7.1 The largest opportunities for reduction of
underwater noise will be during the initial design of the ship. For
existing ships, it is unlikely to be practical to meet the underwater
noise performance achievable by new designs. The following design
issues are therefore primarily intended for consideration for new
ships. However, consideration can also be given to existing ships
if reasonable and practicable. While flow noise around the hull has
a negligible influence on radiated noise, the hull form has influence
on the inflow of water to the propeller. For effective reduction of
underwater noise, hull and propeller design should be adapted to each
other. These design issues should be considered holistically as part
of the overall consideration of ship safety and energy efficiency.
7.2.1 Propellers should be designed and selected
in order to reduce cavitation. Cavitation will be the dominant radiated
noise source and may increase underwater noise significantly. Cavitation
can be reduced under normal operating conditions through good design,
such as optimizing propeller load, ensuring as uniform water flow
as possible into propellers (which can be influenced by hull design),
and careful selection of the propeller characteristics such as: diameter,
blade number, pitch, skew and sections.
7.2.2 Ships with a controllable pitch propeller
could have some variability on shaft speed to reduce operation at
pitch settings too far away from the optimum design pitch which may
lead to unfavourable cavitation behaviour (some designs may be able
to operate down to a shaft speed of two thirds of full).
7.2.3 The ship and its propeller could be model
tested in a cavitation test facility such as a cavitation tunnel for
optimizing the propeller design with respect to cavitation induced
pressure pulses and radiated noise.
7.2.4 If predicted peak fluctuating pressure at
the hull above the propeller in design draft is below 3 kPa (1st harmonic
of blade rate) and 2 kPa (2nd harmonic) for ships with a block coefficient
below 0.65 and 5 kPa (1st harmonic) and 3 kPa (2nd harmonic) for ships
with a block coefficient above 0.65, this could indicate a potentially
lower noise propeller. Comparable values are likely to be 1 kPa higher
in ballast condition.
7.2.5 Noise-reducing propeller design options
are available for many applications and should be considered. However,
it is acknowledged that the optimal propeller with regard to underwater
noise reduction cannot always be employed due to technical or geometrical
constraints (e.g. ice-strengthening of the propeller). It is also
acknowledged that design principles for cavitation reduction (i.e.
reduce pitch at the blade tips) can cause decrease of efficiency.
7.3.1 Uneven or non-homogeneous wake fields are
known to increase cavitation. Therefore, the ship hull form with its
appendages should be designed such that the wake field is as homogeneous
as possible. This will reduce cavitation as the propeller operates
in the wake field generated by the ship hull.
7.3.2 Consideration can be given to the investigation
of structural optimization to reduce the excitation response and the
transmission of structure-borne noise to the hull.