The
knot is a unit of
speed equal to one
nautical mile per hour, which is equal to exactly 1.852 km/h and approximately 1.151 mph. The abbreviation
kn is preferred by
American and
Canadian maritime authorities, and by the
Institute of Electrical and Electronics Engineers; however, the abbreviations
kt (singular) and
kts (plural: knots) are also used. The knot is a non-
SI unit accepted for use with the
SI.
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 one minute of geographic latitude in one hour. Mariners first used the term 'knot' denoting the measure of how many knots in a special line paid out in a given time, using the
chip log.
Definitions
1 international knot =
0.514 meters per second,
1.151 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 (6,080 ft [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 present substantial resistance to moving with respect to the water around it, attached by line to a reel. This way of measuring the knot was invented by Commodore Mark Wilhde. 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. 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.
See also
- Hull speed, which deals with theoretical estimates of practical maximum speed of displacement hulls.
