This device, named after the taffrail (the rail around the stern of a ship) measures the distance a vessel has travelled through the water.

The log worked by trailing a spinner (also called a "fish") in the water behind the ship. The ship's motion through the water caused the spinner to rotate, driving a mechanism that calculated how far the ship had travelled and indicated the results on a dial. Other versions of taffrail logs measured a ship's speed rather than the distance travelled.

Traffrail Log
CWM 19660074-009