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
Wind-powered vehicle sail. Both land yachts and ice boats have low forward resistance to speed and high lateral resistance to sideways motion. Aerodynamic forces on sails depend on wind speed and direction and the speed and direction of the craft ( "V" ). The direction that the craft is traveling with respect to the "true wind" (the wind direction and speed over the surface – "V" ) is called the "point of sail". The speed of the craft at a given point of sail contributes to the "apparent wind" ( "V" )—the wind speed and direction as measured on the moving craft.