| Knot (speed) |
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DEFINITION 1 knot = 1 Nautical Mile /hour = 1.852 km/h 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 equivalent to:
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 Knot s placed at a distance of 47 Feet 3 Inch es (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%. 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. Nautical examples Displacement Hull s (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. AERONAUTICAL TERMS In some circumstances relatively small discrepancies in measured and actual airspeeds can be significant.
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