Forces on sails: Difference between revisions

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[[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'') &mdash; 0-30°<br />B. Close-Hauled (''lift'')&mdash; 30-50°<br />C. Beam Reach (''lift'')&mdash; 90°<br />D. Broad Reach (''lift–drag'')&mdash; ~135°<br />E. Running (''drag'')&mdash; 180°<br />True wind ('''V_T''') is the same everywhere in the diagram, whereas boat velocity ('''V_B''') and apparent wind ('''V_A''') vary with point of sail.]]

'''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 [[Windmillwindmill sail|wind mill sails]]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.

Forces on sails depend on wind speed and direction and the speed and direction of the craft. The direction that the craft is traveling with respect to the "true wind" (the wind direction and speed over the surface) is called the [[point of sail]]. The speed of the craft at a given point of sail contributes to the "[[apparent wind]]"—the wind speed and direction as measured on the moving craft. The apparent wind on the sail creates a total aerodynamic force, which may be resolved into [[Drag (physics)|drag]]—the force component in the direction of the apparent wind—and [[Lift (force)|lift]]—the force component [[normal (geometry)|normal]] (90°) to the apparent wind. Depending on the alignment of the sail with the apparent wind, lift or drag may be the predominant propulsive component. Total aerodynamic force also resolves into a forward, propulsive, driving force—resisted by the medium through or over which the craft is passing (e.g. through water, air, or over ice, sand)—and a lateral force, resisted by the underwater foils, ice runners, or wheels of the sailing craft.