Reducing underwater noise from large commercial ships: Current status and future directions

Concerns about the effects of underwater noise pollution from shipping have led to a number of initiatives to develop quieting technologies for large commercial ships. These include the development of non-mandatory technical guidelines for reducing ship noise by the International Maritime Organization. For most merchant vessels the noise generated by cavitation will dominate all other sources of noise from that vessel. In addition, the noisiest vessels will contribute most of the shipping related hydroacoustic noise. The noisiest vessels are also likely to suffer excessive cavitation and may be operating inefficiently. Such vessels may be identified on the basis of efficiency indices or noise measurements and are most likely to benefit from remedial action to reduce noise which may also improve fuel efficiency. Slow steaming practices since 2007 resulted in an observed reduction in mean speeds from 15.6 (sd = 4.2) knots in 2007 to 13.8 (sd = 3.0) knots in 2013 for ships transiting the major shipping route in the eastern Mediterranean. Based on general observed relationships between speed and noise for vessels with fixed pitch propellers, we estimated that slow steaming in the last five years has likely reduced the overall broadband acoustic footprint from these ships by over 50%. There is still a lack of data on noise levels from individual ships and how these relate to different factors such as loading and trim and also a need for noise measurements from new propeller design concepts that offer improvement in fuel efficiency through reduced cavitation. Ongoing research on these issues should help develop further practical and economic quieting technologies in addition to measures already identified in the IMO guidelines.