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 sail and the apparent wind ("α" ) exceeds the point of maximum lift on the "C"–"C" polar diagram, separation of flow occurs. The separation becomes more pronounced until at "α" = 90° lift becomes small and drag predominates. In addition to the sails used upwind, spinnakers provide area and curvature appropriate for sailing with separated flow on downwind points of sail. Again, in these diagrams the direction of travel changes with respect to the apparent wind (V), which is constant for the sake of illustration, but would in reality vary with point of sail for a constant true wind. In