The word knot as a unit of speed has an interesting beginning. The first method of calculating the velocity of a ship was by Dutchman's log. A chip of wood thrown from the fore-castle was timed as it passed down the side of the ship; the calculation of speed was based on the length of time the chip took to travel between the forward and after marks, since distance divided by time equals speed.
But it was from a later type, the hand log, first used in the 16th century, that the word knot develops. A triangular piece of wood called a log-ship is weighted at the bottom, and slung by means of a three-legged rope crows-foot, one leg of which is secured to the log-ship with a removable wooden plug, in such a way as to present resistance to the water when towed astern of a ship on a log-line. This plaited line of about l50 fathoms is marked every ten fathoms. In the days of sail the hand log was streamed once an hour by the midshipman-of-the-watch and the boatswain's mate.
The latter rigged the log with a plug in securely enough to remain in against the water pressure to be expected, and streamed it astern, As the log-line slipped through his fingers, at the first knot that passed after the log was clear of the wake, the boatswain's mate called out “turn” and the midshipman inverted his hour-glass. When each subsequent knot passed the boatswain's mate sang out its number. As the last of the sand fell into the bottom half of the glass the midshipman gave the order “check”; the boatswain's mate stopped letting the line run out, noting the number of the knot nearest his hand. Comparing the number of the knot against the time on a chart gave the speed of the ship. By jerking the log-line the plug was removed and the log recovered.
Thus it was that knots in a line became associated with nautical miles per hour. Thus you will understand from this description that the land-lubber’s ‘knots per hour’ is meaningless.
