Origin
Until the mid-19th century vessel speed at sea was measured using a chip log. This consisted of a wooden panel, weighted on one edge to float perpendicular to the water surface, and thus present substantial resistance to moving with respect to the water around it, attached by line to a reel. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out. Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another sailor used a 30 second sand-glass (28 second sand-glass is the current accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.
Read more about this topic: Knot (unit)
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