Page 25 - Electronic Navigation Cyber Book

length of the vessel and the only satisfactory forward site may be within
the bulb. In oil tankers the after position is invariably chosen, usually
under the fore part of the engine room. Classification Society Lloyd
Register does not permit oscillators to be fitted underneath cargo space
on vessels classed for carrying petroleum in bulk.

A position in the forepeak may appear to be the best, but in bad weather
and lightship it would be unlikely to give good results and may also be
difficult to fit there. In a laden ship of normal design, a position about a
quarter of the length from the stem will often be found to give satisfactory
results. Ships often making long passages in ballast e.g. tankers, often
find an after position about three-quarters of the length from the stem
gives better results. If two are fitted, one is fitted at one quarter and one
at three-quarter length abaft the stem.

Care must be taken to make sure a receiving transducer is a sufficient
distance from the propeller, and tests should be carried out to ensure this.
They need to be sufficiently separated to prevent interaction between
them, but the separation should be as small as possible to ensure
accurate sounding in shallow water. Positions either side of the keel is
often satisfactory.

Other factors, which should be borne in mind, are: fit in a horizontal
position, sometimes slightly projecting but faired off to avoid aeration.
Avoid sites near bow thruster units, water intake pipes, and underwater
log units.

Internal access to the transducer should be possible for maintenance.
Any junction box should be in a dry space and if possible the transducer
should be in a dry place.
NOISE

All transmission systems are subjected to interfering signals of some
kind.

CROSS NOISE
It is caused by vibration of the energy, which is transmitted out by a ship
and goes directly to the echo sounder receiver. The recorder shows a
broad line on zero reading and this can mask echoes totally.

THERMAL NOISE
It is generated in electronic devices by random movement of electrons in
components and this is amplified in the receiver in any radio system. In a
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