Page 32 - Electronic Navigation Cyber Book

All nautical instruments designed to measure the speed of a ship through
water are known as logs. This nomenclature dates back to days of sail
when sailors tossed a log attached to rope knotted at regular intervals off
the stern of a ship. The sailors would count the number of knots that
passed through their hands in a given period of time. Today sailors still
use the unit of knots to express a ship's speed. The speed of the ship
was needed to navigate the ship using dead reckoning, which was
standard practice in the days before modern navigation instruments
like GPS.

A Speed log consists of a wooden board
attached to a line (the log-line). The log-
line has a number of knots tied in it at
uniform spacings. The log-line is wound
on a reel to allow it to be paid out easily in
use.
Over time, the log was standardized in
construction. The shape is a
quarter circle, or quadrant, and the log-
line is attached to the board with a bridle
of three lines connected to the vertex and
to the two ends of the quadrant's arc. In
order to ensure that the log submerges
and is oriented correctly, the bottom of the log is weighted with lead. This
provides for more resistance in the water and a more accurate and
repeatable reading of speed. The bridle is attached in such a way that a
strong tug on the log-line results in one or two of the bridle's lines
releasing, allowing the log to be retrieved with relative ease.
When the navigator wished to determine the speed of his vessel, a sailor
dropped the log over the stern of the ship. The log would act as
a drogue and remain roughly in place while the vessel moved away. The
log-line was allowed to run out for a fixed period of time. The speed of
the ship was indicated by the length of log-line passing over the stern
during that time.

The first known device to measure speed is often claimed to be the
Dutchman's Log. However this invention is attributed to
thePortuguese Bartolomeu Crescêncio, who designed it in the end of
15th century or in the beginning of 16th century. An object that would float
was thrown overboard and the time required to pass between two points

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