Page 51 - Electronic Navigation Cyber Book

SUMMARY

• To be accurate, speed must be calculated with reference to a known
datum.

• At sea, speed is measured with reference to the ocean floor (ground-
tracking (G/T)) or water flowing past the hull (water-tracking (W/T)).
• Traditionally, maritime speed logging devices use water pressure,
electromagnetic induction, or the transmission of low frequency radio
waves as mediums for indicating velocity.

• A water pressure speed log, occasionally called a Pitot log:

(a) measures W/T speed only;

(b) requires a complex arrangement of pressure tubes and chambers
mounted in the engine room of a ship and a Pitot tube protruding through
the hull;
(c) produces a non-linear indication of speed which must be converted to
a linear indication to be of any value. This is achieved either mechanically
or electrically in the system;

(d) speed indication is affected by the non-linear characteristics of the
vessel's hull and by the vessel pitching and rolling;

(e) possesses mechanical sections that require regular maintenance.
• An electromagnetic speed log:

(a) measures W/T speed only;

(b) produces a linear speed indication;

(c) operates by inducing a magnetic field in the salt water flowing past the
hull and detecting a minute change in the field;

(d) produces a varying speed indication as the conductivity of the
seawater changes.

(e) Indication may be affected by the vessel pitching and rolling in heavy
weather.

• Speed logs that use a frequency or phase shift between a transmitted
and the received radio wave generally use a frequency in the range 100-
500 kHz. They also use a pulsed transmission format.
• A log using the acoustic correlation technique for speed calculation:
© 2018 Digital Galaxy Index 51