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 power the volume of air passing through "A" (which is considered perpendicular to the direction of the wind); "Avtρ" is therefore the mass "m" passing through "A". Note that ½ "ρv" is the kinetic energy of the moving air per unit volume. Power is energy per unit time, so the wind power incident on "A" (e.g. equal to the rotor area of a wind turbine) is: Wind power in an open air stream is thus "proportional" to the "third power" of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electric power therefore