When sailing, the forces that propel a boat forward are determined by the roughly constant force of the wind in its sails and the resistance to its motion through water. This resistance is a result of both drag (which decreases with speed) and pressure differences that form along the surface of the water. The latter are generated by the flow around the longer-path side of a keel or sail, and can be reduced by designing a feature such as a “winged” keel that moves some of its weight to the upwind side.
Depending on the conditions, sailboats can produce a bow wave with crests at the front and troughs at the rear that extend to about half their length from the water line. The shape of this wave determines in part how the boat turns, and can be modified by designing features such as a “spreader” that increases the power of the shrouds or spreaders that support the mast.
Most sailboats have a displacement hull, which are flat at the stern or back and move through the water by pushing it aside rather than by flowing over it. This results in a lower average speed than is possible for motor boats, but provides excellent stability and a great deal of control when maneuvering. The hull is also designed to cut through waves and troughs by the shape of its prow, or bow.
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