Installation manual

Simrad PI54

851-165187 / Rev.A

Propeller noise

A vessel’s main propeller is the dominant source of underwater

acoustic noise. When ever possible, hydrophone(s) should be

located as far a way as possible from the main propeller and

never closer than ten meters. Hydrophone(s) should not be

mounted in the dir ect acoustic path (line-of-sight) of the main

propeller unless absolutely necessary.

The primary cause of propeller noise is cavitation ( small

bubbles generated by the partial vacuum created by the blades as

they pass through the water). The resulting underwater acoustic

noise from cavitation is normally weakest on the side of the

vessel were the propeller blades rotate toward the surface and

most pronounced on the side were they rotate toward the

bottom. Most vessels have clock-wise rotating propellers

resulting in their port sides being less effected by cavitation

induced noise than their starboard.

To minimise the negative effect of cavitation noise on

hydrophone performance, installation is generally recommended

as follows:

• Single hydrophone - if only one hydrophone is to be

installed on a vessel with a clock-wise rotating propeller, it

should be located on the port side of the hull.

• Dual hydrophones - if two hydrophones ar e to be installed,

they should be placed on either side of the vessel’s keel.

When in doubt about the best fore-and-aft location for

hydrophones, they can be placed at different distances from

the bow ( for example the por t hydrophone can be a little

further aft than the starboard, approximately three to five

meters for a thirty-five meter vessel). When trawling in both

deep and shallow water the hydrophones should also be tilted

differently with respect to each other. The hydrophone that is

closest to the propeller should have the greatest tilt and be

located on the port side of the hull for a vessels with

clock-wise rotating propellers.

Bow/sternthruster noise

Bow and sternthruster operation may severely effect

hydrophone r eception. Hydrophone installation closer than four

meters to either is strongly discouraged.