Page 34 - Electronic Navigation Cyber Book

Mechanical speed logs called patent logs or taffrail logs, operating on
physical principles in a manner similar to a car's odometer by towing a
vane or rotor from the stern (or taffrail) by a long line were developed in
the eighteenth century (or earlier) but became practical in the nineteenth
[6]
century and replaced the traditional chip log. Since the second half of
the twentieth century, more modern mechanical and electro-mechanical
versions based upon a small impeller or paddle wheel attached to (or
through) the bottom of the hull continue to be used especially on smaller
yachts.

In recent years ultrasonic speed sensors have become available. These
new type use two ultrasonic transducers one forward, one aft that send
ultrasonic pulses through the water flow past the hull. By calculating the
time differential in pulse propagation from one sensor to the other, the
speed of the hull through the water can be calculated directly.

Today, the most accurate means of maritime speed measurement comes
from Doppler measurement, either derived acoustically by use
of Doppler Sonar or radio interferometrically by Doppler measurement of
satellite signals such as those from Global Positioning System (GPS).
Most commercial GPS systems are not configured to operate in this
mode, however.

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