Installation manual

Trawl hydrophone installation

851-165187 / Rev.A

Considerations

Correct installation of PI54 hydrophone(s) is vital to system

performance. Several variables must be taken into consideration,

the most important of which is the vessel’s construction. This

guide is for use in selecting the best location for the hydrophone

and includes a brief description of areas to be avoided.

Note: Simrad strongly suggests that this information is read

thoroughly, and that the instructions are understood and

followed. Proper hydrophone placement is difficult to achieve,

but essential for correct system operation.

Depth

Water just below the sea surface contains a myriad of small air

bubbles created by the turbulence of breaking waves. The first

five to ten metres may be heavily saturated in moderate seas

with the greatest concentration and largest bubbles closest the

surface. Air bubbles disrupt sound waves in water. The degree

to which they absorb and reflect such energy vary, but in some

cases they can block hydrophone reception. It is therefore

recommended to mount the unit as deep as possible.

Pounding danger

When a vessel is in ballast and pitching in heavy seas, it is

important that the hydrophone is not lifted out of the water.

Should a vessel pound so heavily that the hydrophone be

exposed, sound reception will be interrupted and the unit may

be damaged on impact.

The boundary layer

The flow of water in the immediate vicinity of the hull of a

moving vessel is known as a boundary layer. This flow is

responsible for underwater noise that can disturb hydrophone

reception and its thickness is contingent on a vessel’s:

• Hull form

• Size and number of underwater protrusions

• Velocity

• Hull roughness

The boundary layer is thin (laminar flow) near the vessel’s bow

and becomes thicker (turbulent flow) as it moves aft. Laminar

flow is smooth with streamlines approximately parallel to the

hull and contributes relatively little to noise created by flow.

Conversely, turbulent flow is more disorderly and in tur n

contributes to a greater extent.