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
Tip-speed ratio and then converts that signal to an AC signal with the grid/transmission system frequency. As already mentioned, variable speed wind turbines cannot be directly connected to a transmission system. One of the drawbacks of this is that the inertia of the transmission system is reduced as more variable speed wind turbines are put online. This can result in more significant drops in the transmission system's voltage frequency in the event of the loss of a generating unit. Furthermore, variable speed wind turbines require power electronics, which increases the complexity of the turbine and introduces new sources of failures. It has