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 total aerodynamic force (F) on sails, which is a function of apparent wind velocity (V) and varies with point of sail. The forward driving force (F) component contributes to boat velocity (V), which is, itself, a determinant of apparent wind velocity. Absent lateral reactive forces to F from a keel (in water), a skate runner (on ice) or a wheel (on land), a craft would only be able to move downwind and the sail would not be able to develop lift. At a stable angle of heel (for a sailboat) and a steady speed, aerodynamic and hydrodynamic forces are in