Apparent wind in knots and degrees. Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following: In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the "ground" or "geographical wind". The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane
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