Knot (unit)

For other uses, see Knot (disambiguation).

The knot (pronounced not) is a unit of speed equal to one nautical mile (which is defined as 1.852 km) per hour, approximately 1.151 mph.^[1] The abbreviation kn is preferred by the International Hydrographic Organization (IHO), which includes every major sea-faring nation;^[2] however, the abbreviations kt (singular) and kts (plural) are also widely used.^[3] However, use of the abbreviation kt for knot conflicts with the SI symbol for kilotonne. The knot is a non-SI unit accepted for use with the International System of Units (SI).^[4] 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 one minute of geographic latitude in one hour. Etymologically, the term knot 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),

1.852 kilometres per hour (exactly),^[4]

0.514 metres per second,

1.151 miles per hour (approximately).

1,852 m is the length of the internationally-agreed nautical mile. The U.S. adopted the international definition in 1954, having previously used the U.S. nautical mile (1,853.248 m).^[5] The U.K. adopted the international nautical mile definition in 1970, having previously used the U.K. Admiralty nautical mile (6,080 ft [1,853.184 m]).

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.

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.)

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 upright, 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.^[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

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

Although the unit knot does not fit within the primary 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 is, for practical purposes, identical to 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.

Speed is sometimes incorrectly expressed as "knots per hour", which would actually be a measure of acceleration, as in "nautical miles per hour per hour".

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^[7] were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.

The following abbreviations^[8] are used to distinguish between various measurements of airspeed.

• KTAS is "knots true airspeed", the airspeed of an aircraft relative to undisturbed air.
• KIAS is "knots indicated airspeed", the speed shown on an aircraft's pitot-static airspeed indicator.
• KCAS is "knots calibrated airspeed", the indicated airspeed corrected for position error and instrument error.
• KEAS is "knots equivalent airspeed", the calibrated airspeed corrected for adiabatic compressible flow for the particular altitude.

See also

Nautical portal

• Beaufort scale
• Hull speed, which deals with theoretical estimates of practical maximum speed of displacement hulls.
• Metre per second
• Nautical mile
• Orders of magnitude (speed)

References

1. ^ Bartlett, Tim (2003, reprinted July 2008). RYA Navigation Handbook. Southampton: Royal Yachting Association. 
2. ^ Chart No. 1, Positions, Distances, Directions, Compass. Jointly by NOAA and Department of Commerce, USA. http://www.nauticalcharts.noaa.gov/mcd/chartno1.htm.  The cited book incorporates IHO Chart INT 1 and therefore represents the practice of the member states;
3. ^ "Google calculator". http://www.google.com/search?q=kt. Retrieved 2009-04-05. "1 kt = 0.514444444 m/s" 
4. ^ ^{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. http://www.bipm.org/en/si/si_brochure/chapter4/table8.html. "The knot is defined as one nautical mile per hour." 
5. ^ Louis E. Barbrow and Lewis V. Judson (1976). "Appendix 4 The international nautical mile" (PDF). Weights and Measures Standards of the United States, A brief history. NIST Physics Laboratory. http://physics.nist.gov/Pubs/SP447/app4.pdf. Retrieved 2007-08-02. 
6. ^ Oxford Companion to Ships and the Sea. Page 454.
7. ^ For example, Part 23 of the Federal Aviation Regulations, amendment 23-7, September 14, 1969
8. ^ U.S. Federal Aviation Regulations, Part 1 Definitions and Abbreviations

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