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
Variable speed wind turbine formula_24 formula_31 "For further details, see Blade Element Momentum Theory" Consider the following figure: This is the depiction of the apparent wind speed, as seen by a blade (left of figure). The apparent wind speed is influenced by both the free-stream velocity of the air, and the rotor speed. From this figure, we can see that both the angle formula_32 and the apparent wind speed formula_33 are functions of the rotor speed, formula_2. By extension, the lift and drag forces will also be functions of formula_2. This means that the axial and tangential forces that act on the blade vary