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 mainsail causes the stagnation line on the jib to be shifted forward and allows the boat to point closer to the wind, owing to higher leeward velocities of the air over both sails. Sails characteristically have a coefficient of lift ("C") and coefficient of drag ("C") for each apparent wind angle. The planform, curvature and area of a given sail are dominant determinants of each coefficient. Sails are classified as "triangular sails", "quadrilateral fore-and-aft sails" (gaff-rigged, etc.), and "square sails". The top of a triangular sail, the "head", is raised by a halyard, The forward lower corner of the sail,