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
North Sails the 3DL line of upwind sails in 1992. In the 3DL manufacturing process, aramid or carbon fiber yarns and layers of polyester (PET) film are laid over a computer-controlled mold that is configured to assume the airfoil shape of the sail, and then thermo-formed to produce a laminated sail with the intended three-dimensional shape. North Sails manufactures 3DL sails in facilities in Minden, Nevada, and Sri Lanka. In 2009, North Sails began using pre-impregnated tapes of carbon and UHMPE fibers on its full-sized articulating molds. When thermomolded, the layers of reinforcing material consolidate to form a seamless composite sail membrane,