Simrad EQ42 Manual page 64

EQ42/50/52 Installation and service
It is of profound importance for good performance of the EQXX that the water
flowing over the transducer be free of bubbles and aeration. If the transducer face
is clean but the performance degrades with increasing vessel speed, then aeration
of the water flowing under the transducer may be the cause of the poor
performance.
Due to the varying design of ship's hulls and different operating speeds, there can
be great variation in the amount of air bubbles which are carried beneath the hull.
These bubbles tend to be carried close to the hull as they pass aft. For this reason,
it is desirable for the transducer to be mounted on a fairing block which holds the
transducer away from the hull and which directs the flow of aerated water around
the sides of the transducer rather than over the face of the transducer.
On deep keeled vessels, care must be taken to ensure that the transducer beam
will not be blocked by any part of the keel. Although the appropriate mounting
location that meets all requirements depends on the type of vessel and its normal
operating speeds, a practical choice is usually somewhere between one third and
one half of the vessel's water line length from the bow. Leveling blocks may be
designed accordingly to meet this requirement.
The more the transducer protrudes from the hull, the better the results will be.
Particularly the lower frequency operation, interference from propeller noise can
be a significant problem. This can be seen as an increase in the "noise" on the
echosounder display when the propeller speed is increased. To help reduce this,
the transducer'' mounting face may be angled slightly forward on the order of 5°
for the 50 kHz transducers and 3° for the 200 kHz transducers. The goal is to
incline the transducer so that a line of sight along the transducer's radiating
surface passes below the propeller.
Keeping the propeller clean and free of any nicks or roughness will assist in
minimizing interference from propeller noise due to cavitation.
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Chapter 9