screw log
Noun: A screw log is a historical nautical instrument used to measure a ship's speed through water. It is a cigar-shaped (cylindrical) log with rotary fins (a type of propeller or impeller) that is towed behind a ship. As the ship moves, the water causes the fins to rotate. The rotations are mechanically transmitted to a counter on the ship's deck, indicating the distance traveled, which can then be used to calculate speed.
The term screw log is a specific, technical term for a type of ship's log (a speed-measuring device). It is primarily used in historical, maritime, or engineering contexts when discussing the evolution of nautical navigation technology.
- The captain ordered the deployment of the screw log to get an accurate reading of their speed in the calm sea.
- Before the invention of modern electronic logs, many ships relied on a screw log for navigation.
- The maritime museum has an excellent exhibit on 19th-century navigation, featuring a fully restored screw log.
- Patent Log: The screw log is a type of patent log, a class of mechanical logs invented in the 19th century to improve upon the older chip log (or common log).
- Taffrail Log: A screw log is often mounted on the taffrail (the rail at the stern of a ship), leading to the common alternative name "taffrail log."
- Patent Log: A general term for mechanical towed logs, including the screw log.
- Taffrail Log: Often synonymous with screw log, named for its typical mounting position.
- Chip Log / Common Log: An older, simpler device using a rope with knots, which the screw log was designed to replace.
- Impeller Log: A modern electronic version that uses a small impeller (a type of screw or rotor) mounted on the hull.
- Ship's Log: Can refer to the speed-measuring device or, more commonly today, the official record of a ship's voyage.
- Taffrail log (specific context)
- Patent log (broader category)
- Mechanical log (broader category)
- Towed log (descriptive)
- To stream the log: The action of deploying a towed log, like a screw log, into the water behind the ship.
- To heave the log: The action of using a chip log; this phrase is not typically used for a screw log.
The screw log is an obsolete technology. The term is not used in modern maritime practice where electromagnetic or acoustic Doppler speed logs are standard. Its name derives directly from its screw-like (helical) fins that rotate in the water.
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a cigar-shaped log with rotary fins that measure the ship's speed
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