Ship Quieting Technologies
Ship propellers, motors, and gears create sound. The sounds produced depend on numerous factors including ship type, size, hull shape, propulsion system, ship speed, and transit conditions. Unless close to a vessel, most of the sounds that ships produce are low frequency (below 500 Hz) and contribute to ocean ambient noise. Studies have shown that marine animals may alter their behavior in response to ship noise. Multiple activities have focused on reducing ship-related underwater noise. These activities include the publication of international acoustic standards, the European Union Marine Strategy Framework Directive, and the International Maritime Organization (IMO) non-mandatory guidelines.
Propeller-induced cavitation (the formation and rapid collapse of bubbles) is the main source of underwater sound produced by ships. The IMO recommends that propellers be designed to reduce cavitation by appropriate selection of propeller diameter, blade number, and pitch.
The shape of a ship’s hull and the resulting wake affect propeller performance and resultant noise production. The IMO recommends hull and propeller design be considered together to achieve noise reduction. For example, hulls can be designed and structures added to produce a uniform wake.
Machinery onboard a vessel also produce sounds that can propagate through the hull. The properties of underwater-radiated sounds will vary depending on the type of machinery, its location on the vessel, and other factors related to vessel design. Usually, machinery that is directly bolted to the hull produce the highest levels of underwater sound. Larger machinery such as engines, turbines, diesel generators, etc. also have a greater influence on underwater sound production.
Selecting equipment with inherently low noise and vibration levels is one of the most effective methods for reducing machinery noise. The transmission of machinery vibration into the water can be reduced by vibration isolators and/or isolation mounts. These use “soft” or “elastic” materials (usually steel coil springs) located between the machinery and the ship’s hull. Commonly-used thermal and fire insulation materials, such as fiberglass and mineral wool, also help to reduce radiated noise.
Operational modifications and maintenance also help reduce noise production for both new and existing ships. Reducing ship speed is one operational procedure that can reduce ship noise. Regular propeller cleaning removes marine fouling and reduces surface roughness, helping reduce propeller cavitation. Maintaining a smooth underwater hull surface and smooth paintwork helps reduce a ship’s resistance and may help reduce underwater noise produced, as well as increase energy efficiency for the ship.
Efforts to moderate the effects of ship noise include the use of underwater acoustic propagation models to assess the “noise footprint” of individual vessels and to produce “noise maps” that show the contributions of multiple vessels to the local ambient noise. For example, to help inform management decisions about vessel noise in Glacier Bay National Park, Alaska, scientists estimated the acoustic exposure of humpback whales to vessel noise under a variety of scenarios. Model simulations showed cruise ship speed to be the dominant factor affecting noise exposure in Glacier Bay, with fast ships producing the highest maximal sound pressure level and cumulative sound exposure levels over a 24-hour period. (For more information, please see the Advanced Topic on Sound Pressure Levels and Sound Exposure Levels) Cruise ships traveling at 13 knots produced cumulative sound exposure levels 3 times lower than those traveling at 20 knots; maximal sound pressure levels also decreased for the slower vessels. Therefore, rather than decreasing the allowable number of cruise ships in Glacier Bay, or synchronizing the timing of their arrivals, the scientists suggested that reducing the speeds of the cruise ships, and/or otherwise quieting the ships, offered the best potential to lessen potential impacts on humpback whales.
Additional Links on DOSITS
- Science of Sound > How does shipping affect ocean sound levels?
- Science of Sound > Advanced Topics > Ocean Noise Variability and Noise Budgets
- Science of Sound > Advanced Topics > Sound Pressure Levels and Sound Exposure Levels
- Audio Gallery > Humpback Whale
- Audoly, C., & Rousset, C. (2014). European project for achieving quieter oceans by shipping noise footprint reduction (p. 070012). Presented at the 167th Meeting of the Acoustical Society of America, Providence, Rhode Island. https://doi.org/10.1121/1.4904407.
- Achieve Quieter Oceans by shipping noise footprint reduction (AQUO Project)
- M. Bahtiarian, Society of Naval Architects and Marine Engineers (SNAME). 2014 Marine Vessel Environmental Performance (MVEP) Assessment Guide, General Measures: Ocean Health and Aquatic Life – Underwater Noise.
- NOAA Fisheries, Understanding Sound in the Ocean
- Southall, B. L., Scholik-Schlomer, A. R., Hatch, L., Bergmann, T., Jasny, M., Metcalf, K., … Wright, A. J. (2017). Underwater Noise from Large Commercial Ships-International Collaboration for Noise Reduction. In J. Carlton, P. Jukes, & Y. S. Choo (Eds.), Encyclopedia of Maritime and Offshore Engineering (pp. 1–9). Chichester, UK: John Wiley & Sons, Ltd. https://doi.org/10.1002/9781118476406.emoe056.
- Frankel, A., & Gabriele, C. (2017). Predicting the acoustic exposure of humpback whales from cruise and tour vessel noise in Glacier Bay, Alaska, under different management strategies. Endangered Species Research, 34, 397–415. https://doi.org/10.3354/esr00857
- Guidelines for the Reduction of Underwater Noise from Commercial Shipping to Address Impacts on Marine Life. (2014). International Maritime Organization. http://www.imo.org/en/MediaCentre/HotTopics/Documents/833 Guidance on reducing underwater noise from commercial shipping,.pdf.
- Leaper, R., Renilson, M., & Ryan, C. (2014). Reducing underwater noise from large commercial ships: Current status and future directions. The Journal of Ocean Technology, 9(1), 51–69.
- McWhinnie, L., Smallshaw, L., Serra-Sogas, N., O’Hara, P. D., & Canessa, R. (2017). The Grand Challenges in Researching Marine Noise Pollution from Vessels: A Horizon Scan for 2017. Frontiers in Marine Science, 4. https://doi.org/10.3389/fmars.2017.00031.
- Spence, J. H., & Fischer, R. W. (2017). Requirements for Reducing Underwater Noise From Ships. IEEE Journal of Oceanic Engineering, 42(2), 388–398. https://doi.org/10.1109/JOE.2016.2578198.