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Preliminary version 2022-07-26
Preparations
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 water 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.
Turbulent flow
Laminar flow
Air bubbles
A
B
C
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.
Related topics
Preparations, page 29
Where to install the transducer, page 39
110-0006927/A (Preliminary)
41
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