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In order to navigate at sea, it is absolutely essential to know your speed. With no landmarks for reference, you must know have far you have gone in order to know where you are on a map. In order to know how far you have gone, you need to know how fast you've traveled and for how long.

In Lost Without Longitude, we present one of the most famous examples for measuring one's speed: the chip log, which sailors used to count their "knots."

Chip Log


Description: Chip logs were used to measure a ship's speed through the water. A plate or other object tied to the end of the rope (the "chip") was thrown into the water, and the rope spooled out as the ship moved along. After a certain amount of time (typically measured by sand glass), the rope was hauled out of the water and the number of knots that came out was counted. This gave the ship's speed in "knots", now known as nautical miles.

Chip logs were an important navigational tool. In the days before reliable timekeeping, these were the best method for determining longitude. By combining the speed in knots and the amount of time the ship had sailed, the navigator could determine east/west distance with varying degrees of accuracy.

Related Instruments: Dead reckoning

Usage Dates: Mid-1500s to late 1800s.


Chip Log

Chip Log

A large chip log on display at the Harvard Museum of Historical Scientific Instruments.

Chip Log Plate

Chip Log Plate

View of the same chip log from the opposite side, showing the wooden plate that would drag through the water.

Chip Log Knot

Chip Log Knot

Close-up on the same chip log, showing one of the knots.



  • The Chip Log from The Mariner's Museum.

  • Wikipedia's page on Chip Logs.

  • Chip Log at Michael Hammond's history of science page.

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