10.1 Although the main components of underwater
noise are generated from the ship design (i.e. hull form, propeller,
the interaction of the hull and propeller, and machinery configuration),
operational modifications and maintenance measures should be considered
as ways of reducing noise for both new and existing ships. These include,
among others:
Propeller polishing done properly removes marine fouling
and vastly reduces surface roughness, helping to reduce propeller
cavitation.
10.3
Underwater hull surface
Maintaining a smooth underwater hull surface and smooth
paintwork may also improve a ship's energy efficiency by reducing
the ship's resistance and propeller load. Hence, it will help to reduce
underwater noise emanating from the ship. Effective hull coatings
that reduce drag on the hull, and reduce turbulence, can facilitate
the reduction of underwater noise as well as improving fuel efficiency.
10.4
Selection of ship speed
10.4.1 In general, for ships equipped with fixed
pitch propellers, reducing ship speed can be a very effective operational
measure for reducing underwater noise, especially when it becomes
lower than the cavitation inception speed.
10.4.2 For ships equipped with controllable pitch
propellers, there may be no reduction in noise with reduced speed.
Therefore, consideration should be given to optimum combinations of
shaft speed and propeller pitch.
10.4.3 However, there may be other, overriding
reasons for a particular speed to be maintained, such as safety, operation
and energy efficiency. Consideration should be given in general to
any critical speeds of an individual ship with respect to cavitation
and resulting increases in radiated noise.
10.5
Rerouteing and operational decisions
to reduce adverse impacts on marine life
Speed reductions or routing decisions to avoid sensitive
marine areas including well-known habitats or migratory pathways when
in transit will help to reduce adverse impacts on marine life.