Knot (unit) fluids (tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG) (ground speed (GS) in aircraft) and rate of progress towards a distant point ("velocity made good", VMG) are also given in knots. Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the
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