Page 33 - Electronic Navigation Cyber Book

on deck was measured with a sandglass. However, the first reference to
a Dutchman's log is in 1623, after the ship log. The Dutchman's log could
be used with a brass tobacco box, rectangular with rounded ends. This
box had tables on it to convert log timing to speed.

The log has been used by mariners for a long time. The first occurrence
of a description of the device in print was in A Regiment for the
Sea by William Bourne in 1574.

Initially, the log-line was not knotted and the length was measured directly
on the line. With the introduction of the nautical mile as a standard unit of
measure at sea in the 15th century, the line began to be marked at equal
intervals proportional to the nautical mile and to the time interval used for
measurement. Initially, the markings were in the form of knots in the line.
Later, knotted cords were worked into the log-line.

Originally, the distance between marks was 7 fathoms or 42 feet used
with a sandglass with a 30 second running time. Later refinements in the
length of the nautical mile caused the distance between knots to be
changed. Eventually, the distance was set to 47feet, 3 inches
(14.4 meters) for a standard glass of 28 seconds.

Accuracy and considerations by the navigator[edit]

The use of a log did not give an exact measure of speed. A number of
considerations had to be taken into account, for example:

• the amount of following sea
• the effect of currents
• stretch of the line
• inaccuracies in the measurement of elapsed time. Sandglasses were
affected by ambient temperature, humidity, and sea state.

Frequent measurements helped in mitigating some of these inaccuracies
by averaging out individual errors, and experienced navigators could
determine their speed through the water with a fair degree of accuracy.

Because a log measures the speed through the water, some errors,
especially the effect of currents, that is the movement of the water itself,
can not be corrected for. Navigators rely on position fixes to correct for
these errors.

Modern navigation tools such as GPS report speed over ground and in
general will not give the same results as a log when currents are present.

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