Kongsberg Simrad SN90, Installation Manual

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3 The transducer must never be lifted free of the water surface.
Transmitting into open air may damage the it beyond repair. Mounting the
transducer at a deep position on the hull will in most cases prevent this.
4 If the transducer is lifted up from the water during heavy seas, it may be damaged
when the hull strikes back at the sea surface.
Avoid protruding objects
Objects protruding from the hull will generate turbulence and flow noise. This will
reduce the SN90 performance.
Such objects may be zinc anodes, sonar transducers or even the vessel's keel. Holes
and pipe outlets are also important noise sources, as well as rough surfaces caused by
bad welding. All these protruding objects may act as resonant cavities amplifying the
flow noise at certain frequencies.
Do not place a transducer in the vicinity of such objects, and especially not close behind
them. For the same reason, it is very important that the hull area around the transducer
face is as smooth and level as possible.
Even traces of sealing compound, sharp edges, protruding bolts or bolt holes without
filling compound will create noise.
Stay far away from the propellers
The propulsion propellers is the dominant noise source on most vessels. The noise
is transmitted through the sea water, and may often reduce the performance of your
SN90 system.
For this reason, the transducer must be placed far away from the propellers, which means
on the fore part of the hull. Positions outside the direct line of sight from the propellers
are favourable.
On small vessels with short distances it is advised to mount the transducer on that side of
the keel where the propeller blades move upwards, because the propeller cavitation is
strongest on the other side. The cavitation starts most easily when the water flows in
the same direction as the propeller blade, and that is to some degree the case at that side
of the keel where the propeller blades move downwards.
Choose a position far away from the bow thruster(s)
Bow thruster propellers are extremely noisy.
When in operation, the noise and cavitation bubbles created by the thruster may make
your SN90 Purse Seine and Trawl Sonar useless, almost no matter where the transducer
is installed.
Simrad SN90