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 aerodynamics, the aspect ratio of a sail is the ratio of its length to its breadth (chord). A high aspect ratio indicates a long, narrow sail, whereas a low aspect ratio indicates a short, wide sail. For most sails, the length of the chord is not a constant but varies along the wing, so the aspect ratio "AR" is defined as the square of the sail height "b" divided by the area "A" of the sail planform: Aspect ratio and planform can be used to predict the aerodynamic performance of a sail. For a given sail area, the aspect ratio,