A knot is a unit of speed. 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.
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. 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 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
1 (international) knot is exactly equal to 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
- 1.150779 mile (statute) per hour (mph)
- 0.99936 Admiralty knot
- 0.5144444 recurring metres per second
Origin
Until the mid-19th century vessel speed at sea was measured using a chip log. This device consisted of a wooden panel, weighted on one edge to float upright, 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%.
The chip log is also sometimes called the Knotted Rope. [citation needed]
Discussion
Although knots do not fit within the primary SI system, their 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
The following abbreviations are used to distinguish between various measurements of airspeed.
- KTAS is "knots true airspeed", a measure of an aircraft's true speed through the air
- KIAS is "knots indicated airspeed", meaning the airspeed shown on the airspeed indicator
- KCAS is "knots calibrated airspeed", or indicated airspeed corrected for position error
- KEAS is "knots equivalent airspeed", which is calibrated airspeed corrected for compressibility effects.
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.
- OnlineConversion.com: What is a knot? What is a nautical mile?
See also
- Hull speed, which deals with theoretical estimates of practical maximum speeds of displacement hulls.
- Metre per second
- Nautical mile
- Orders of magnitude (speed)
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 on 2007-08-02.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
