Knot (unit)

The knot (/nɒt/) is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately 1.151 mph.^[1] The ISO Standard symbol for the knot is kn.^[2] The same symbol is preferred by the IEEE; kt is also common. The knot is a non-SI unit that is "accepted for use with the SI".^[3] 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 approximately one minute of geographic latitude in one hour.

Etymologically, the term 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),^[3]

0.514 metres per second (approximately),

1.151 miles per hour (approximately),

20.254 inches per second (approximately).

7003185200000000000♠1852 m is the length of the internationally agreed nautical mile. The US adopted the international definition in 1954, having previously used the US nautical mile (7003185324800000000♠1853.248 m).^[4] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (7003185318400000000♠6080 ft [7003185318400000000♠1853.184 m]).

(Values in bold face are exact.)

Usage

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, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out.^[5] Knots placed at a distance of 8 fathoms - 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently 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 6999514350000000000♠1851.66 m/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 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, for practical purposes, is equivalent to about 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. Recent British Admiralty charts have a latitude scale down the middle to make this even easier.^[7]

Speed is sometimes incorrectly expressed as "knots per hour",^[8] which is in fact a measure of acceleration.

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 were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.^[9]

The following abbreviations are used to distinguish between various measurements of airspeed:^[10]

• 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

The indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At 11000 m (36000 ft), an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.

See also

• Beaufort scale
• Hull speed, which deals with theoretical estimates of practical maximum speed of displacement hulls
• Knot count
• Knotted cord
• Metre per second
• Orders of magnitude (speed)
• Rope (unit)

References

Further reading

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