Knot (speed)
From Wikipedia, the free encyclopedia
A knot is a unit of speed, abbreviated kt or kn. It is a non-SI unit accepted for use with the SI.[1] It is used around the world for maritime and aviation purposes.
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[edit] Definition
1 international knot = 1.000 nautical mile per hour = 1.852 kilometre per hour exactly. This is based on the internationally agreed length of the nautical mile, as adopted by the US in 1954 (which previously used the US nautical mile of 1852.249 m), the UK in 1970 (which previously used the UK or Admiralty nautical mile of 1853.184 m) and other countries. This is the definition used in most, if not all, modern circumstances. Knot is sometimes mistakenly used to refer to the nautical mile itself, but this is incorrect.
The speed of a vessel relative to the fluid itself, and not land, is referred to as 'boat speed' or 'vessel speed' or, for aircraft, 'air speed' and is usually measured in knots. The combination of direction of motion and speed is called velocity. If the medium is flowing, boat velocity differs from the ground velocity by the velocity of the medium. Velocity made good refers to ground velocity and may also be expressed in knots in navigation.
The velocity of an object relative to the ground is equal to the sum of its velocity in the medium and the velocity of the medium relative to the ground.
vog = vmg + vom, where the velocities are vectors.
[edit] 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
- 0.5144444 meters per second (m·s−1)
- 1.150779 mile (statute) per hour (mph)
- 0.99936 Admiralty knot
Bear in mind that there is no relationship between speed (knots) on water and speed (mph or km/h) on land as there are several factors to be considered. Speed/distance on land is always fixed as land is immovable, whereas water has current flowing with or against the vehicle. Thus, the speed relationship between the vehicle and the surface it travels on alters constantly.
[edit] Origin
In some sailing ships, speed was measured by casting the chip log from the stern. The log was relatively immobile, and attached by line to a reel. Some sources suggest that knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another used a 28 second sandglass 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 accepted value today is a bit less than 0.02%.
[edit] Discussion
Because a knot is defined as a nautical mile per hour, the expression "knots per hour" is considered incorrect as a unit of speed, since this suggests 'nautical mile per hour²', which would be a measure of acceleration.
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.
[edit] Nautical examples
Displacement hulls (for example those not planing on the surface of the water) have a maximum speed beyond which they tend to 'dig in', with their bows high and sterns low, and become increasingly wasteful of propulsive power. This is known as their hull speed and it depends mainly upon waterline length. For various displacement boat hulls the following table relates waterline lengths to hull speeds and so gives some examples of usage of the unit knots.
| Waterline length (ft) | Waterline length (m) | Hull speed (knots) | Type of Boat |
|---|---|---|---|
| 10 | 3.0 | 4.4 | Dinghy |
| 18 | 5.5 | 5.9 | Small fishing or pleasure boat |
| 28 | 8.5 | 7.3 | Small yacht |
| 36 | 11.0 | 8.2 | Family yacht |
| 50 | 15 | 9.8 | Small commercial fishing boat or ferry |
| 200 | 61 | 20 | Small commercial ship |
| 400 | 122 | 28 | Typical cruise or container ship |
[edit] Aeronautical terms
In some circumstances relatively small discrepancies in measured and actual airspeeds can be significant.
- KTAS is "knots true airspeed", a measure of an aircraft's true airspeed 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.
[edit] 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?
