Propeller design versus slow steaming of a low-noise container ship

A propeller design optimization study was performed for a single-screw container ship using an automated multiobjective design method. The propellers were optimized for maximum in-behind efficiency and minimum underwater radiated noise (URN), while satisfying many design constraints. A clear trade-off between efficiency
and URN is found, whereby the overall URN reduces by 3.7dB for a relative efficiency penalty of 1% with reference
to the most efficient designs. This effect reduces for larger efficiency reductions; while the contribution of the
cavitation noise reduces, the contribution of the ship machinery noise remains unaffected thereby acting as a
noise floor.
The various propeller designs were evaluated in a hypothetical scenario for the ship transiting a marine
protected area with noise limitations requiring a slowdown to meet (self-imposed) noise limits, with the ship sailing
faster outside this region to maintain arrival time. For each 1% reduction in efficiency of the propeller design, the
required slowdown decreased by 1 knot. The more quiet propellers with a 2-4% efficiency loss have a lower net
energy expenditure when this region is 10% or more of the total journey, by virtue of a lower ship speed outside the
marine protected area.