taffrail log

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. See patent log.

Wiktionary

1. n. nautical A mechanical device dragged from the stern of the vessel that indicates the vessel's speed through the water; usually consists of a propeller, a reading unit and a stiff line connecting them.

WordNet 3.0

1. n. a cigar-shaped log with rotary fins that measure the ship's speed