Chip log

From Glossary of Meteorology

chip log

A line marked at intervals [commonly about 15 m (50 ft)], and paid out over the stern of a moving ship.

By timing the intervals at which the markers appear as the line is pulled out by a drag (the "chip"), the ship's speed can be determined. The wave length of ocean waves can be estimated by noting the position of wave crests relative to the markers.