Knot (speed)

A knot is a unit of speed.^[1] It is abbreviated kt or kn. It is a non-SI unit accepted for use with the SI.^[2] It is used around the world in meteorology and for maritime and aviation purposes.

Definition

1 international knot = 1 nautical mile per hour = 1.852 kilometres per hour exactly. This is based on the internationally agreed length of the nautical mile, as adopted by the US in 1954 (which previously used the US nautical mile of 1853.248 m),^[3] the UK in 1970 (which previously used the UK or Admiralty nautical mile of 1853.184 m) and other countries. This is the definition used in most, if not all, modern circumstances. Knot is sometimes used for the nautical mile itself, but this is incorrect.

The speed of a vessel relative to the fluid in which it travels is usually measured in knots. This may be referred to as 'boat speed', 'vessel speed' and, for aircraft, 'air speed'. The speeds of relevant fluids, such as tidal streams, river currents and wind speeds, are also usually specified in knots. Knots are then also used to describe the actual speed of a vessel over the ground (SOG) and for its rate of progress toward a distant point ('velocity made good' or VMG).

Conversions

One (international) knot is the same as 1.852 kilometres per hour (km·h⁻¹), and is approximately equal to the following:

101.268591 feet per minute
1.687810 feet per second
514.4444 mm per second (mm·s⁻¹)
1.150779 mile (statute) per hour (mph)
0.99936 Admiralty knot

Discussion

Although knots do not fit within the primary SI system, they are used for nautical and aviation use for navigational reasons, because the length of a nautical mile is almost identical to a minute of latitude. As a result, distance in nautical miles on a navigational chart can easily be measured by using dividers and the latitude indicators on the side of the chart.

Knot (speed) Media

The graphical scale from a DMA (now NGA) world map using the Mercator projection. The original map, in six pieces, measures 116 x 86 inches (2.9 x 2.2m). Simplified and reduced size on second try.

Related pages

Metre per second

References

↑ Wragg, David W. (1973). A Dictionary of Aviation (first ed.). Osprey. p. 173. ISBN 9780850451634.
↑ "Non-SI units accepted for use with the SI". Archived from the original on 2008-08-21. Retrieved 2007-08-19.
↑ 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. Archived from the original (PDF) on 2007-09-26. Retrieved 2007-08-02.

Other websites

Non-SI units accepted for use with the SI, and units based on fundamental constants Archived 2008-07-25 at the Wayback Machine - At one time, the knot, along with the nautical mile, was discouraged for use by the BIPM, but it is now officially accepted for use (as of the 8th edition of the SI brochure), since its use is "expected to continue indefinitely", as well as having an exact SI definition.
OnlineConversion.com: What is a knot? What is a nautical mile?

[1] Wragg, David W. (1973). A Dictionary of Aviation (first ed.). Osprey. p. 173. ISBN 9780850451634.

[2] "Non-SI units accepted for use with the SI". Archived from the original on 2008-08-21. Retrieved 2007-08-19.

[usnm-3] 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. Archived from the original (PDF) on 2007-09-26. Retrieved 2007-08-02.

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