Kongsberg Simrad SN90 Reference Manual page 641

Concept descriptions
Propeller noise is usually water-borne. In some cases, however, shaft vibrations or
vibrations in the hull near the propeller may be structure-borne to the transducer. If a
propeller blade is damaged, this may increase the noise considerably.
Propeller cavitation is a severe source of noise. "Singing" propellers might be a source of
noise, which interferes at discrete frequencies. In some cases static discharge from the
rotating propeller shaft may be quite disturbing.
Cavitation
Cavitation is the formation of small air bubbles
close to the transducer face. The bubbles appear
because the local pressure becomes negative
during parts of the acoustic pressure cycles.
The cavitation threshold increases with the
hydrostatic pressure. The noise is made when
the bubbles implode.
Cavitation noise may appear near extruding
objects at higher speeds, but more often it is
caused by the propellers. Propeller cavitation is a severe source of noise. The cavitation
starts when the water flows in the same direction as the propeller blades. This is where
the propeller blades move downwards.
In some cases a resonant phenomenon is set up in a hole near the hull. This sound will
have a discrete frequency, while all other flow noise will have a wide frequency spectrum.
(Image from U. S. Navy in the public domain.)
Flow noise
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.
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