Knot (speed)

From Free net encyclopedia

A knot is a unit of speed, abbreviated kt or kn. It is not an SI unit. It is accepted (although discouraged) for use with the SI. It is used around the world for maritime and aviation purposes.

1 Definition
2 Conversions
3 Origin
4 Discussion
5 Terms
6 External links

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Definition

1 knot = 1 nautical mile/hour = 1.852 km/h. This is the definition used in most, if not all, modern circumstances. Knot is sometimes used to mean nautical mile, though this is incorrect.

By convention, knots measures the speed of the vessel relative to the fluid itself and not land. If the medium is flowing at all, then the nautical velocity differs from the ground velocity by the velocity of the medium.

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Conversions

1 knot is equivalent to:

101.2686 foot/minute
1.687810 foot/second
0.5144444 meter/second
1.852 kilometer/hour (exact)
30.86667 meter/minute
1.150779 mile (statute)/hour

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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 1851.66 m/h. The difference from the accepted value today is a bit less than 0.02%.

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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 SI system their retention for nautical and aviation use is important for navigational reasons because 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.

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Terms

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.