Forces on sails to stay within the limitations of a craft's stability and power requirements, which are functions of hull (for boats) or chassis (for land craft) design. Sails derive power from wind that varies in time and with height above the surface. In order to do so, they are designed to adjust to the wind force for various points of sail. Both their design and method for control include means to match their lift and drag capabilities to the available apparent wind, by changing surface area, angle of attack, and curvature. Wind speed increases with height above the surface; at the same
Forces on sails balance. Integrated over the sailing craft, the total aerodynamic force (F) is located at the centre of effort ("CE"), which is a function of the design and adjustment of the sails on a sailing craft. Similarly, the total hydrodynamic force (F) is located at the centre of lateral resistance ("CLR"), which is a function of the design of the hull and its underwater appendages (keel, rudder, foils, etc.). These two forces act in opposition to one another with F a reaction to F. Whereas ice boats and land-sailing craft resist lateral forces with their wide stance and high-friction contact with