle with sheet block track (fair lead) position,  jib  halyard tension, jib Cunningham tension, or forestay tension.<ref>http://media.wiley.com/product_data/excerpt/0X/04705165/047051650X.pdf</ref><ref>[http://www.sailingusa.info/sail_shape.htm Sail Shape<!-- Bot generated title -->]</ref>
 
===Influence of the roughness of the sail===
 
As on a hull or wing, roughness plays a role on the performance of the sail. Small humps and hollows may have a stabilizing effect or facilitate stalls as when switching from laminar to turbulent flow. They also influence friction losses.
 
This area is the subject of research in real and wind  tunnel conditions. It is currently not simulated numerically. It appears that at high Reynolds number, well chosen roughness prolongs the laminar mode incidence a few degrees more.<ref>http://www.usna.edu/naoe/people/SCHULTZ%20PAPERS/Miklosovic,%20Schultz%20&%20Esquivel%20JoA%202004.pdf</ref><ref>http://www.usna.edu/naoe/people/SCHULTZ%20PAPERS/Schultz%20JFE%202002.pdf</ref>
 
===Influence of the Reynolds number===
 
The [[Reynolds number]]  is a measure of the ratio of inertial forces to viscous forces in  moving fluids.
It  also  indicates degrees  of  laminar  or  turbulent flow.
Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant,
and is characterised by smooth, constant fluid motion. Turbulent flow occurs at high
Reynolds numbers and is dominated by inertial forces.
 
The stronger the wind, the more the air particles tend to continue moving in a straight line,
so are  less likely to stick to the wing, making the transition to turbulent mode nearer.
The higher the Reynolds number  the better the performance of the sail (within other optimal parameters.)<ref>[http://aerade.cranfield.ac.uk/ara/arc/rm/3726.pdf figure 6 page 23]</ref>
 
The lift force formula  <math>F = \frac12 \times \rho \times S \times C \times V^2</math>  is  practical and easy to use.
The aerodynamic lift coefficient, C, depends on  wind speed, V, and surface characteristics.
The lift coefficient depends on Reynolds number as shown in the tables and polar diagrams.
The Reynolds number is defined by <math> \mathrm{Re} = {{{\bold \mathrm U} L} \over {\nu}}</math> .
The Reynolds number depends on wind speed, U, and length, L, travelled by the air (characteristic chord length) and [[kinematic viscosity]], <math>{\nu}</math>.
But the influence of the Reynolds number is second order relative to other factors.
The performance of the sail changes very little for a variation of the Reynolds number.
The influence of very low Reynolds number is included within the tables (or chart)
by plotting the lift coefficient (or drag) for several values of the Reynolds number (usually three values).
 
Increasing the incidence or the maximum lift coefficient by good choice of the Reynolds number
is very interesting but secondary. The Reynolds number depends only on three parameters: speed, viscosity and length:
 
Viscosity is a physical constant, it is not an input variable for optimisation.
 
Wind speed is a variable of optimisation. It is obvious that we look for  the highest possible wind speed on the sail
for sailing maximum force much more than for reasons of Reynolds number. This parameter has already been optimised.
 
The sail is inherently inelastic and of fixed size. So, the characteristic length is fixed for a given sail.
Length optimisation is the responsibility of the naval architect, except for sail changes by  the  sailor.
Performance tuning of the sails by varying the characteristic length of
the Reynolds number is masked by the optimisation of other parameters, such as looking for better sailing performance
by adjusting the weight of the sails. The weight of the sail is an important point for the balance of the ship.
Just a little more weight in the higher part of the sail may create a major change affecting the balance of the ship.
Or, for high winds, the sail fabric must resist tearing, so be heavy. The sailor is looking for a set of sails adapted
to each range of wind speeds for reasons of weight more than for reasons of Reynolds number: jib, storm sail, main sail,
spinnaker, light genoa, heavy genoa, etc. Each wind speed has its sail. Higher winds tend to force small characteristic
lengths.
The choice of the shape of the sails and therefore the characteristic length is guided by other criteria
more important than the Reynolds number.
The price of a sail is very high and therefore, limits the number of sails.
 
The coefficients of lift and drag, including the influence of the Reynolds number,
are calculated by solving the equations of physics governing the flow of air over a wing using
computed simulation models. The results found are well correlated with reality, less than 3% error.<ref>http://bbaa6.mecc.polimi.it/uploads/validati/TR02.pdf Viola, I.M., Fossati, F. Downwind sails aerodynamic analysis.
BBAA VI International Colloquium on: Bluff Bodies Aerodynamics & Applications. Milano, Italy, July, 20-24 2008.
</ref>
 
==Lift/Drag ratio and Power==
[[File:420er 002.jpg|thumb|400px|Example of sailboat racing upwind, making the crew trapeze to decrease the heel.]]
Polar curves of lift versus drag initially have a high slope.
This is very well explained by the [[Lifting-line theory|theory of thin profiles]].
The initial constant drag and lift slope becomes more horizontal, as maximum lift is approached.
Then at higher angles of incidence a dead zone appears, reducing the effectiveness of the sail.
The goal of the sailor is to set the sail in the incidence angle where the pressure is maximum.
Considering the proper  tuning for a Bermuda-rigged boat, it is rare to set a sail with theoretical optimum L/D.
The apparent wind is not constant for two reasons: wind  and  sea.
The wind itself is not constant, or even simply variant.
There are swings in the wind, there are gusts of wind and wind shifts.
Even assuming constant wind, the boat can be raised with the swell or wave,
the top of the sail finding faster winds, or in the troughs there is less wind.
Up or down a wave the boat pitches, that is to say, the top of the sail is propelled forward and back constantly
changing the apparent wind speed, relative to the sail.
The apparent wind changes all the time and very quickly. It is often impossible to adapt to sea conditions with
correspondeningly fast adjustments of the sails. Therefore, it is impossible to be at the theoretical optimum. This  is  not  necessarily a  disadvantage  as  the "pumping"  phenomenon of abrupt changes in incidence has  been  shown  to increase  lift  beyond  the  steady  flow  situation.<ref name=Marchaj4>{{cite book|last=Marchaj|first=C. A.|title=Sail performance : techniques to maximise sail power|year=2003|publisher=Adlard Coles Nautical|location=London|isbn=978-0-7136-6407-2|pages=343–350|edition=Rev. ed.}}</ref> 
Nevertheless, setting to the maximum optimum  may prove quickly disastrous for a small change in wind.
It is best to find an optimum setting more tolerant to changing conditions of apparent wind, state of equipment and weather.
 
The important parameter influencing the type of sail trim is the shape of the hull.
The hull shape is elongated to provide a minimum of resistance to progress.
We need to consider effects of  wind on direction of  the hull tilt: forward ([[Flight dynamics (aircraft)|pitch]]) or
heel ([[Flight dynamics (aircraft)|roll]]).
Downwind, sailing thrust is oriented in the direction of travel so will result in a forward pitch.
Maximizing sail area may be important as the heeling force is  minimal.
The situation changes if part of the force is perpendicular to the vessel.
For the same force as  sailing downwind, the force perpendicular to the vessel may result in a substantial heel.
Under heavy list, the top of the sail does not take advantage of stronger winds at altitude,
where the wind can give maximum energy to sail and boat.<ref name="Keikki">http://heikki.org/publications/ModernYachtLePelleyHansen.PDF</ref><ref>[http://www.gidb.itu.edu.tr/staff/insel/Publications/Cesme.PDF voir figure3b]</ref><ref>[http://hal.archives-ouvertes.fr/docs/00/45/37/30/PDF/articlecorrige.pdf see the figures]</ref><ref>[http://www.sailingworld.com/experts/heel-for-speed FTE: Heel For Speed | Sailing World<!-- Bot generated title -->]</ref><ref>[http://www.sailingworld.com/experts/how-heel-affects-speed-and-handling Sailing World<!-- Bot generated title -->]</ref>
 
The heel phenomenon is much more sensitive than  sail induced pitching.
Accordingly, to minimize the list, the type of setting will be different close to the wind versus downwind:
Close hauled, the setting is for L/D. When sailing downwind, the setting is for power.
 
===Performance limitations of a sail===
 
A sail can recover [[Energy (physics)|energy]] of the wind.
Once the particles have passed their energy to the sail,
they must give way to new particles that will in turn give energy to the sail.
As the old particles transmitting energy to the sail evacuate, these particles
have retained a certain energy in order to escape. The remaining energy of the particle is not negligible.
If the old particles evacuate too soon to make way for new particles,
these particles carry with them a lot of energy.
They then hand the sail less energy. So there is little energy per unit time, or [[Power (physics)|power]], transmitted to the sail.
Conversely, if the old particles evacuate too slowly they certainly convey a lot of energy to sail
but they prevent the new transmission of power.
So there is little power transmitted to the sail.
There is a balance between incoming particle speed and exit velocity, giving maximum power to the sail.
This limit is called the [[Betz limit]]  :
:<math> P_{extractable}^{max}=\frac{16}{27}.P_{arriving on sail}</math>
 
with <math> P_{arriving on sail} =
P_{kinetic} = \frac{1}{2}.\rho.S.v_{incident}^3 \,</math>
 
with
 
:<math>\rho</math>  : fluid density (1.23 kg / m³ in air at 20&nbsp;°C)
 
:S: surface wind "cut" by the sail m²
 
:<math>v_{incident}</math>  : speed incident (upstream) of the fluid in m / s, ie the apparent wind speed.
 
So the sail can not recover more than 60% of the energy in the wind.
The rest being used to evacuate the air parcels off the surface of the sail.
Note that the surface of the Betz limit is not the surface of the sail but the surface wind "cut" through the sail.<ref>A true wind approach will be more rigorous than used a surface ''cut''.</ref>
 
The formula for the force on the sail is
<math>F = \frac12 \times \rho \times S \times C \times {V_{apparent wind}}^2</math>
 
where
 
<math>\ S </math> is a characteristic surface in the case of sail on the surface of the chord.
 
<math>\ C </math> is the aerodynamic coefficient.
 
<math>\ C \times S </math> represents the percentage of energy recovered over the upper(outer) surface
multiplied by the upper(outer) surface area plus the percentage of energy recovered from the lower surface
multiplied by the surface area of the lower(inner) surface.
By definition for a sail, the fabric is thin, so the upper surface area is identical to the lower surface area.
Considering the sail as inelastic, the sail airfoil is relatively thin.
The camber of the sail can be very important in lift mode lest the airflow comes off the airfoil and thus decreases
the performance of the sail.
Even for a highly deformed spinnaker, the spinnaker must be set to catch maximum wind.
The upper surface area or  the  lower  surface area are approximately equal  to the surface area through the chord.
The surface area of the sail is approximated to the surface area through the chord <math>\ S</math>.
So the drag coefficient <math>\ C</math> has upper limit 2.
 
On the other hand, the apparent wind is related to the true wind from the formula:
:<math> {V_{true wind}}^2 =  {V_{apparent wind}}^2 + {V_{boat}}^2 -2 \times V_{apparent wind} \times V_{boat}
\times cos (\beta  -\pi)  </math>
 
with <math> \beta</math>, the angle between true wind and the direction of movement of the boat in [[radian]].
 
The apparent wind depends on the true wind and boat speed.
The true wind speed is independent of the boat.
The boat can take any apparent wind speed.
So if the sailor increases the apparent wind with the true wind fixed, the boat speed increases, with some practical limits.
 
Research is intended to improve the speed of boats. But improvements are limited by the laws of physics.
With all the advanced technology available, the aerodynamic coefficient has a theoretical limit,
which limits the recoverable force at constant speed.
Recovered energy from the wind intercepted by the canopy is limited to 60%.
The only way for the sailor to go faster is to increase the energy recovered per unit time (or power )
by increasing the surface wind intercepted by the canopy.
Without going into calculations, the faster the boat moves, the more the surface area intercepted increases,
the vessel has more energy per unit of time, it goes even faster.
If the boat is faster, the area intercepted is even greater. It gets even more energy. It goes even faster than before.
The boat then enters a virtuous cycle. If the apparent wind increased indefinitely. with no heeling problem and hull
resistance, the boat would accelerate indefinitely. The other possibility is to increase the area of the sails.
But, the sailor can not increase the surface of the sails indefinitely. 
Increasing the surface area of sail, the responsibility of the naval architect, is limited by the strength of
materials.
 
===Lift/drag. Upwind sail cut and trim===
 
[[File:Sail force parts.svg|thumb|300px|Decomposition of force on sailing upwind: Apparent wind (W) at incidence, (α) and angle to course sailed (β). Aerodynamic force (A). Lift (C), perpendicular to flow. Drag (B), parallel to flow. C1 is portion of lift propelling the boat and C2 the portion causing heeling and leeway (λ). Drag (B) will also contribute to heeling, leeway and reduce propulsion.]]
 
In  the  example of  upwind  sailing, the apparent wind, with incidence, α, to the sail chord, is at an angle, β, to the course sailed.  This means that:
*a (small) part of the drag slows the boat.
*the other part of the drag of the sail is involved in the vessel's [[heeling (sailing)|heel]] and [[leeway]], λ.
*much of the lift of the sail contributes to the advancement of the vessel,
*the other part of the lift of the sail is involved in the vessel's heel and leeway.<ref name="Finot_1">[http://www.finot.com/ecrits/Damien%20Lafforgue/article_voiles_english.html Les voiles<!-- Bot generated title -->]</ref>
 
Sailing high to the wind generates a perpendicular heeling force. Naval architects plan optimum heel to give maximum forward drive. Technical means used to counter the list include ballast, [[hydrofoil]]s and counter  ballasted keels. Heel can be almost completely offset by the counter- heel technology such as boom / swing keel, type of hydrofoil, etc. These technologies are costly in money, weight, complexity and speed of change of control, so they are reserved for elite competition. In normal cases, the heel remains as extra ballast begins to decrease forward drive.  The architect must find a compromise between the amount of resources used to reduce the heel and the heel remaining reasonable. The naval architect often sets the optimal heel between 10° and 20° for monohulls.<ref>[http://c_r_y_a.tripod.com/Sterne%20How%20to.htm#4%20Non%20Optimum Bob Sterne How to Sail Fast<!-- Bot generated title -->]</ref> As a result, the sailor must stick as much as possible to the best heel chosen by the architect. Less heel may mean that the boat is not allowing maximum sail performance. More heel means that the head of the sail drops, thus  reducing pressure, in which case the sailing profile is not the best.
 
The sailor desires optimum heel, a heel giving optimum perpendicular force for the best resulting driving force and  to minimise the ratio of perpendicular force to driving force.
 
This ratio depends on the point of sail, incidence, the drag and lift for a given profile.
 
As the lift is the main contributor to the force that drives the boat, and drag  usually the main contributor to  perpendicular heeling and  leeway forces, it is desirable to maximise the L/D.
The point of sail depends on the course chosen by the sailor. The point of sail is a fixed parameter, not a variable of optimisation. But each apparent wind angle relative to the axis of the ship has a different optimum settings.
 
The sail trimmer will first select the trim profile giving maximum lift. Each  profile corresponds to a different polar  diagram.
A sail is generally flexible, the sailor changes the trim through:<ref>[http://modelismepassion.ibelgique.com/voiles.htm voiles<!-- Bot generated title -->]</ref> 
* the position of the draft of the sail by adjusting the elements acting on the tension of the fabric of the sail
* adjusting twist of the sail using the leech tension lines such as boom vang or jib sheet angle adjustment.
 
Many polar curves exist for the possible sail twists and draft positions. The  goal is choosing the optimal one.
 
The twist will be set for constant incidence angle along the luff for maximum sail performance, remembering that apparent wind strength and angle varies with altitude.
 
The best L/D is usually obtained when the draft is as far forward as possible. The more  forward  the draft, the greater the angle of  incidence over the luff area.  There comes an attack angle when the air streams do not stick to the sail, creating a dead zone of turbulence which reduces the efficiency of the sail. This inefficient zone is located just after the luff on the windward side.  The tell tales in this area become unstable.<ref name="BAY_1">[http://www.bretagne-atlantic-yachting.eu/peda/reglages_de_voile.html Les réglages de voile - Réglage de grand voile, réglage de génois, réglage de spi<!-- Bot generated title -->]</ref> The flatter  the stretched fabric over the sail is, the less the draft. The yacht has several elements acting on the tension of the fabric of the sail:
 
* Cunningham tension,
* the tack,
* the head point,
* the clew of the sail.
* the backstay,
* shrouds. They act indirectly.
 
These elements can interact.  For example, backstay tension also affects the tension of the head point and therefore the shape of the luff. Both high clew sheet tension tightening the foot and a tighter backstay cause slackening of the leech.
 
For a flexible sail, the [[Camber (aerodynamics)|camber]] of the sail and position of the draft are linked. This is a result of their dependence on shape of the cut of the sail. The camber is a major factor for maximising lift. It is the naval architect or sail maker that sets the cut of the sail for the draft-camber relationship. The thickness of the airfoil profile corresponds to the thickness of sail fabric. Variations in thickness of a sail  are negligible compared to the dimensions of the sail. Sail thickness is not a variable to optimise. Contrast mast thickness and profile which are much more important.<ref>[http://vincent.chapin.free.fr/publications/MarineTechnology_VGC_Jan2005.pdf Viscous  Computational Fluid Dynamics as a Relevant Decision-Making Tool for Mast-Sail Aerodynamics]</ref>
 
For the naval architect the sail-shape offering a large L/D is one with a large aspect ratio.  (see previous polar diagram) This explains why  modern boats use the Bermudan rig.
 
Sail drag  has three influences:
 
* induced drag (see influence of aspect ratio on the lift). As the profile is not of infinite length, the ends of the sail, foot and head, equalise the depression of the leeward surface  with the pressure of the upwind surface.  This dissipated pressure balance becomes the induced drag.
* friction drag, related to boundary layer laminar turbulent flow and roughness of fabric
* form drag,  related  to choice of the airfoil profile, camber, draft position, and mast  profile
* (parasitic drag is related to parts extraneous to the sail, but may influence rigging, boat and  sail design)
 
[[Lifting-line theory|Prandtl's lift theory]] applied to  thin profile is less complex than the resolution of [[Navier-Stokes equations]], but clearly explains the aspect ratio's effect on induced  drag. It shows that the principal factor influencing L/D is induced drag. This theory is very close to reality for a low-impact thin profile.<ref>[http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml Aerospaceweb.org | Ask Us - Drag Coefficient & Lifting Line Theory]</ref>  Smaller secondary terms include the form drag and friction drag. This theory shows that the factor with  main  influence is aspect ratio.<ref name="aeroelasticite">http://www.ltas-mct.ulg.ac.be/who/stainier/docs/aeroelasticite.pdf page 28</ref> The architect chooses the best aspect ratio for best sailing, confirming the choice of Bermudan rig.  The sailor's choice of sail trim affects the factors of secondary importance.
 
::{| class="toccolours collapsible collapsed" width="90%" style="text-align:left"
!Explanation
|-
|
[[Lifting-line theory|Thin profile theory]]<ref>[http://j.haertig.free.fr/aerodyn_theorique/ligne_portante_3D.pdf calcul avec la méthode des lignes portantes avec les deux vortex d'extrémité de profil]</ref> is applied to a 3D profile. A conventional infinitely extended 2D profile of the thin profile theory is truncated. At each end section of the truncated profile there is a vortex that swirls over its whole periphery.
The theory then gives  the drag:<ref>[http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html Induced Drag Coefficient]</ref> 
:<math>Ci = {{Cz^2} \over {\pi \times \lambda \times e}} </math>
with
* Cz : [[Lift coefficient]] of profile
* <math>\pi</math> : the circle circumference to diameter ratio, pi or 3.1416
* λ : Aspect ratio (dimensionless) <math>\lambda = {b^2 \over S}</math> with ''b'' the length of the luff and ''S'' the surface area of the sail.
* e : Oswald efficiency number
 
The theory only models the induced drag. The  form and friction drag are neglected.<ref>[http://www.grc.nasa.gov/WWW/K-12/airplane/dragco.html The Drag Coefficient]</ref> To set the order of magnitude:
* Cz ranges from about 0 to 1.5, a value of 1 is taken
* e is between 0 and 1 for a sail is at about 0.8
* λ aspect ratio.
 
For an  Edel 2 sloop, the mainsail is 10 m2 with a foot of 2.5 m is λ = 1.6
<math>\ Ci </math> for an  Edel 2 is  0.2.
 
But the sail's reflection in the sea must be taken into account. If we{{Who|date=January 2012}} neglect the distance between the sea and  the  foot, the sail's surface area and  length doubles.
So <math>\ Ci </math> is 0.1.
 
In reality, <math>\ Ci </math> lies between these two values. This value varies depending on the state of the sea, that is depending on the quality of the mirror.
 
The calculation of the Oswald efficiency number is based on integral calculus. It is not addressed in the literature because it is calculated indirectly. The formulas are not all written with the  Oswald efficiency number. There is also another notation
<math>\ \delta</math> :
 
:<math>Ci = {{Cz^2} \over {\pi \times \lambda \times e}} = {{Cz^2} \over {\pi \times \lambda }} (1+ \delta) </math>
 
Thin profile theory applied in 3D gives the formulas for the induced drag.<ref>http://s6.aeromech.usyd.edu.au/aero/liftline/liftline.pdf</ref> Note the case of a large cambered sail such as a genoa.  The recalculation based on this camber is no longer negligible. Also note that  can be calculated not using a Fourier series, but via the calculus.<ref>[http://www.engbrasil.eng.br/index_arquivos/art52.pdf voir (3.2.1) page 38]</ref>
 
The lift/drag ratio is directly derived from the formula of the induced drag:
 
:<math>L/D = Cz / Ci = {{\pi \times \lambda \times e} \over Cz } </math>
 
The theory then gives for the lift:<ref>[http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-100-aerodynamics-fall-2005/study-materials/1liftdrag.pdf aerodynamic lift]</ref><ref>[http://www.aoe.vt.edu/~neu/aoe5104/23%20-%20LiftingLineTheory.pdf Lift]</ref>
 
:<math>Cz = Cz' * ({{\lambda } \over {\lambda + 2}}) </math>
 
with
* λ: Aspect  ratio (dimensionless) <math>\lambda = {b^2 \over S}</math> with ''b'' is the length of the luff, ''S'' the surface area of the sail.
* Cz:  lift coefficient calculated by the theory with an infinite extension profile.
 
It should be noted that in the case of a sail wind speeds are far removed from Mach, it follows that the correction factor of the Mach number is approximated to 1.
 
The theory then gives for the  infinite extension lift profile:
 
:<math>\ Cz' = 2 * \pi * (\alpha + \alpha 0) </math>
 
with
α incident angle between the chord of the sail and the apparent wind.
α0 3D coefficient is actually slightly different from the 2D calculation.
 
hence
 
:<math>L/D = ({{e} \over {\alpha + \alpha 0}}) * ({{\lambda + 2 } \over {2}}) </math>
 
The naval architect fixes e and λ, while the sailor sets α and α0.
The sailor does not have a choice of total factor α0, i.e. She can not choose the full form of the profile. The set of possible profiles is limited. Indeed, the sail maker sets a cut of the sail therefore defines a set of profiles that can be made possible depending on sail trim. To illustrate, the sail may be cut to give a profile NACA0009 but if the sail is not taut it can become NACA0012 NACA0015 NACA0018 or in between, though the general shape remains the same.  The relationship between draft position and camber, camber / chord, etc.  are fixed. The choice of the general form is the responsibility of the sail maker or naval architect.
 
A high L/D is in the range of possible profiles of the sail set to a maximum draft, forward on the sail,  with the sail taut.<ref>[http://oa.upm.es/2203/2/INVE_MEM_2008_53562.pdf figures 27 and 29]</ref> Trimming to advance the draft position is  desirable. But that does not mean it can be placed anywhere. The choice of the profile by the sail maker could for example result  in maximum draft positioned at 30% or 50% of chord. In the first case flexibility of  draft position will be  between 30% and 100%, while in the second case between 50% and 100%.
 
Form drag and friction drag, have a secondary but significant impact for competition sailing.<ref>''Principles of yacht design'', by Lars Larsson and Rolf E Eliasson ISBN 0-7136-5181-4 or 9 780713 651812 page 151 figure 7.20 This figure shows well the different types of drag</ref>  The surface perpendicular to the wind is a factor, the greater the depth of the draft (camber) the smaller the L/D due to  increased  drag. Similarly increasing twist increases  drag.<ref>[http://oa.upm.es/2203/2/INVE_MEM_2008_53562.pdf figure 26]</ref> A flatter sail is preferable to a  balloon shape. This implies that the general shape of a sail is set properly by a tense sail. But the sail should not be too tight as  too much flatness decreases the lift.<ref>[http://www.futureship.net/downloads/KrebberHochkirchHPYD06.pdf figure 17]</ref>
 
These calculations are approximations of reality. They are still relatively simple. They avoid the very heavy 3D calculations (see Forces on sails# Several sails : multi-dimensional problem resolution). They are handy for sizing a rig or to model  its behaviour in full sail.
|}
 
A higher L/D means less drag,  for the same heeling force. The maximum L/D will be preferred. So among the remaining profiles that give maximum lift, the sailor selects  the profile with maximum L/D (draft forward on the sail). Now that the profile of the sail is set, it remains to find the point of the polar diagram of the profile giving the maximum forward force to the vessel, that is to say the choice of the angle of incidence.
 
On a triangular sail the zone of maximum lift coefficient (0.9 to 1.5) has two characteristic points (see Marchaj's  polar diagram above, and<ref name=Lafforgue1 />):
*Point 1: the maximum L/D (0 to 5° incidence, the correct zone)
*Point 2: maximum lift (15° incidence on the polar diagram).
 
As total aero and hydrodynamic drag slows the boat, it is necessary that the portion of the lift that moves the boat is greater than the contribution of the total drag:
 
:<math>{F_{p}} = L \times sin(\beta) -{D_{t}} \times cos(\beta) > 0</math>
 
and <math> {D_{t}} = {(L/D)_{\alpha}} ^{-1} \times L </math>
 
hence <math>\ {(D/L)_{\alpha}}  < tan(\beta) </math>
 
with:
 
* <math>\ \alpha </math> incident angle between the chord of the sail and the apparent wind,
* <math>\ \beta </math> angle between apparent wind and boat's course made good (including  leeway).
* <math>\ {(L/D)_{\alpha}} </math>  Lift/Drag of sail at  α
* <math>\ {F_{p}} </math> propulsive force
* <math>\ {D_{t}} </math> total of  aerodynamic and hydrodynamic  drag
* <math>\ {L} </math> lift
 
This means that we{{Who|date=January 2012}} should not increase incidence beyond the point of the polar curve or decrease the tangent at this point less than the tangent of β. Hence between the maximum L/D noted point 1 (end of correct zone) and a L/D of tan(β) noted point 2.
The evolution of the propelling force is as follows:
at 0° incidence to point 1 both forward force and heel increase linearly.
Point 1 to the optimum forward force  still leaves the polar curve flattening, which mean that the drag slowed the boat's progress more than lift adds. But overall as the heel increased, the sail has a lower apparent wind. The top of the sail is no longer at the altitude of fast winds.
From the optimum point to point 2, forward force decreases until it becomes zero, the ship stands up.
Optimal adjustment of incidence is between point 1 and point 2. The optimum point depends on two factors:
* changes in the L/D
* changes in the heel.
 
The sailor will find a compromise between these two factors between points 1 and 2. The optimum operating point is close to point 1 and close-hauled, where the heel is dominant factor. Since it is difficult to heel on a broad reach, the optimum will be closer to point 2.
 
Note that the L/D is determined through the polar of the sail. The polar is determined regardless of the apparent wind speed, yet the heel is involved in setting speed (wind in the sail), so the L/D of the polar of the sail does not depend on the heel.
 
[[File:Matchrace kolk.jpg|thumb|300px|Oops! Heel is too much for the smooth running of the yacht.]]
The position of the draft is the dominant factor in the search for the optimum. All the knowledge of ocean racing is to advance the draft forward. With a setting of "too much", the sail answers. The optimum trim is always on the verge of dropping out. The jib luff lift and main leech lift are so very important. At this optimum the main leech and jib luff tell tales are horizontal and parallel to the surface of the sail.<ref>[http://voilehabitable.org/cms/tiki-index.php?page_ref_id=116&PHPSESSID=10939f64b633cbc77fdf4b7710a44e5f Voile-habitable : Réglage et conduite au portant sous spi<!-- Bot generated title -->]</ref><ref>[http://www.wb-sails.fi/news/95_11_Tellingtales/Tellingtales.html WB-Sails Ltd<!-- Bot generated title -->]</ref><ref>[http://www.sailtheory.com/tuning.html tuning @ sailtheory.com<!-- Bot generated title -->]</ref>
 
The purpose of trimming the boat is to have maximum propulsive force (Fp). A simple way might be to set a giant sail, except the boat will capsize due to Fc, the capsize force. The ratio Fp/Fc is an important consideration.
 
In summary, at points  of sail where lift acts, the L/D is determined by the height of the sail, sail fabric and cut, but especially good sail trim. Close-hauled, there can be variations in the L/D of 100%  comparing one sailing crew to another. In the race, boats are often close in performance (the role of racing rating rules). The dominant factor for the speed of the boat is the crew. The L/D is not a secondary concept.<ref>[http://marc.donneger.free.fr/Voile&Mer/propulse.htm MD / Voile & Mer<!-- Bot generated title -->]</ref><ref>http://arxiv.org/ftp/arxiv/papers/1002/1002.1226.pdf</ref><ref>[http://www.finot.com/ecrits/vitessecoq/chap4/chap4.htm Capacité de porter de la toile<!-- Bot generated title -->]</ref>
 
A sail boat can drift, this leeway creates lift from the submerged form, the force used to counteract the force pushed perpendicular to the sail. So in other words, minimising the heel also amounts to minimising the leeway of the ship. Minimising leeway gives  better upwind performance.  The L/D of a yacht enhances its ability to go upwind.
 
Similarly, the concept of balancing L/D, is in various forms:
* trimming the  boat for optimal sailing upwind,
* Fp / Fc, the inverse of capsizing  tendency,
* design capacity of the boat to go upwind,
* L/D of the sail, or slope of the polar.
 
===Power. Downwind sail cut and trim===
 
[[File:Zeiltheorie2.png|thumb|300px|Decomposition of the aerodynamic sail force to forward, and lateral components at different points of sail.]]
 
Downwind sailing forces tend  to pitch  the boat forward. Heeling (rolling axis)  forces are  less important (in  theoretical  steady state conditions only).
The [[apparent wind]] is at an acutely aft angle to the axis of the ship. The chord of the sail is  roughly square to the axis of the ship. So:
 
* much of the sail's drag  contributes to the advancement of the ship
* the other part of the sail drag  is involved in the vessel's heel
* much of the lift is involved in slowing the vessel,
* the other part of the lift is involved in the vessel's  heel.<ref name=Lafforgue1 />
 
The optimum setting depends on the apparent wind angle relative to the course.  The sail profile is chosen for maximum drag.
The  heel is not a big factor reducing boat speed. The L/D is not a  factor in applying the right profile. The overriding factor is to get the sail profile to give the maximum forward drive based on drag or  "power".
To maximize the power or maximize propulsive effort are equivalent.
 
::{| class="toccolours collapsible collapsed" width="65%" style="text-align:left"
!Explanation
|-
|
'''Power''' in physics is defined for the sail:
:<math>P = \vec{F} \cdot \vec{v} </math>
 
Where:
 
<math>\vec{F}</math> ( In [[newton (unit)|N]] ) is the force vector on the sail.
 
<math>\vec{v}</math> (In m / s ), boat velocity vector over the ground with  <math>\vec{v}  \times \vec{v} = V_b^2 </math>.
With <math>\ V_b </math> boat speed over the ground,
 
<math>\ P</math> is the instantaneous power (in W ).
 
At constant boat velocity, forward force is exactly balanced by hull drag:
:<math> \frac{d(m\vec{V})}{dt} = \sum{\vec{\mathrm{F}}_i} = 0 </math>;
 
Hence,
:<math> \vec 0 = \vec\mathrm{Fsail} + \vec\mathrm{Fhull} + \vec\mathrm{Fgravity}</math>.
 
Force projected on the course:
:<math>\ 0 = F_{forward} + F_{hullcourse}</math>
 
The hull acts as an underwater profile. The hull lift is perpendicular to the advancement of the ship. So, it does not affect propulsion. The hull's driving resistance is due to its drag. For simplicity, this formula is chosen:
:<math>  F_{hullcourse} =  \frac12 \times \rho_{water} \times S_b \times C \times V_b^2</math>.
 
With <math>\ \rho_{water}</math>, the water  density, C,  the  hull drag coefficient, <math>\ S_b </math>, the hull wetted  surface  area and  <math>\ V_b </math>, the boat speed over the ground.
 
The following factors depend somewhat on the speed of the boat:
:<math> \ b = \frac12 \times \rho_{water} \times S_b \times C </math>
 
Hence,
 
:<math>  F_{hullcourse} =  b \times V_b^2 = - F_{forward}</math>
 
Hence,
 
:<math>P = (F_{forward})^{ \frac{3}{2}} \times ( \frac{1}{b})^{\frac{1}{2}} </math>
 
Therefore, optimising  [[Power (physics)|power]] amounts to optimising the propulsive effort.
:<math> F_{forward} = lift \times sin(\beta) -drag \times cos(\beta) </math> with <math> lift = {(L/D)_{\alpha}}  \times drag </math>
 
With:
* α Incidence, angle between the chord of the sail and the apparent wind,
* β angle between apparent wind and the course (including drift).
 
Hence, the complete formula is:
:<math>P = ( lift \times ( sin(\beta) -{(L/D)_{\alpha}} ^{-1} \times cos(\beta)) )^{ \frac{3}{2}} \times ( \frac{1}{b})^{\frac{1}{2}} </math>
To vary the power the sailor sets only three factors:
 
* L/D
* incidence
* lift.
 
This means that in some cases we must take into account the lift in optimising the speed of the boat.
The lift equation is <math>  lift =  \frac12 \times \rho_{air} \times S \times C \times V^2</math> with C the lift coefficient, S the surface area of the sail, and V is the apparent wind speed. The speed is not identical along the luff.  A weighted average, determined through testing, is used. The  lift depends on several factors including primarily the apparent wind speed, averaged over the entire sail,  which depends heavily on the vessel's list.
 
'''The stability of a sail boat'''
 
Physical power can not increase indefinitely. The boat will capsize if the heel or the pitch is too large.  The power is limited by the capacity of the ship to withstand the heel, i.e. the righting moment.
 
The righting moment is  the  work needed  to keep the boat upright.<ref>[http://www.johnsboatstuff.com/Articles/estimati.htm Estimating Stability<!-- Bot generated title -->]</ref><ref>[http://www.hawaii-marine.com/templates/stability_article.htm Stability and Trim for Ships, Boats, Yachts and Barges – Part I<!-- Bot generated title -->]</ref>
This prevents the boat from capsizing.  Hull shape, ballast and keel all contribute to the righting moment. This moment exactly balances the moment generated by the force of the wind in the sails when the ship is at constant speed. This constant speed simplification will avoid  a too complex formula:
 
:<math>  \vec 0 = \vec\mathrm{M}_{sail/G} + \vec\mathrm{M}_{hull/G} </math>
 
with
 
* G centre of gravity of the boat
* E centre of  effort of sail
* <math>\  \vec\mathrm{M}_{sail/G} = \vec{F_{forward}} \times distance_{G - E} </math>
* <math>\  \vec\mathrm{M}_{hull/G} </math> righting moment of the boat, this time including the effects of ballast, keel of the hull and so on.
 
These moments are decomposed along the axes of the ship: roll, pitch, yaw. Each axis has its limits (Fc low to high downwind heel), namely:
 
Maximum moment for heeling: <math>\ M_{heel_{max}} </math>
 
Maximum moment for pitch: <math>\ M_{pitch_{max}} </math> .
 
Of course, it is not possible to capsize at the yaw axis.  The pitch axis will not have the same righting moment as the heel axis. A hull is made to offer the least resistance to movement. So, the limit along the axis of pitch is considerably more important than along the heel:
 
:<math>\ M_{heel_{max}} << M_{pitch_{max}} </math>
 
To set the order of magnitude of a monohull, <math>\ 10 \times M_{heel_{max}} = M_{pitch_{max}} </math>. Although this value is lower for a multihull.
 
A sail boat is usually composed of several sails. But for the  calculation, a single equivalent sail will approximate multiple sails. This sail will have its own polar plot. The results for this sail are applicable to multiple sails as the sails influence each other. The correct trim is different from the equivalent multiple sail trim. This difference can be determined via computation or experience of the sailor on his yacht. (See case of several sails: multi-dimensional resolution of the problem).
 
The force in the sails changes direction and intensity depending on the point of sail. The force is divided into sail lift and drag.  <math>\ D_{pitch} </math> is the lever arm of the pitch and <math>\ D_{heel} </math> is the lever arm of the heel acting on the  centre of  effort of the sail in relation to the centre of gravity.
 
Hence,
 
:<math>\ M_{pitch}= D_{pitch} \times  (lift \times sin(\beta) -drag \times cos(\beta) ) =  D_{pitch} \times lift \times ( sin(\beta) -{(L/D)_{\alpha}} ^{-1} \times cos(\beta)) </math>
:<math>\ M_{heel} = D_{heel} \times (lift \times cos(\beta) +drag \times sin(\beta) ) =  D_{heel} \times lift \times (cos(\beta) +{(L/D)_{\alpha}} ^{-1} \times sin(\beta))</math>
 
with
<math>\ \beta </math> course to  apparent wind angle.<ref>[http://www.orc.org/rules/ORC%20VPP%20Documentation%202009.pdf page 42 equation 47  breakdown identical with  other notation]</ref>
 
These moments should be expressed in conjunction with the means against heel. Analysis of hull heel countering measures is beyond the scope of this article. Nevertheless, the results of calculation show that <math>\ M_{heel}</math> and <math>\ M_{pitch} </math>  evolve respectively along the angle of heel and pitch. The value of moments rise linearly to pass through a maximum and then decrease. The [[metacentre]] approach is generally used:
 
: <math>\ M_{pitchorheel} =  A \times GM \times sin(\phi) </math>
 
with
 
* <math>\ \phi </math>  heel or pitch angle
* <math>\ GM </math> distance to the centre of gravity [[metacenter]]
* <math>\ A </math>heel or pitch constant<ref>http://www.towage-salvage.com/files/stab014.pdf</ref>{{,}}.<ref>[http://www.authorstream.com/Presentation/Garrick-25629-1-3-Dynamic-Stability-Objectives-Heeling-Moments-Moment-Curve-Static-Righting-CGC-JARVIS-November-15-197-as-Entertainment-ppt-powerpoint/ 1 3 Dynamic Stability Ppt Presentation<!-- Bot generated title -->]</ref>
 
The relevant moments are: for the sail, the heeling moment and for the hull, the  righting moment.
 
The literature often uses a simplified equation for calculating the heeling moment or <math>\ M_{heel} </math> sail.
:<math>\ M_{heel} =  D_{heel} (lift \times cos(\beta) + drag \times sin(\beta) ) </math>.
 
Lift and drag are of the form:
:<math>F = \frac12 \times \rho \times S \times C \times V^2</math>
 
The wind speed is not constant with altitude. Speed depends on the heel. Different simplified formulas are used:
* <math> V= a \times cos(\phi)</math>
*: which leads to:
*: <math>\ M_{heel} =  pressure \times S \times A {cos(\phi)}^n </math>
 
with
*:: <math>\ pressure  </math> average pressure on the sail.
*:: <math>\ A {cos(\phi)}^n </math> is called the heeling arm. (Moment divided by the displacement of the vessel. Not to be confused with wind heeling arm.)
*:: <math>\ \phi </math> angle of heel or pitch.
*:: <math>\ S </math> surface area of the sails.
*:: <math>\ A </math> constant.
*::n a  coefficient to be determined.<ref>[http://www.formsys.com/extras/FDS/webhelp/hydromax/heeling_arm_definition.htm Heeling arm definition<!-- Bot generated title -->]</ref>
*or <math> V^2= b\times (1 - a \times \phi)</math><ref>http://www.gidb.itu.edu.tr/staff/insel/Publications/Cesme.PDF</ref> will give another formula,
*or other formula.<ref>[http://202.114.89.60/resource/pdf/1044.pdf PII: 0169-5983(94)00027-1<!-- Bot generated title -->]</ref>
 
The higher the heel, the greater the boat approaches its safety limit. Heel is not desirable because it reduces the forces for the good performance of the boat. The two phenomena, speed and safety, act in the same direction. Taking into account the effect of wind speed in the rest of the explanation does nothing. It would reinforce the results found without bringing anything new. In the following explanation angles (pitch heel) will be low, therefore neglected.
 
From these curves are determined <math>\ M_{pitch_{max}} </math> and <math>\ M_{heel_{max}} </math>. These are the maximum moments and  not  for  practical use because if exceeded, then the boat is in danger. In addition, the angles are too high and the sail does not take advantage of fast winds at altitude. For these reasons, the linear region of  the curve is used where there are lower angle limits for optimum heel and pitch.
 
Upwind lift propels  the  boat  and  drag  slows it.  Downwind the  reverse is  true. The sail tunings are to optimize respectively the  driving  force. At the transition between these two  at a  transverse wind, the setting mode then switches to search for maximum drag at maximum lift. The transition also corresponds to a flow on the sail of turbulent (search for drag) to laminar (search for lift).
 
To consider low heel (i.e. a GM approach), if the yacht is well designed, then it is neither tender nor stiff. So, the sailing centre is approximately above the centre of gravity on the same vertical:
:<math>\ D_{heel} \approx D_{pitch} </math>
 
More <math>\ D_{heel} = D_{pitch} </math> is still somewhat varying in first approximation:
* The sailing centre is near the geometric centre of the sails
* The most common case is the boat without ballast over a ton. So, the major part of the weight is fixed, the centre of gravity moves slightly.
 
'''Downwind <math>\ \beta \approx 180 \deg</math>'''
 
Downwind, the leading edge is the leech and the trailing edge is the luff. The situation is reversed close hauled. The downwind sail profile works in reverse. As downwind drag moves the boat, and lift slows it, so you have a maximum drag, a sail that blocks the wind set to a high incidence, therefore, a mode of turbulent air on the sail.
 
Consider the simple case of downwind <math>\ \beta = 180  </math>. Hence,
:<math>\ M_{pitch}=  D_{pitch} \times drag </math>
:<math>\ M_{heel} = - D_{heel} \times lift</math>
 
Or downwind sailing works exclusively in drag. The lift is zero then:
:<math>\ M_{pitch}=  D_{pitch} \times drag \times  1  </math>
 
This simplification suggests that the heel is not a problem.  But,  other phenomena in non steady state conditions like oscillations  set  up  from spinnaker vortex  shedding and rudder induced roll may cause "death roll" [[Broach (sailing)|broaching]].<ref>{{fr icon}} http://chazard.org/emmanuel/cours-de-catamaran-couples-de-rotation-dessalage-heel-enfournement</ref><ref name="MarchajRoll">{{cite book|last=Marchaj|first=C. A.|title=Sail performance : techniques to maximise sail power|year=2003|publisher=Adlard Coles Nautical|location=London|isbn=978-0-7136-6407-2|pages=351–360|edition=Rev. ed.|chapter=Part 2 Ch 7 Sailing Downwind (Rolling)}}</ref>  Too much power  could  also  cause pitchpoling. Downwind, the boat can not go faster than the wind. The more the boat approaches the true wind speed the lower the apparent wind pushing the sail. The goal is therefore to safely raise a maximum sail area to move the boat as close as possible to the true wind speed.
 
As soon as the wind is not exactly aft, the apparent wind effect appears. As the apparent wind increases, so, also the force on  the  sail. If the sails were hoisted for a calculated safe limit charging downwind, this limit is exceeded. Setting smaller sails moderates this [[apparent wind]] effect. The boat is always at a lower speed than the true wind. Speed limits of the hull are affected by high wind shifts (from breeze to storm). In such conditions, the sailor is more reasonable to  temper speed in a search for maximum boat security. The loss of speed compared to the case of maximum sail area will be minimal because the downwind boat can not go faster than the wind.
 
'''From downwind to a broad reach <math>\ \beta \approx 120 \deg</math>'''
 
The lift slows the ship, so, minimize it. The caution profile is reversed. The polar plot of the sail giving maximum drag and minimum lift is for an incidence of 90°.<ref>[http://www.orc.org/rules/ORC%20VPP%20Documentation%202009.pdf Figure 19 on page 34 and Figure 17 and Figure 20 an incidence value of 90° is slightly wrong. This is due to the fact that for a flexible sail the sailor can not place the entire surface of the sail perpendicular to the wind (to cut the wind on the full sail). For a jib, maximum drag  is at 160° and the incidence of lift is zero. For a mainsail, max 170°. For a headsail genaker type or large genoa, 180° max.]</ref> So, keep the incidence perpendicular to the apparent wind. In this case the lift is zero. Then the constraints are:
 
:<math>\ M_{pitch}=  -D_{pitch} \times drag \times  cos (\beta)  </math>
:<math>\ M_{heel} =  D_{heel} \times drag \times sin(\beta) </math>
 
Furthermore, the lift is no longer parallel to the course. The perpendicular part causes the heel.
 
With the apparent wind, the boat accelerates. The boat's heel is compensated by the leeway. Leeway increases the resistance efforts of the hull. From downwind to a broad reach, the boat is more rapid, taking advantage of the increasing apparent wind effect. Approaching a  beam reach, the resistance of the hull takes over. The boat slows down a bit.<ref>[http://www.grain-de-sel.org/technique/voile/coursvoile.htm Grain de Sel : Navigation à la voile<!-- Bot generated title -->]</ref>
 
Like <math>\ M_{heel_{max}} << M_{pitch_{max}} </math>, here <math>\ 10 = \frac{M_{pitch_{max}} } { M_{heel_{max}} }</math>.
 
There is a tipping point or constraint design changes from the stress of pitching to the stress of heel:
 
So,
:<math>\ \beta = 174.3 \deg</math>
The angle is close to downwind. For a low ratio <math>\ 3.5 = \frac{M_{pitch_{max}} } { M_{heel_{max}} }</math>, it is still 165 degrees.
 
So, the constraint is:
:<math>\ M_{heel} =  D_{heel} \times drag \times sin(\beta) </math>
 
'''Downwind, the transition zone'''
 
Downwind, if the boat keeps the same profile of drag at a broad reach, sailing well adjusted bearing capacity is zero. The propulsive effort follows the formula:
:<math>\ F_{propulsive}=  - drag \times  cos (\beta)  </math>
 
So the more the boat approaches the beam reach, the more propulsive drag decreases to zero. So, the  choice comes to running downwind or  to switching to lift on the beam.
 
Similarly in reverse, if the boat keeps the same profile as for beam reaching, as the incidence is not too close to zero, the more the sail keeps the profile, and propels the boat in lift mode, the more the point of sail moves  from beam  to broad reach, the more the incidence decreases, and the propulsive effort is reduced. So, is it better to switch to drag, positioning the wing to cut the path of maximum wind like on a broad reach? This particular point differs between sailboats and sets of sails available. For example, using a multihull with better anti heel properties than a monohull, the tipping point will be different.
 
'''Beam Reach <math>\ \beta \approx 90 \deg</math>'''
 
Near a  beam  reach, the lift is reversed. It contributes to the advancement of the ship, and drag slows the boat. The propulsive effort follows the formula:
:<math>\ F_{propulsive}= lift \times sin(\beta) - drag \times cos(\beta) </math>
 
As the angle is not perfectly <math>\ \beta = 90 </math> in the ideal case would be to find a point on the polar of the sail with no drag and maximum lift. Unfortunately, unlike running downwind with drag and maximum lift is  zero, the theory of thin profiles shows that drag exists. The choice of the correct incidence of the sail is going to depend on the L/D of the sail. For a sail working in lift <math>\ lift  >> drag </math>.
 
The constraints are:
:<math>\ M_{pitch}= D_{pitch} \times  lift </math>
:<math>\ M_{heel} = - D_{heel} \times  drag  </math>
 
As <math>\ lift  >> drag </math>, the constraint is:
:<math>\ M_{pitch_{max}}  > D_{pitch} \times  lift </math> if sailing through work lift
 
On beam  reach, we must use lift as much as possible. So the choice of the profile turns to a sail which can be the most hollow.<ref>[http://digilander.libero.it/eapisa/download/conferenza%202g.ppt see page 31]</ref>  But as the sail is more hollow, flow becomes more turbulent. We need to find the limit, because in turbulent flow the lift breaks down.
 
The profile selected, then the right incidence is  selected. The good incidence is the point of the polar plot with the higher lift (an incidence of 20°, which varies according to the sails).
 
The heel does not pose a problem yet because the heel component contains only drag, which is quite low. The drag is not as small as possible because  we selected a profile with a maximum lift with a big draft, therefore generating a lot of drag for a working lift profile.
 
As the optimum incidence is 20°, it is possible to adjust the sail lift mode corresponding to the lower points of  sail <math>\ \beta = 90 \deg</math> . The limit is <math>\ \beta = 70 </math>. At this point of sail incidence is zero. The propulsive effort is zero. The sail is no longer inflated by the wind.
 
It is not uncommon to see a sail set in drag. This situation, as evidenced by the formulas, give a heavy list. The sail is set incorrectly. The propulsive effort is provided by the drag <math>\ F_{propulsive}=  drag \times  cos (\beta)  </math>, with <math>\ cos(\beta) </math> close to zero. The propulsive effort is low. Almost all of the drag makes the boat heel.
 
'''Close reach'''
 
At  points  of  sail closer to the wind, the sail works to lift. As the heel becomes more important, we must limit the heel by improving the L/D (see L/D this Wikipedia page). It should be sailing with a less and less draft.
 
The constraints are:
 
:<math>\ M_{pitch}= D_{pitch} \times  (lift \times sin(\beta) -drag \times cos(\beta) ) =  D_{pitch} \times lift \times ( sin(\beta) -{(L/D)_{\alpha}} ^{-1} \times cos(\beta)) </math>
:<math>\ M_{heel} = D_{heel} \times (lift \times cos(\beta) +drag \times sin(\beta) ) =  D_{heel} \times lift \times (cos(\beta) +{(L/D)_{\alpha}} ^{-1} \times sin(\beta)) </math>
 
As <math>\ M_{heel_{max}} << M_{pitch_{max}} </math>, here  <math>\ 10 = \frac{M_{pitch_{max}} } { M_{heel_{max}} }</math>.
 
There is a tipping point where design constraint changes from the pitching factor to the heel factor:
:<math>\ \frac {D_{pitch} \times lift \times ( sin(\beta) -{(L/D)_{\alpha}} ^{-1} \times cos(\beta)) }{D_{heel} \times lift \times (cos(\beta) +{(L/D)_{\alpha}} ^{-1} \times sin(\beta))} = 10 </math>
 
Hence,
:<math>\ \beta =  \pi - tan^{-1}(\frac{1}{10}) - tan^{-1}(L/D) </math>
 
with
:<math>\ tan^{-1} </math> the [[Inverse function]] of the [[tangent]] <math>\ tan(tan^{-1}(x)) = x</math>.
:<math>\ \beta  </math> in radians.
 
However, <math>\ L/D >> 1 </math>, in practice  <math>\ L/D > 10 </math> then <math>\ tan^{-1}(L/D) \approx \frac{\pi}{2} </math>.
 
The tipping point is near the beam reach, even with a low ratio <math>\ 3.5 = \frac{M_{pitch_{max}} } { M_{heel_{max}} }</math>. So, soon after the beam reach, the sail, set to the greatest lift (deep draft), creates an excessive heel. The setting needs  gradual adjustment to obtain the highest possible L/D (sail more flat).
 
The constraint is:
:<math>\ M_{heel} =  D_{heel} \times lift \times (cos(\beta) +{(L/D)_{\alpha}} ^{-1} \times sin(\beta))</math>
 
'''Close hauled'''
 
The formulas are the same as above. As against the position of the polar of the sail changes, the incidence decreases. Indeed, the angle with the wind becomes weaker and weaker, until it becomes so weak that the sail is not inflated. So, there is no profile. The sail flaps in the wind.
 
'''Analysis of results'''
 
Constraints following the gaits are:
:<math>\ M_{pitch_{max}}  >  D_{pitch} \times drag </math> downwind
:<math>\ M_{pitch_{max}}  > D_{pitch} \times  lift </math> beam reach
and
:<math>\ M_{heel_{max}} > - D_{heel} \times drag \times sin(\beta) </math>  on a broad reach
:<math>\ M_{heel_{max}} >  D_{heel} \times  lift \times (cos(\beta) +{(L/D)_{\alpha}} ^{-1} \times sin(\beta))</math> upwind.
 
So, each will look at a different limit.
Or,
:<math>\ F_{forward} = drag \times cos(\beta)  </math> on a broad reach
:<math>\ F_{forward} = drag </math> downwind
:<math>\ F_{forward} = lift </math> beam reach
:<math>\ F_{forward} = lift \times ( sin(\beta) -{(L/D)_{\alpha}} ^{-1} \times cos(\beta))</math> upwind and partly reaching
Therefore,
:<math>\ M_{heel_{max}}  > D_{heel} \times  F_{forward} \times \frac{sin(\beta)}{cos(\beta)}</math> on a broad reach
:<math>\ M_{pitch_{max}}  > D_{pitch} \times  F_{forward} </math> beam reach
:<math>\ M_{pitch_{max}}  >  D_{pitch} \times F_{forward} </math> downwind
:<math>\ M_{heel_{max}} > D_{heel} \times F_{forward} \times sin( \pi - \beta -tan^{-1}(L/D)) </math> upwind
 
The point of sail is not an optimization variable but the incoming data. It depends on the chosen course. The point of  sail varies from near or close to downwind of about 30° to 180°.
 
The L/D of a sail using lift is similar to a thin profile. The theory of thin profile gives the formula of L/D. But the naval architect or sail maker to increase the performance of the boat will try to have a L/D as high as possible (see L/D and  sail trim). This setting is by design quite high. Factor <math>\ sin( \pi - \beta - tan^{-1}(L/D) ) \approx sin( \frac{\pi}{2} - \beta ) </math> is quite independent of the forward drive and varies little in point of sail. But, sufficiently varied so that, as the formula shows,  optimizing the performance also depends on the optimization of L/D. This optimization will give  settings different from the pure power settings, called L/D.
 
So, the constraints are directly related to the forward drive. But the physical power is directly related to the forward drive <math>P = (F_{forward})^{ \frac{3}{2}} \times ( \frac{1}{b})^{\frac{1}{2}} </math>. So, the stability is directly related to the power.
 
Finally, we return to the same concept as the physical power. The harder it is to capsize the boat, the more the boat supports a large area of canvas for the propulsive force, the more the boat is moving fast, so the more powerful . It is well understood that the explanation is a guide to load.  If the reader wishes, it is possible to obtain more complete formulas  operating  less severely than the approximations made in this guide. (See the scientific literature from the University of Southampton)<ref>[http://www.wumtia.soton.ac.uk/papers/HISWA2008ARC.pdf example on ORC class]</ref>{{,}}<ref>{{PDF|[http://www.wumtia.soton.ac.uk/papers/RINA1990BD.pdf The Development of Stability Standards for UK Sailing Vessels, B. Deakin]}}</ref>{{,}}<ref>http://www.orc.org/rules/ORC%20VPP%20Documentation%202009.pdf</ref>
 
On the other hand, the apparent wind is the vector sum of the true wind minus the boat speed. The mathematics shows that:
:<math> {V_{true wind}}^2 =  {V_{apparent wind}}^2 + {V_{boat}}^2 -2 \times V_{apparent wind} \times V_{boat} \times cos (\beta  -\pi)  </math>
 
The formula clearly shows the potential gain following the apparent wind speed of the boat; potential gain that the yacht will build between close hauled and beam reach. The propulsive effort is determined by: <math>F = \frac12 \times \rho \times S \times C \times {V_{wind}}^2</math> and the wind speed <math>V_{wind} = \frac {\mu'} {k}  \ ln ( \frac {z + z0} {z0}) </math>. Without going into details, the effort of the sail is balanced by gravity, hydrodynamic forces of the hull and buoyancy. So, by including all formulas, it is possible to determine the maximum speed of a sailboat with good accuracy according to its set of sails and point of sail. The software that performs this calculation are named the [[Velocity prediction program]]. The results indicate that the ship is the fastest in about beam reach (wide at close reach), that is to say the area where the lift of the sails is felt, and the apparent wind gained without compensation efforts for heel (drift leading to a strong resistance of the hull) too high.
 
The common thread: to maximize the speed of the boat. But according to course conditions, other choices are possible, leading to another type of setting.  Settings will differ in a gale, or for  a sailboat one is sailing against.
 
In reality, downwind must be respected at best an incidence of 90° with the apparent wind, simple enough for a mainsail with boom but, for headsail, it is more difficult, even with outriggers. But, this condition is not met then the lift is not zero. Performance is then lower than expected.
 
This approach is for the sail. It helps to understand the settings and optimizations to be made. For the naval architect the same process is conducted using the formulas including the hull .<ref>http://www.wumtia.soton.ac.uk/papers/FAST2005WHM2BD.pdf</ref> For  more completeness, this optimization can  include, rather than static (constant speed of the boat and wind), dynamic variables such  as sea swell and wind gusts.
 
A moment is a force multiplied by its lever arm. So, for the hull the  arm length should be as high as possible and for the sail as low as possible. The sailor has little control over the length of the lever arm. The bulk of the optimization work will be done by a naval architect for the hull (keel ballast) and sailmaker for the sail. This optimization is of course not independent. It is linked to other elements. It is limited for example by looking for winds aloft giving maximum propulsive effort. The end result will be one between all the constraints:
* Upwind, L/D is the major factor for the sail maker.
* With a tailwind, minimising  the  lever arm of  the sail's centre of effort is the major factor.
|}
 
The polar "power" plots have a higher maximum propulsive effort compared to polar "L/D" plots. The polar plot giving the maximum drag is a draft located behind the sail. Unlike the optimum setting for  close hauled, there is no sudden drop in pressure if the trough is set a little too far. The setting of the sail is wider, more tolerant .<ref name="BAY_1"/>
 
The power of the sail depends almost solely on the part of the sail force contributing to the advancement of the ship (along the axis of vessel speed or course made good). The power is treated as part of the sail force contributing to the advancement of the boat. The power is determined by the polar plot of the sail. The polar plot is independent of the apparent wind speed. Nor, in  steady state theory as opposed to  dynamic reality,<ref name="MarchajRoll"/> does the heel on the sail intervene with the speed setting. So the heel is not taken into account in the polar plot (same for the L/D of a polar plot).  The profile of maximum power is not the profile of maximum L/D, where a setting of "power" creates too much heel, a fairly standard error.<ref>[http://sinousparlionsassiette.blogspot.com/ Si nous parlions assiette<!-- Bot generated title -->]</ref>
 
==Several sails: multidimensional problem resolution==
 
The previous method for estimating the thrust of each sail is not valid for boats with multiple sails, but it remains a good approximation.
 
Sails close to each other influence each other. A two-dimensional model explains the phenomenon.<ref>[http://www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf The Aerodynamics of sail interaction]</ref> In the case of a sloop-rigged sailboat, the foresail changes air flow entering onto the mainsail. The conditions of a stable fluid, constant and uniform, necessary for tables which give lift coefficient, are not respected with  multiple sails. The cumulative effect of several sails on a boat can be positive or negative. It is well known that for the same total surface sail, two sails properly set are more effective than a single sail set correctly. Two sails can increase the  sailing thrust 20% compared a single sail of same area.<ref name= Al_Atabi>http://jestec.taylors.edu.my/Issue%201%20Vol%201%20June%2006/p89-98.pdf Al_Atabi, M.  The  Aerodynamics  of  wing  tip  sails. Journal of Engineering Science and Technology.Vol. 1, No. 1 (2006) 89-98. Multiple sails figure on page 94 of  article</ref><ref name=RichardsLasher1>{{cite journal|last=Richards|first=Peter|coauthors=Lasher, William|title=WIND TUNNEL AND CFD MODELLING OF PRESSURES ON DOWNWIND SAILS|journal=BBAA VI International Colloquium on: Bluff Bodies Aerodynamics & Applications|date=20–24 July 2008|url=http://bbaa6.mecc.polimi.it/uploads/validati/TR08.pdf|accessdate=2 June 2012}}</ref>
 
==See also==
*[[Sail]]
*[[Sailing]]
*[[Sailcloth]]
*[[Points of sail]]
*[[Sail-plan]]
*[[Rigging]]
*[[Wing]]
*[[Sail twist]]
*[[Stays (nautical)]]
*[[Sheet (sailing)]]
 
==Notes and references==
{{reflist|2}}
 
==Bibliography==
*{{cite book
|title=Royce's Sailing Illustrated: The Sailors Bible Since '56
|last=Royce
|first=Patrick M.
|year=1993
|publisher=Prostar
|isbn=978-0-911284-08-9
}}
 
*{{cite book
|title=Single-handed Sailing
|last=Mulville
|first=Frank
|year=1991
|publisher=Seafarer Books
|isbn=978-0-85036-410-1
}}
 
*{{cite book
|title=Sailing Theory and Practice, Revised edition
|last=Marchaj
|first=C.A.
|year=1985
|publisher=Putnam
|isbn=978-0-396-08428-0
}}
 
*{{cite book
|last=Marchaj
|first=C. A.
|title=Sail performance : techniques to maximise sail power
|year=2003
|publisher=Adlard Coles Nautical
|location=London
|isbn=978-0-7136-6407-2
|edition=Rev. ed.
}}
 
* {{cite book
  |last = Bethwaite
  |first = Frank
  |title = High Performance Sailing
  |publisher = Waterline (1993), Thomas Reed Publications (1996, 1998, et 2001), and Adlard Coles Nautical (2003 and 2007)
  |year = first edition in 1993; next in 1996, last in 2007
  |isbn = 978-0-7136-6704-2}}
 
* {{cite book|last=Eliasson|first=Lars Larsson & Rolf E.|title=Principles of yacht design|year=2007|publisher=International Marine|location=Camden, Me|isbn=978-0-07-148769-6|edition=3rd ed.}}
 
* {{cite book|last=Fossati|first=Fabio|title=Aero-hydrodynamics and the performance of sailing yachts : the science behind sailing yachts and their design|year=2009|publisher=International Marine /McGraw-Hill|location=Camden, Maine|isbn=978-0-07-162910-2|coauthors=translated by Martyn Drayton}}
* {{fr icon}} {{cite book
  |last = Curry
  |first = Manfred
  |title = L'aérodynamique de la voile et l'art de gagner les régates
  |publisher = Etienne Chiron, Ed. nouv. with new document (1 juillet 1991)
  |year = 1930
  |isbn = 978-2-7027-0027-3}}
 
* {{fr icon}} {{cite book
  |last = Bertrand
  |first = Chéret
  |title = Les Voiles. Comprendre, régler, optimiser
  |publisher = Gallimard
  |date = June 2010
  |isbn = 978-2-7424-0767-5}}
 
* {{fr icon}} [[Leonhard Euler]] ''[http://books.google.fr/books?id=0qwWAAAAQAAJ&pg=PA166&lpg=PA166&dq=centre+v%C3%A9lique&source=bl&ots=lm1-n-v_7o&sig=UB58BXVE7ruNiDy9HpnhmWTVbpc&hl=fr&ei=HI7FS5raCYKROJ6fwb4P&sa=X&oi=book_result&ct=result&resnum=10&ved=0CBwQ6AEwCTgo#v=onepage&q=centre%20v%C3%A9lique&f=false Théorie complète de la construction et de la manoeuvre des vaisseaux]'' printed by Claude-antoine Jombert at Paris in 1773
 
* {{la icon}} [[Leonhard Euler]] ''[http://www.math.dartmouth.edu/~euler/pages/E110.html E110 Scientia navalis]'' full title is ''Scientia navalis seu tractatus de construendis ac dirigendis navibus Pars prior complectens theoriam universam de situ ac motu corporum aquae innatantium. Auctore Leonhardo Euler prof. honorario academiae imper. scient. et directore acad. reg. scient. Borussicae. Instar supplementi ad tom. I. novorum commentar. acad. scient. imper. Petropoli typis academiae scientiarum MDCCXLIX.''
 
==External links==
* {{PDF|[http://www.aero.us.es/adesign/Documentation/NACA%20TR%20824%20-%20Summary%20of%20airfoil%20data.pdf Rapport NACA n° 824]|18.6&nbsp;MB}}
* {{PDF|[http://naca.central.cranfield.ac.uk/reports/1955/naca-report-1218.pdf Rapport NACA n° 1218]|4.01&nbsp;MB}}
* {{PDF|[http://naca.central.cranfield.ac.uk/reports/1955/naca-report-1217.pdf Rapport NACA n° 1217]|4.57&nbsp;MB}}
* [http://www.sailboat-technology.com/links/online_articles.php listing of interesting article on Sailboat-technology.com]
* [http://www.finot.com/ecrits/Damien%20Lafforgue/article_voiles_english.html Sails: from experimental to numerical Damien Laforgue]
 
{{DEFAULTSORT:Forces On Sails}}
[[Category:Naval architecture]]
[[Category:Sailing]]
[[Category:Marine propulsion]]

Revision as of 22:39, 23 February 2014

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