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Sailboats use sails to obtain driving force from the wind while moving through water. Their appearance ranges from small open boats to larger vessels with complex arrangements of masts and sails. The basic problem is shared: the forces acting above and below the surface must work together. The visible canvas is therefore only one part of a system that also includes the hull, steering, and submerged surfaces. A sail changes the movement of air around it and experiences a resulting force. The useful direction of that force depends on the sail's shape and its position relative to the wind. A boat [...]