A chip log, also called a common log or ship log, is a navigational instrument mariners used to estimate a vessel's speed through the water, made of a wooden board tied to a line, the log-line, knotted at regular intervals. A sailor threw the log over the stern, and because the log itself stayed roughly still in the water as the ship moved away, counting how many knots ran out through the sailor's hands in a fixed span of time gave the ship's speed. The first printed description of the method appears in William Bourne's 1574 book A Regiment for the Sea, and the practice is the direct source of the nautical term knot for a unit of speed, since sailors were literally counting knots on a rope to gauge how fast their ship was moving.
Facts
SignificanceA navigation tool mariners use to estimate the speed of a vessel through water. 1 FAQs
How did sailors time the line when using a chip log?
A half-minute sandglass that William Bourne devised.
They used a half-minute sandglass. William Bourne, whose 1574 book A Regiment for the Sea gave the first known printed description of the device, devised a half-minute sandglass for the timing. The sailors counted the knots that ran out in that span. At the time a mile was reckoned as 5,000 feet, so in 30 seconds at one mile per hour a ship would travel about 42 feet, which is how the length of line paid out could be read as a speed.
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In Field
Source Chip log (Wikipedia)
Sources
1. Chip log (Wikipedia)
Origins section
The first known description of the device in print is in A Regiment for the Sea by William Bourne, in 1574.
Introduction
A chip log is a navigation tool mariners use to estimate the speed of a vessel through water.
In Field: Marine Engineering, Categories
Marine navigation
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