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 Variable speed wind turbine Original models of wind turbines were fixed speed turbines; that is, the rotor speed was a constant for all wind speeds. The tip-speed ratio for a wind turbine is given by the following formula: formula_1 where formula_2 is the rotor speed (in radians per second), formula_3 is the length of a blade, and formula_4 is the wind speed. That is to say, for a fixed-speed wind turbine, the value of the tip-speed ratio is only changed by wind speed variations. In reference to a formula_5-formula_6 graph, which illustrates the relationship between Tip-speed ratio and efficiency, it