Simrad ES70 - INSTALLATION REV A Installation Manual page 116

Simrad ES70
1 The upper water layers of the sea contain a myriad of small air bubbles created by the
breaking waves. In heavy seas the upper 5 to 10 metres may be filled with air, and
the highest concentrations will be near the surface. Air bubbles absorb and reflect the
sound energy, and they may in worst cases block the sound transmission altogether.
2 Another reason to go deep is the cavitation in front of high power transducers.
Cavitation is the formation of small bubbles in the water due to the resulting local
pressure becoming negative during parts of the acoustic pressure cycles. The
cavitation threshold increases with the hydrostatic pressure.
The transducer must never be lifted free of the water surface. Transmitting into
3
open air may damage the transducer beyond repair. Mounting the transducer at a
deep position on the hull prevents 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. This is especially important for low
frequency transducers with large faces.
Mount the transducer midway between the bow and the stern to avoid heave effects
Heave is the up and down movement of the vessel. It disturbs the echo traces in the
echogram, so that a flat bottom is displayed as a wave. A transducer location in the
middle of the vessel minimises the influence of vessel roll and pitch.
Mount the transducer away from protruding objects on the hull
Objects protruding from the hull, such as zinc anodes, sonar transducers or even the
vessel's keel, generate turbulence and flow noise. Holes and pipe outlets are also
important noise sources. They may act as resonant cavities amplifying the flow noise
at certain frequencies. Do not place an echo sounder 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.
Mount the transducer at the forward part of the hull to minimise the effects from
the boundary water layer
When the vessel forces its way through the sea, the friction between the hull and the
water creates a boundary layer. The thickness of the boundary layer depends upon vessel
speed and the roughness of the hull. Objects protruding from the hull, and dents in the
hull, disturb the flow and increase the thickness of the boundary layer. The flow in this
boundary layer may be laminar or turbulent. A laminar flow is a nicely ordered, parallel
movement of the water. A turbulent flow has a disorderly pattern, full of eddies. The
boundary layer increases in thickness when the flow goes from laminar to turbulent. The
figure below illustrates the boundary layer of a vessel moving through the water.
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