Page 31 - Electronic Navigation Cyber Book

CHAPTER 3 SPEED LOG

3.1 Introduction

Speed measurement has always been of the utmost importance to the
navigator. The accuracy of a dead reckoning position plotted after a long
passage without star sights being taken, is dependent upon a sound
knowledge of the vessel's heading and speed.

To be of value, the speed of any object must be measured relative to
some other point. At sea, speed may be measured relative to either the
seabed (ground reference speed) or to the water flowing past the hull
(water reference speed). Both of these types of speed measurement are
possible and both have their place in modern navigation systems.

This chapter deals with the methods of speed logging that are in general
use on board modern vessels. One of these, the pressure tube log, is old
but it still gives a satisfactory performance. Another, the electromagnetic
log, is often used on smaller vessels and the popular Doppler speed log
is to be found everywhere.

History

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