Knot (unit)

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The knot (pronounced not) is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately 1.151 mph.[1] The ISO Standard symbol for the knot is kn.[2] The same symbol is preferred by the IEEE; kt and NMPH are also seen. The knot is a non-SI unit that is "accepted for use with the SI".[3] Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels 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.

Definitions[edit]

1 international knot =
nautical mile per hour (exactly),
1.852 kilometres per hour (exactly),[3]
0.514 metres per second (approximately),
1.151 miles per hour (approximately),
20.2537 inches per second (approximately).

1852 m is the length of the internationally agreed nautical mile. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m).[4] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft [1853.184 m]).

Conversions between common units of speed
m/s km/h mph knot ft/s
1 m/s = 1 3.6 2.236936 1.943844 3.280840
1 km/h = 0.277778 1 0.621371 0.539957 0.911344
1 mph = 0.44704 1.609344 1 0.868976 1.466667
1 knot = 0.514444 1.852 1.150779 1 1.687810
1 ft/s = 0.3048 1.09728 0.681818 0.592484 1

(Values in bold face are exact.)

Usage[edit]

The speeds of vessels relative to the fluids in which they travel (boat speeds and air speeds) are measured in knots. For consistency, the speeds of navigational 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.

Origin[edit]

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.[5] Knots placed at a distance of 47 feetinches (14.4018 m) passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.[6] 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%.

Modern use[edit]

Graphic scale from a Mercator projection world map, showing the change with latitude

Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because standard nautical charts are on the Mercator projection and the scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier.[7]

Speed is sometimes incorrectly expressed as "knots per hour",[8] which is in fact a measure of acceleration.

Aeronautical terms[edit]

Prior to 1969, airworthiness standards for civil aircraft in the United States Federal Aviation Regulations specified that distances were to be in statute miles, and speeds in miles per hour. In 1969 these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.[9]

The following abbreviations are used to distinguish between various measurements of airspeed:[10]

Note that the indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At 11000 m (35000 ft), an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.

See also[edit]

References[edit]

  1. ^ Bartlett, Tim (2003, reprinted July 2008). RYA Navigation Handbook. Southampton: Royal Yachting Association. 
  2. ^ "ISO 80000-3:2006". International Organization for Standardization. Retrieved 20 July 2013. 
  3. ^ a b "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI brochure (8th ed.). International Bureau of Weights and Measures. "The knot is defined as one nautical mile per hour. There is no internationally agreed symbol, but the symbol kn is commonly used." 
  4. ^ Louis E. Barbow and Lewie V. Judson (1976). "Appendix 4 The international nautical mile" (PDF). Weights and Measures Standards of the United States, A brief history. NIST Physics Laboratory. Retrieved 2007-08-02. 
  5. ^ Jacob Abbott (1858). Rollo on the Atlantic. DeWolfe, Fiske, & Co., Publishers. Retrieved 30 November 2011. 
  6. ^ Oxford Companion to Ships and the Sea. p. 454.
  7. ^ e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002
  8. ^ Wilson, Alastair (22 July 2009). "'Knots an hour'". The Kipling Society. Retrieved 1 December 2012. "Since the 1890s or thereabouts, it has been drummed into the young seaman that a knot is a unit of speed, namely, one nautical mile per hour; and that consequently only the uneducated speak of "knots per hour" or "knots an hour". It was therefore inevitable that Kipling’s frequent use of this expression should grieve a number of seafaring readers, as the pages of the Kipling Journal testify." 
  9. ^ For example, Part 23 of the Federal Aviation Regulations, amendment 23-7, September 14, 1969
  10. ^ U.S. Federal Aviation Regulations, Part 1 Definitions and Abbreviations

Further reading[edit]

  • Kemp, Peter (editor). The Oxford Companion to Ships and the Sea. Oxford university Press, 1976. ISBN 0-19-282084-2