Log Line

The log line measured a ship's speed. A weighted wooden chip was thrown over the stern on a line knotted at regular intervals; a sailor counted the knots that ran out while a small sandglass of about 28 to 30 seconds emptied, and the count gave the speed in knots. The figure went into the ship's logbook, which took its name from the log.

The log line measured a ship's speed through the water. A flat wooden board shaped like a quarter circle, the log-ship or chip, was weighted with lead along its curved edge so it floated upright and stayed still in the water. It was tied to a long line wound on a reel, with knots tied in the line at regular spacing.

A sailor threw the chip over the stern; as the line ran out, another turned a small sandglass of about 28 to 30 seconds. When the sand ran out the line was stopped, and the number of knots that had passed gave the speed in knots, nautical miles per hour. The speed and course were chalked on a log board each hour and copied into the logbook, which took its name from the log.

With speed, course and time, the navigator could work out the ship's position by dead reckoning.

Parts & Labels

Note
Weighted wooden board.
Title
Log-ship or chip
Note
Knotted line on a reel.
Title
Log line
Note
Markers tied at fixed intervals.
Title
Knots
Note
Sandglass of about 28 to 30 seconds.
Title
Log glass

Historical Overview

Note
The English writer William Bourne described the log in A Regiment for the Sea in 1574.
Title
First description
Note
Richard Norwood measured the length of a degree in 1635 and published The Seaman's Practice (1637), improving the spacing of the knots.
Title
Better spacing

Timeline

1574Bourne describes the log

1637Norwood's The Seaman's Practice

1769Falconer defines the log and log line

Comparison Panel

Note
The log measured speed; the logbook recorded it with course, weather and events.
Title
Log and logbook
Note
Dead reckoning works forward from speed and course; a fix comes from landmarks or the sky.
Title
Dead reckoning and fixing

Interesting Facts

Note
Ships' speeds are still given in knots, and records are still called logs and logbooks.
Title
A word that stayed
Note
Currents, a stretched line or a careless count made the log only an estimate, so navigators checked it against the sun and stars.
Title
Errors
Note
The first few fathoms of line, the stray line, were let run before timing began so the chip was clear of the ship's wake.
Title
Stray line

Sources

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First printed description of the log.
Title
William Bourne, A Regiment for the Sea (London, 1574)
Note
Knot spacing and navigation.
Title
Richard Norwood, The Seaman's Practice (London, 1637)
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Period definitions of ship parts, watches and the log.
Title
William Falconer, An Universal Dictionary of the Marine (London, 1769)
Note
Log glasses and log-ships.
Title
Royal Museums Greenwich, Navigation collections