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.
1 international knot =
1 nautical mile
metres per hour (exactly),
0.51444 metres per second (approximately),
1.15078 miles per hour (approximately),
20.25372 inches per second (approximately)
1.68781 feet per second (approximately).
The length of the internationally agreed nautical mile is 1852
m. The US adopted the international definition in 1954, having previously used the US nautical mile (1853
The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty
nautical mile (6080
ft or 1853
Conversions between common units of speed
(* = approximate values)
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.
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. Knots
tied at a distance of 47 feet
(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.
The knot count would be reported and used in the sailing master
's dead reckoning
. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.
Derivation of knots spacing:
, so in
seconds that is
metres per knot.
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 the length of a nautical mile
, upon which the knot is based, is closely related to the longitude
/latitude geographic coordinate system
. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.
Standard nautical charts are on the Mercator projection
and the horizontal (East–West) 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.
Speed is sometimes incorrectly expressed as "knots per hour",
which would mean "nautical miles per hour per hour" and thus would refer to acceleration
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.
The following abbreviations are used to distinguish between various measurements of airspeed
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.
- ^ Bartlett, Tim (July 2008) . RYA Navigation Handbook. Southampton: Royal Yachting Association.
- ^ "ISO 80000-3:2006". International Organization for Standardization. Retrieved 20 July 2013.
- ^ International Standards and Recommended Practices, Annex 5 to the Convention on International Civil Aviation, “Units of measurement to be Used in Air and Ground Operations”, ICAO, 4th Edition, July 1979.
- ^ a b "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI brochure (8th ed.). International Bureau of Weights and Measures. The knot is defined as one nautical mile per hour. There is no internationally agreed symbol, but the symbol kn is commonly used.
- ^ Louis E. Barbow and Lewie 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 26 September 2007. Retrieved 2 August 2007.
- ^ Jacob Abbott (1858). Rollo on the Atlantic. DeWolfe, Fiske, & Co., Publishers. Retrieved 30 November 2011.
- ^ Kemp, Peter, ed. (1976). The Oxford Companion to Ships and the Sea. Oxford University Press. p. 454. ISBN 0-19-282084-2.
- ^ E.g. BA Chart 73, Puerto de Huelva and Approaches, 2002
- ^ Wilson, Alastair (22 July 2009). "'Knots an hour'". The Kipling Society. Retrieved 1 December 2012. Since the 1890s or thereabouts, it has been drummed into the young seaman that a knot is a unit of speed, namely, one nautical mile per hour; and that consequently only the uneducated speak of "knots per hour" or "knots an hour". It was therefore inevitable that Kipling’s frequent use of this expression should grieve a number of seafaring readers, as the pages of the Kipling Journal testify.
- ^ For example, Part 23 of the Federal Aviation Regulations, amendment 23–7, 14 September 1969
- ^ "Abbreviations and symbols". edocket.access.gpo.gov.
Last edited on 30 April 2021, at 22:23
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