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
Apparent wind to the combined speed and direction of each the craft and the true wind. Apparent wind is important to sailors in order to set sail angle with respect to the wind and to anticipate how much power the wind will generate on a point of sail. Apparent wind differs in speed and direction from the true wind that is experienced by a stationary observer and composed of the true wind speed (TWS) and true wind direction (TWD) or the TWS and true wind angle (TWA) relative to the boat if it were stationary. In nautical terminology, apparent wind is measured