Knot (speed)

A knot is a unit of speed, abbreviated kt or kn. It is a non-SI unit accepted for use with the SI.Non-SI units accepted for use with the SI It is used around the world for maritime and aviation purposes.

Definition

1 knot = 1 nautical mile/hour = 1.852 km·h⁻¹ exactly. This is based on the internationally agreed length of the nautical mile, as adopted by the US in 1954, the UK in 1970 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, for aircraft, 'air speed' and is usually measured in knots. If the medium is flowing, this 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.

Conversions

1 knot is roughly equivalent to:

101.268591 foot/minute
1.687810 foot/second
0.5144444 metres per second (m·s⁻¹)
1.852 kilometres per hour (km·h⁻¹) (exact)
1.150779 mile (statute)/hour (mph)

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. 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 %.

Discussion

Because a knot is defined as a nautical mile/hour, the expression "knots per hour" is considered incorrect as a unit of speed, since this suggests 'nautical mile/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.

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 normally-shaped boat hulls the following table relates waterline lengths to hull speeds and so gives some examples of usage of the unit knots.

Hull speeds
Waterline length (ft)	Waterline length (m)	Hull speed (knots)	Comment
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

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.

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.

References

Navigation | Units of velocity

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