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 length of the blade, formula_4 is the velocity of the wind, and formula_5 is the power coefficient for the wind turbine. The power coefficient is a representation of how much of the available power in the wind is captured by the wind turbine. The torque, formula_15, on the blades is given by the ratio of the power extracted to the rotor speed, formula_2: formula_17 The rotor speed can be related to the wind speed, formula_4, through the tip-speed ratio, formula_6: formula_1 Thus we can get the following expressions for torque and power: formula_21 and formula_22 From the above equation, we