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 power the volume of air passing through "A" (which is considered perpendicular to the direction of the wind); "Avtρ" is therefore the mass "m" passing through "A". Note that ½ "ρv" is the kinetic energy of the moving air per unit volume. Power is energy per unit time, so the wind power incident on "A" (e.g. equal to the rotor area of a wind turbine) is: Wind power in an open air stream is thus "proportional" to the "third power" of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electric power therefore