- Author
- A.N. Other
- Subjects
- History - general
- Tags
- RAN Ships
- None noted.
- Publication
- September 1989 edition of the Naval Historical Review (all rights reserved)
The log is a device for measuring a ship’s speed through the water. An early log, said to have been invented about 1620, consisted of a float or chip designed to assist towing, but so fitted that it could be tripped for recovery, a sand glass and a line mark, with a knot at intervals. The float could be thrown overboard and allowed to fall astern until the first mark on the line was reached, at which time the sand glass was inverted. The number of knot marks to pass over the stern were counted until the sand ran out.
Based on a nautical mile, the knots in the line were usually 47-5 feet apart and the time interval 28 seconds, hence the number of knots that passed over the stern represented the speed of the ship in nautical miles per hour. The length of the line between the knots bore the same proportion to the nautical mile as the time interval to the second in one hour.
The word ‘knot’ used to denote the ship’s speed in nautical miles per hour originated from the use of this early log.