MARINE TERMINOLOGY ORIGINATION: KNOT
By Capt. Do Ngoc Hoai
Vessel’s Speed Unit – Knot
The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. The ISO standard symbol for the knot is kn. The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE); kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travel approximately one minute of geographic latitude in one hour.
Etymologically, the term derives from counting the number of knots in the line that unspooled from the reel of a chip log in a specific time.
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 moving vessel and the line allowed to pay out. Knots tied at a distance of 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass to time the operation. This method gives a value for the knot of approximately 1.85166 km/h. The difference from the modern definition is less than 0.02%.