The first known device to measure speed is
often claimed to be the Dutchman's Log. An object that would float was
thrown overboard and the time required to pass between two points 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.
Although the unit knot does
not fit within the primary SI system, its retention for nautical
and aviation use is important for navigational reasons, since
the length of a nautical mile is almost identical to a minute of
latitude. As a result, distance in nautical miles on a
navigational chart can easily be measured by using dividers and
the latitude indicators on the side of the chart.
Nautical speed is sometimes erroneously expressed as "knots per
hour" which would actually be a measure of acceleration, as in
"nautical miles per hour per hour".
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More Data and
Information on Speed Logs 
