A Knot (K) is 1 Nautical Mile per Hour.
A Nautical Mile (NM), has various definitions. It is defined
as 1 Minute of latitude at the equator.
The circumference at the equator is approximated as = 360 *
60 = 21600 NM
It had been defined as: 6080’ (UK) or 6080.2’ (US).
A statute mile is about 7/8 of a nautical mile. This is more accurate than the usual 5/6 approximation.
The word “knot” likely comes from the method used to measure speed on a ship.
This was done by casting a log (literally any large piece of wood) attached to a rope into the water. When the log passed the ship’s stern, a sandglass (an hourglass which measured a shorter interval) was started.
The log was heavy enough to resist moving through the water, while the rope was released or “paid-out” from the ship. Knots or rags in the rope were counted as they passed the ship’s stern (also called the “taffrail”), until the sand ran out.
For example, a ship moving at 5 NM/h would pay out rope including 5 knots from the ship during the time interval. The count of markers was noted in the ship’s diary, which is still called the log.
Before the advent of the ship's chronometer, it was not possible to find a ship's longitude. Thus constantly recording or logging the ship's speed along with the time spent on a course was essential for estimating the ship's position. This information was kept in ship's log book.
This method of navigation is called "dead reckoning", which is short for “Deduced Reckoning”.
Conservative navigators still check more sophisticated methods and instruments, using this method.
The correct usage is "The ship is going 3 knots". It is incorrect to say, "The ship is going 3 knots per hour". (This would be a measure of an acceleration, which is not commonly used).
1 Knot = 47.25 ft * 3600 sec / 28 sec = 6075 ft/hr. This is an old but excellent approximation of the current (agreed 7/1/59) international standard:
6076.11549 ft. or 1852 meters.
The most recent practice used intervals of 47 ft. 3 in and 28 seconds. Thus a ship moving at 5 NM/h would pay out 5 knots from the ship in 28 seconds
Since setting markers in the log line at any regular interval
is not difficult it is surprising to me that the time interval is not something
more common like 30 seconds or 1 minute. The then the interval would be 50’
7.56” or 101’ 3.12”. Perhaps the fractional inches where considered to be a
problem, compared to waiting 28 seconds with a modern time piece.
Converting 1852/2.54 * 100/12m to 6076.115486 ft shows a small error (4.0E-10)
The American Practical Navigator – Bowditch (H.O. Pub. No. 9) 1962 U.S. Navy Hydrographic Office (best $6 I ever spent on a book)
http://en.wikipedia.org/wiki/Knot_(speed)
http://en.wikipedia.org/wiki/Nautical_mile
https://en.wikipedia.org/wiki/Hourglass
Written 8/6/96 A. Dennison
Updated 9/14/2022