Log Line

A device used historically by mariners to measure a vessel’s speed through the water.

The log line consists of a wooden board, known as the log, attached to a long, knotted rope. The log was thrown overboard and allowed to float behind the ship while the rope was paid out. Mariners would measure the vessel’s speed by counting the number of knots that passed overboard in a set amount of time, typically measured using a sandglass. The knots were spaced at regular intervals along the rope, and the number of knots that passed through the sailor’s hands during the timed interval would give the ship’s speed in nautical miles per hour, or “knots.”

The log line was one of the earliest tools used for navigation at sea and was crucial for helping sailors estimate their position and progress during long voyages. Before the advent of more modern instruments, such as the chronometer and sextant, the log line was an essential part of a ship’s navigational toolkit. The term “knots” as a measurement of speed in nautical miles per hour originates directly from this practice.

The log line was typically deployed from the stern of the ship, and the wooden log was designed to float and create enough drag to pull the rope taut. This drag ensured that the line would pay out smoothly and consistently, allowing for an accurate measurement of speed. The sandglass used to time the log line was usually calibrated to a specific duration—often 28 seconds—so that the number of knots counted during that time could be directly translated into the ship’s speed in knots.

While the log line is no longer in common use today, having been replaced by more advanced technologies like GPS and electronic speed logs, it remains an important part of maritime history and terminology. The phrase “logbook” also has its origins in this practice, as sailors would record the results of their log line measurements in a book, along with other navigational data, to track the ship’s progress over time.