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
USA 17 velocity made good was about , or about 1.8 times wind speed. "USA 17" took 63 minutes to reach the downwind mark, so her velocity made good downwind was about , or about 2.5 times wind speed. The measured wind velocity is taken at near sea level, and does not account for effects of wind gradient with reported true wind speed of at mast head height of over . The measured sea-level wind direction shifted away from 180 degrees between the time of setting of the course and commencement of sailing of the downwind leg, by which time wind direction