Kongsberg EM 710, Installation Manual

Page: 33 / 155

Transducer arrays
4.3 Locating the transducer array
Correct location of the system’s transducer is vital for the
operational performance.
A single answer to the question of where to locate the transducer
arrays cannot be given. It depends very much on the vessel’s
construction. However, there are some important guidelines
which are generally applicable.
4.3.1 The boundary water layer
The upper water layers of the sea contain a myriad of small air
bubbles created by breaking waves. In heavy seas the uppermost
5-10 metres may be air-filled, with the highest concentrations
near the surface. Air bubbles absorb and reflect sound waves, and
may in the worst conditions block sound transmission totally.
When a vessel moves through the sea, the friction between the
hull and the water creates a boundary layer. The thickness of the
boundary layer depends upon the vessel speed and the roughness
of its hull. Any objects protruding from the hull, and any dents
in the hull, will 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.
Furthermore, air bubbles in the sea water are pushed down below
the hull and mixed into the boundary layer. The boundary layer
is thin underneath the forward part of the vessel, and increases in
thickness as it moves aftwards. If the sides of the hull are steep,
some of the air bubbles in the boundary layer may escape to the
sea surface along the vessel sides. It is our experience that a wide
and flat bottom, with a rising angle less than about 13 degrees
athwarthship, is prone to cause air problems for a transducer.
Sketch of boundary layer underneath the vessel on page 32 shows
in principle the boundary layer of a vessel moving through the
water.
The conclusion is that the transducer array should be mounted as
deep as possible, and in the forward part of the hull.
4.3.2 Propeller noise
The propulsion propeller is the dominant noise source on most
vessel types. The noise is transmitted through the sea water, and
may in extreme cases reduce the maximum range capability of
the EM 710.
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