Knot (unit)

Knot is a unit of speed, equal to one nautical mile per hour.

The abbreviation preferred by maritime authorities in the USA^[1], and Canada ^[2], as well as the Institute of Electrical and Electronics Engineers and the International Bureau of Weights and Measures is kn.^[3] ^[4] The abbreviation kt is also in widespread use, sometimes with kts for the plural (knots). It is a non-SI unit accepted for use with the SI.^[3] It is used around the world in meteorology and for maritime and aviation purposes. A vessel traveling at 1 knot along a meridian, covers one sixtieth of a degree geographic latitude in one hour. The term knot was first used by mariners who measured how many knots on the line payed out in a given time period when the chip log was deployed.

Definition

1 international knot = 1 nautical mile per hour = 1.852 kilometres per hour exactly = 1.1507794 miles per hour.

This is based on the internationally agreed length of the nautical mile. The international definition was adopted by the US in 1954 (which previously used the US nautical mile of 1853.248 m).^[5] The international definition was adopted by the UK in 1970 which previously used the UK or Admiralty nautical mile of 1853.184 m. This is the definition used in most, if not all, modern circumstances. A common error that should be avoided is to use the term 'knot' to refer to the nautical mile itself.

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'. For consistency in navigation, the speed of relevant fluids, such as tidal stream, river current and wind speed, is 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).

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 have substantial water resistance, 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 sandglass (28 second sandglass is the current accepted timing) to time the operation.^{[citation needed]} 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⁻¹. The difference from the modern definition is less than 0.02%.

Discussion

Although the unit "knots" does not fit within the primary SI system, its retention for nautical and aviation use is important for navigational reasons, since 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.

Aeronautical terms

Prior to 1969, airworthiness standards for civil aircraft in the USA Federal Aviation Regulations specified that distances were to be in statute miles, and speeds in miles per hour. In 1969 these standards^[6] were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.

The following abbreviations^[7] 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.

External links

Non-SI units accepted for use with the SI, and units based on fundamental constants - 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.

References

^ Positions, Distances, Directions, Compass; Office of Coastal Survey, NOAA, USA
^ POSITIONS, DISTANCES, DIRECTIONS, COMPASS, Ministry of Fisheries and Oceans, Canada
^ ^a ^b See BIPM website on accepted non-SI units, in particular note f.
^ IEEE guidelines for authors
^ 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. Retrieved 2007-08-02.
^ For example, Part 23 of the Federal Aviation Regulations, amendment 23-7, September 14, 1969
^ USA Federal Aviation Regulations, Part 1, §1.1 and §1.2

[1] Positions, Distances, Directions, Compass; Office of Coastal Survey, NOAA, USA

[2] POSITIONS, DISTANCES, DIRECTIONS, COMPASS, Ministry of Fisheries and Oceans, Canada

[nonSi-3] See BIPM website on accepted non-SI units, in particular note f.

[4] IEEE guidelines for authors

[usnm-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. Retrieved 2007-08-02.

[6] For example, Part 23 of the Federal Aviation Regulations, amendment 23-7, September 14, 1969

[7] USA Federal Aviation Regulations, Part 1, §1.1 and §1.2

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