Aerodynamic forces on sails depend on wind speed and direction and the speed and direction of the craft ( VB ). The direction that the craft is traveling with respect to the true wind (the wind direction and speed over the surface – VT ) is called the point of sail. The speed of the craft at a given point of sail contributes to the apparent wind ( VA )—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—the force component in the direction of the apparent wind—and lift—the force component 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 wheels or ice runners of the vehicle.
