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
Sailing into the wind head to the wind it loses steerage and is said to be "in irons." Thus boats sailing into the wind are actually sailing "Close Hauled" (i.e., with sails tightly trimmed). When one "sails too close to the wind" it is called "pinching." This phrase is also a colloquial expression meaning "to be reckless." Sailing into the wind Sailing into the wind is a sailing expression that refers to a sail boat's ability to move forward despite being headed into (or very nearly into) the wind. A sailboat cannot make headway by sailing directly into the wind ("see" "Discussion," below), so