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{{Short description|none}} <!-- "none" is a legitimate description when the title is already adequate; see [[WP:SDNONE]] -->
{{Main|Sail}}
[[File:Points of sail--close-hauled (right) and down wind (left).jpg|thumb|right|300px|Aerodynamic force components for two points of sail. <br />''Left-hand boat'': Down wind with stalled
to heel.]]
[[File:Points of sail--English.jpg|thumb|right|300px|Points of sail (and ''predominant sail force component'' for a displacement sailboat).<br />A. Luffing (''no propulsive force'')
'''Forces on sails''' result from movement of air that interacts with [[sail]]s and gives them motive power for sailing craft, including [[sailing ship]]s, [[sailboat]]s, [[Windsurfing|windsurfers]], [[ice boat]]s, and [[Land sailing|sail-powered land vehicles]]. Similar principles in a rotating frame of reference apply to [[windmill sail]]s and [[wind turbine]] blades, which are also wind-driven. They are differentiated from [[force]]s on [[wing]]s, and [[propeller]] blades, the actions of which are not adjusted to the wind. [[Kites]] also power [[Kite boarding (disambiguation)|certain sailing craft]], but do not employ a mast to support the airfoil and are beyond the scope of this article.
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{{Main|Apparent wind|Point of sail|High-performance sailing}}
Apparent wind ('''V<sub>A</sub>''') is the air velocity acting upon the leading edge of the most forward sail or as experienced by instrumentation or crew on a moving sailing craft. It is the [[Euclidean vector#Addition and subtraction|vector sum]] of true wind velocity and the apparent wind component resulting from boat velocity ('''V<sub>A</sub>''' = '''
<div class="center">
;Effect of apparent wind on sailing craft at three points of sail
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The values of driving force (''F<sub>R</sub>'' ) and lateral force (''F<sub>LAT</sub>'' ) with apparent wind angle (α), assuming no heeling, relate to the values of lift (''L'' ) and drag (''D'' ), as follows:<ref name=Fabio/>
:<math>
:<math>\ F_{LAT} = L \cdot \cos(\alpha) + D \cdot \sin(\alpha) </math>
===Reactive forces on sailing craft===
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| last = Committee for the National Tire Efficiency Study
| url = http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf
| title = Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance
| access-date = 2007-08-11}}</ref> whereas kinetic friction is normally a constant,<ref>{{cite book|title=Statics: Analysis and Design of Systems in Equilibrium|publisher=Wiley and Sons|year=2005|isbn=978-0-471-37299-8|page=618|author1=Sheppard, Sheri|author2=Tongue, Benson H.|author3=Anagnos, Thalia|author1-link=Sheri D. Sheppard}}
</ref> but on ice may become reduced with speed as it transitions to [[Friction#Lubricated friction|lubricated friction]] with melting.<ref name = Kimball/>
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| date = 2013
| pages = 448
| publisher = A&C Black
| url = https://books.google.com/books?id=WTRLAAAAQBAJ&q=Faster+than+the+wind&pg=PA204
| isbn = 9781472901309 }}</ref>
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|archive-date = 2012-07-11
}}</ref>
*[[Sailing hydrofoil]]s achieve boat speeds up to twice the speed of the wind, as did the [[AC72]] catamarans used for the [[2013 America's Cup]].<ref name=2013first2>{{cite web|url=http://www.americascup.com/en/news/3/news/18009/emirates-team-new-zealand-gets-leg-up-on-oracle-team-usa |title=Emirates Team New Zealand gets leg up on ORACLE TEAM USA |publisher=
*Ice boats can sail up to five times the speed of the wind.<ref name=Boat_Speed>{{Citation
| first = Bob
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| place = Anapolis
| publisher = SNAME
| url = http://www.sname.org/chesapeakesailingyachtsymposiumcsys/pastpapers/16thcsys
| access-date = 2017-01-29 | archive-date = 2020-09-19 | archive-url = https://web.archive.org/web/20200919123639/https://www.sname.org/chesapeakesailingyachtsymposiumcsys/pastpapers/16thcsys | url-status = dead }}</ref><ref>{{cite web|title=Commonly Asked Questions|url=http://iceboat.org/faqiceboat.html|publisher=Four Lakes Ice Yacht Club|access-date=2010-08-25|archive-url=https://web.archive.org/web/20110309200937/http://www.iceboat.org/faqiceboat.html|archive-date=2011-03-09|url-status=dead}}</ref> '''Lateral force''' is a reaction supplied by the underwater shape of a sailboat, the blades of an ice boat and the wheels of a land sailing craft. Sailboats rely on [[keel]]s, [[centerboard]]s, and other underwater foils, including rudders, that provide [[Lift (force)|lift]] in the lateral direction, to provide hydrodynamic lateral force ('''P<sub>LAT</sub>''') to offset the lateral force component acting on the sail ('''F<sub>LAT</sub>''') and minimize leeway.<ref name=Fabio/> Such foils provide hydrodynamic lift and, for keels, ballast to offset heeling. They incorporate a wide variety of design considerations.<ref>
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===Drag predominant (separated flow)===
When sailing craft are on a course where the angle of attack between the sail and the apparent wind (''α'' ) exceeds the point of maximum lift on the ''C<sub>L</sub>''–''C<sub>D</sub>'' polar diagram, separation of flow occurs.<ref>{{Citation
| last1 = Collie
| first1 = S. J.
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| date = 2006
| url = http://syr.stanford.edu/RINA_Steve.pdf
| access-date = 2015-04-04
| archive-date = 2010-07-28
</ref> The separation becomes more pronounced until at ''α'' = 90° lift becomes small and drag predominates. In addition to the sails used upwind, [[spinnaker]]s provide area and curvature appropriate for sailing with separated flow on downwind points of sail.<ref name=Textor>▼
| archive-url = https://web.archive.org/web/20100728123723/http://syr.stanford.edu/RINA_Steve.pdf
| url-status = dead
▲ }}</ref> The separation becomes more pronounced until at ''α'' = 90° lift becomes small and drag predominates. In addition to the sails used upwind, [[spinnaker]]s provide area and curvature appropriate for sailing with separated flow on downwind points of sail.<ref name=Textor>
{{cite book
| last = Textor
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| journal = Proceedings of the Eleventh AIAA Symposium on the Aero/Hydronautics of Sailing
| date = September 12, 1981
| url = http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf
| archive-url = https://web.archive.org/web/20140422225628/http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf
| url-status = usurped
| archive-date = April 22, 2014
| access-date = 2015-04-11 }}</ref>
The two sails cause an overall larger displacement of air perpendicular to the direction of flow when compared to one sail. They act to form a larger wing, or airfoil, around which the wind must pass. The total length around the outside has also increased and the difference in air speed between windward and leeward sides of the two sails is greater, resulting in more lift. The jib experiences a greater increase in lift with the two sail combination.<ref>{{Cite book|last=Anderson|first=Bryon D.
==Sail performance design variables==
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| url = http://www.sailingbreezes.com/Sailing_Breezes_Current/Articles/Feb09/Guidelines_for_Good_Mainsail_Shape.htm
| access-date = 2015-08-01}}</ref>
*For light air (less than 8 knots), the sail is at its fullest with the depth of draft between 13
*For medium air (
*For heavy (greater than15 knots), the sail is flattened and allowed to twist in a manner that dumps lift with a depth of draft set between 9
Plots by Larsson ''et al'' show that draft is a much more significant factor affecting sail propulsive force than the position of maximum draft.<ref name=Principles>
{{Citation
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===Drag variables===
Spinnakers have traditionally been optimized to mobilize drag as a more important propulsive component than lift. As sailing craft are able to achieve higher speeds, whether on water, ice or land, the velocity made good (VMG) at a given course off the wind occurs at apparent wind angles that are increasingly further forward with speed. This suggests that the optimum VMG for a given course may be in a regime where a spinnaker may be providing significant lift.<ref>
{{Citation| title = Downwind Sails
| publisher = Australian Sailing & Yachting
| date = January 2012
| |||