Wiktionary
chip log
n. (context nautical English) A simple device for measuring the speed of a vessel, consisting of a wooden board (drag) attached to a line on a reel.
Wikipedia
Chip log
A chip log, also called common log, ship log, or just log, is a navigation tool mariners use to estimate the speed of a vessel through water. The word knot, to mean nautical mile per hour, derives from this measurement method.