Simrad EQ60 - Manual (Page 185)

Transducer installation
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857-164342 / Rev.C
TRANSDUCER INSTALLATION
Transducer location
General
A single answer to the question where to locate the transducer
cannot be given. It depends very much on the vessel’s
construction. However, there are some important guide lines.
Go deep
The upper water layers of the sea contain a myriad of small air
bubbles created by the breaking waves. In heavy seas the
uppermost 5 to 10 metres may be air-filled, with the highest
concentrations near the surface. Air bubbles absorb and reflect
the sound energy, and may in worst cases block the sound
transmission totally. Therefore, mount the transducer at a deep
position on the hull.
Consider the situation when the vessel is unloaded, and when it
is pitching in heavy seas. The transducer must never be lifted
free of the water surface. Not only will the sound transmission
be blocked, but the transducer may be damaged by slamming
against the sea surface.
Another reason to go deep is 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.
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. Figure 16 (next
page) sketches the boundary layer of a vessel moving through
the water.
Furthermore, air bubbles in the sea water are pressed down
below the hull and mixed into the boundary layer. The boundary