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 example of a permanent magnet generator is the Siemens SWT-2.3-113. A disadvantage of a permanent magnet generator is the cost of materials that need to be included. Consider a variable speed wind turbine with a permanent magnet synchronous generator. The generator produces AC electricity. The frequency of the AC voltage generated by the wind turbine is a function of the speed of the rotor within the generator: formula_39 where formula_40 is the rotor speed, formula_10 is the number of poles in the generator, and formula_42 is the frequency of the output Voltage. That is, as the wind speed varies, the