Apparent wind that measures wind speed in knots and wind direction in degrees relative to the heading of the boat. Modern instrumentation can calculate the true wind velocity when the apparent wind and boat speed and direction are input. In sailboat racing, and especially in speed sailing, apparent wind is a vital factor, when determining the points of sail a sailboat can effectively travel in. A vessel traveling at increasing speed relative to the "prevailing wind" will encounter the wind driving the sail at a decreasing angle and increasing velocity. Eventually, the increased drag and diminished degree of efficiency of a sail
Wind-powered vehicle sail. Both land yachts and ice boats have low forward resistance to speed and high lateral resistance to sideways motion. Aerodynamic forces on sails depend on wind speed and direction and the speed and direction of the craft ( "V" ). The direction that the craft is traveling with respect to the "true wind" (the wind direction and speed over the surface – "V" ) is called the "point of sail". The speed of the craft at a given point of sail contributes to the "apparent wind" ( "V" )—the wind speed and direction as measured on the moving craft.