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
Variable speed wind turbine As of 2003, nearly all grid-connected wind turbines operate at an exactly constant speed (synchronous generators) or within a few percents of constant speed (induction generators). Below is an illustration of the formula_8 curve for a typical wind turbine. Maximum efficiency occurs at one tip-speed ratio only. Since the tip-speed ratio is given by the aforementioned expression, variable speed wind turbines can operate at maximum efficiency over all wind speeds (ideally). For a wind turbine, the power harvested is given by the following formula: formula_9 where formula_10 is the power, formula_11 is the density of the air, formula_3 is the