The taffrail log, also known as a patent log or screw log, was used to measure the distance or speed by which a ship travelled. This device was invented in 1688 by the English instrument maker Humphry Cole. The piece with the rotating fins was dragged behind the ship on a line, whilst the gauge read the rotations of the fins to ascertain speed.

The log is the marine equivalent of a speedometer. Initially, an actual log was attached to a line which had knots in it at regular intervals. The log was dropped into the water near the prow of the ship. The number of knots played out, correlated with a reading from a sandglass, gave the ship's speed, hence the term 'knots' for nautical speed.