Apparent wind in knots and degrees. Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following: In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the "ground" or "geographical wind". The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane
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.