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 turbine design can be approached by modern turbine designs which may reach 70 to 80% of this theoretical limit. The speed at which a wind turbine rotates must be controlled for efficient power generation and to keep the turbine components within designed speed and torque limits. The centrifugal force on the spinning blades increases as the square of the rotation speed, which makes this structure sensitive to overspeed. Because the power of the wind increases as the cube of the wind speed, turbines have to be built to survive much higher wind loads (such as gusts of wind) than those from which