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Mount the transducer at forward part of hull to
minimize the effects from the flow boundary
water layer
The upper water layers of the sea contain a myriad of small air bubbles created by the
breaking waves. When the hull moves through water it will cause a disturbance, and this
will generate friction. The friction zone is called the flow boundary layer.
The flow in this boundary layer may be laminar or turbulent.
• The laminar flow is a nicely ordered, parallel movement of the water.
• The turbulent flow is a disorderly flow pattern, full of eddies.
A
Turbulent flow
B
Laminar flow
C
Air bubbles
Air bubbles absorb and reflect the sound energy, and they may in worst cases block the
sound transmission altogether.
The boundary layer increases in thickness when it becomes turbulent. The boundary layer
is thin in the forward part of the vessel hull, and increases as it moves aft. The thickness
depends on ships speed and on the roughness of the hull. All objects sticking out from
the hull, or dents in the hull, will disturb the flow and will increase the thickness of the
boundary layer. When the flow speed is high, the turbulence can be violent enough to
destroy the integrity of the water. Small voids or cavities in the water will occur and this
is called cavitation.
It is our experience that a wide and flat bottom, with a rising angle less than around 13
degrees, is prone to giving air problems for the transducer. In any case a transducer location
in the forward part of the hull is preferred in order to minimise the influence of the boundary
layer.
404292/D
Where to install the transducer
67
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