Knot (unit)

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Knot
Airspeed Indicator.svg
An airspeed indicator, which shows speed in knots
General information
Unit system
  • meteorology
  • aviation
  • maritime
Unit of speed
Symbolknorkt
Conversions
1 kn in ...... is equal to ...
    km/h    1.852
    mph    1.15078
    m/s    0.514444
    ft/s    1.68781

The knot ( /nɒt/ ) is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 1.15078 mph or 0.514 m/s). [1] The ISO standard symbol for the knot is kn. [2] 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). [3] The knot is a non-SI unit. [4] The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

Contents

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

1 international knot =
1  nautical mile per hour (by definition),
1852.000 metres per hour (exactly), [4]
0.51444 metres per second (approximately),
1.15078 miles per hour (approximately),
20.25372 inches per second (approximately)
1.68781 feet per second (approximately).

The length of the internationally agreed nautical mile is 1852 m. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m). [5] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft or 1853.184 m).

Conversions between common units of speed
m/s km/h mph knot ft/s
1 m/s =13.62.236936*1.943844*3.280840*
1 km/h =0.277778*10.621371*0.539957*0.911344*
1 mph =0.447041.60934410.868976*1.466667*
1 knot =0.514444*1.8521.150779*11.687810*
1 ft/s =0.30481.097280.681818*0.592484*1

(* = approximate values)

Usage

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

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. [6] Knots placed 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 (28-second sand-glass is the currently accepted timing) to time the operation. [7] 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%.

Derivation of knots spacing:

, so in seconds that is metres per knot.

Modern use

Graphic scale from a Mercator projection world map, showing the change with latitude World Scale from DMA Series 1150 map.png
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 the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.

Standard nautical charts are on the Mercator projection and the horizontal (East-West) 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. [8]

Speed is sometimes incorrectly expressed as "knots per hour", [9] which would mean "nautical miles per hour per hour" and thus would refer to acceleration.

Aeronautical terms

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. [10]

The following abbreviations are used to distinguish between various measurements of airspeed: [11]

The indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At 11000 m (36000 ft), an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.

See also

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Airspeed indicator instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot

The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometers per hour (km/h), knots (kn), miles per hour (MPH) and/or meters per second (m/s). The recommendation by ICAO is to use km/h, however knots is currently the most used unit. The ASI measures the pressure differential between static pressure from the static port, and total pressure from the pitot tube. This difference in pressure is registered with the ASI pointer on the face of the instrument.

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References

  1. Bartlett, Tim (July 2008) [2003]. RYA Navigation Handbook. Southampton: Royal Yachting Association.
  2. "ISO 80000-3:2006". International Organization for Standardization . Retrieved 20 July 2013.
  3. International Standards and Recommended Practices, Annex 5 to the Convention on International Civil Aviation, “Units of measurement to be Used in Air and Ground Operations”, ICAO, 4th Edition, July 1979.
  4. 1 2 "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.
  5. 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. Archived from the original (PDF) on 26 September 2007. Retrieved 2 August 2007.
  6. Jacob Abbott (1858). Rollo on the Atlantic. DeWolfe, Fiske, & Co., Publishers. Retrieved 30 November 2011.
  7. Kemp, Peter, ed. (1976). The Oxford Companion to Ships and the Sea . Oxford University Press. p.  454. ISBN   0-19-282084-2.
  8. e.g. BA Chart 73, Puerto de Huelva and Approaches, 2002
  9. 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.
  10. For example, Part 23 of the Federal Aviation Regulations, amendment 23–7, 14 September 1969
  11. "Abbreviations and symbols". edocket.access.gpo.gov.