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 is evident that only one value of formula_6 yields the highest efficiency. That is, the fixed speed wind turbine is not operating at peak efficiency across a range of wind speeds. This was a motivator for the development of variable speed wind turbines. Before the need to connect wind turbines to the grid, turbines were fixed-speed. This was not a problem because turbines did not have to be synchronized with the frequency of the grid. All grid-connected wind turbines, from the first one in 1939 until the development of variable-speed grid-connected wind turbines in the 1970s, were fixed-speed wind turbines.