The knot is a unit of speed equal to one nautical mile per hour. Its kn abbreviation is preferred by American and Canadian maritime authorities, the Institute of Electrical and Electronics Engineers, and the International Bureau of Weights and Measures, however, the kt (knot) and kts (knots) abbreviations also are used. The knot is a non-SI unit accepted for use with the SI. World-wide, the knot is used in meteorology, and in maritime and aviation navigation — for example, a vessel travelling at 1 knot along a meridian, travels one-sixtieth of a degree of geographic latitude in one hour, however, this is only true if the water is static. If a vessel is heading (say) west at three knots (measured on GPS) and a tidal stream is heading east at three knots, the indicated speed from a ships log would be six knots, when the ground speed would be three. Therefore the speed measured is relative to the medium that is being passed through, ie, water or air. Mariners first used the term knot denoting the measure of how many knots of line paid out in a given time using the chip log.

Definitions

1 international knot =

1 nautical mile per hour (exactly),

1.852 kilometres per hour (exactly),

0.514 meters per second,

1.15077945 miles per hour (approximately).

1.852 km 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). The U.K. adopted the international nautical mile definition in 1970, having previously used the U.K. Admiralty nautical mile (6,080ft [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.

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. 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%.

Modern use

Although the unit knot 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.

Nautical speed is sometimes erroneously 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 USA 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.

The following abbreviations 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

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

References

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

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.