Knot (unit)

1 international knot =

1 nautical mile per hour (by definition),

1852.000 metres per hour (exactly),[4]

0.51444 metres per second (approximately),

1.15078 miles per hour (approximately),

20.25372 inches per second (approximately)

1.68781 feet per second (approximately).

The length of the internationally agreed nautical mile is 1852 m. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m).[5] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft or 1853.184 m).

(* = approximate values)

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.

Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out.[6] Knots tied at a distance of 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.[7] 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%.

Derivation of knots spacing:

${\displaystyle 1~{\textrm {kn}}=1852~{\textrm {m/h}}=0.5144~{\textrm {m/s}}}$, so in ${\displaystyle 28}$ seconds that is ${\displaystyle 14.40}$ metres per knot.

Graphic scale from a Mercator projection world map, showing the change with latitude

Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.

Standard nautical charts are on the Mercator projection and the horizontal (East-West) scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier.[8]

Speed is sometimes incorrectly expressed as "knots per hour",[9] which would mean "nautical miles per hour per hour" and thus would refer to acceleration.

• Beaufort scale
• Hull speed, which deals with theoretical estimates of practical maximum speed of displacement hulls
• Knot count
• Knotted cord
• Metre per second
• Orders of magnitude (speed)
• Rope (unit)