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Ships can reduce underwater noise by combining vessel-specific design and propulsion choices with careful operating practices and maintenance. The most useful measures depend on what is making noise on a particular vessel, how it is propelled, and the conditions in which it must operate. The International Maritime Organization (IMO) revised its guidance in 2023 to help designers, shipbuilders, owners, and operators plan those measures.
What causes underwater noise from ships?
Underwater radiated noise can come from several parts of a vessel, and the dominant source can vary with ship design and operating conditions. The IMO identifies propeller cavitation, engine machinery, and hull movement among the main sources; its earlier guideline work also discusses hull design and onboard machinery. IMO’s SDC 8 meeting summary and its overview of publications supporting global action describe these sources.
Cavitation occurs when pressure changes around a propeller cause bubbles to form and collapse. Machinery and vibration can also transmit sound through the hull into the water. Because these sources interact with the vessel’s design and operating state, a measure aimed at the wrong source may have little effect; diagnosis should guide the choice of intervention.
How can ships reduce underwater noise?
Design or refit the hull, propeller, and propulsion system
For a new build or major refit, evaluate the hull form, propeller design, and how well the propeller is matched to the ship’s wake. Review propulsion settings and how machinery vibration is transmitted through the vessel. These are engineering choices that need to be assessed for the specific ship, rather than universal hardware additions. The IMO’s revised guidelines cover design and operational approaches for stakeholders.
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Set speed and propulsion according to propeller type
For fixed-pitch propellers, reducing vessel speed, shaft revolutions per minute (RPM), or engine output can reduce cavitation noise. The IMO advises operators to understand a ship’s cavitation inception speed—the speed at which cavitation begins—and, where practicable, operate below it in designated protected areas.
For controllable-pitch propellers, reducing speed by itself may not reduce noise. Shaft speed and propeller pitch may need to be adjusted together. Operators should use ship-specific knowledge or measurements when available, while accounting for safety, operating requirements, and energy efficiency. The IMO guidance does not make one speed setting suitable for every vessel.
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Maintain the propeller, hull, and machinery
Keep the propeller and hull in good condition, and maintain moving machinery and vibration-isolation measures so deterioration does not undermine their intended performance. Hull cleaning is one operational or maintenance example identified in the IMO material. The relevant work and intervals depend on the vessel and its maintenance regime.
Optimize trim, draught, handling, and voyage plans
Consider trim and draught—the vessel’s fore-and-aft balance and how deeply it sits in the water—alongside ship handling and voyage planning. An appropriate setup can reduce power demand and associated propeller cavitation noise. Operational adjustments should be assessed in context rather than treated as fixed prescriptions for all routes or loading conditions.
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How should operators choose among measures?
Compare candidate measures against the vessel’s likely noise sources and operating profile. The IMO does not establish a universal best measure or a comparable cost-effectiveness ranking. Its workshop on energy efficiency and underwater radiated noise considered possible co-benefits and trade-offs, so assess the effects together rather than assuming that every noise measure will also save fuel.
- Expected noise effect: Is the measure aimed at a source known to matter on this vessel, and can the result be assessed?
- Safety and operational feasibility: Can the ship use the measure without compromising safe navigation or essential operating needs?
- Energy and fuel consequences: Could the adjustment increase or reduce power demand under the ship’s actual conditions?
- Implementation burden: What capital work, maintenance, training, or changes to voyage planning would be needed?
Where data are available, use measurements and ship-specific operating knowledge to evaluate outcomes. The reviewed IMO material does not provide a broadly applicable percentage or decibel reduction for these measures, so a general numerical promise would be misleading.
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What does IMO guidance require?
The IMO’s Marine Environment Protection Committee approved revised Guidelines for the Reduction of Underwater Radiated Noise from Shipping in July 2023; the IMO says they took effect in October 2023. They support action and underwater-noise management planning by stakeholders. The reviewed IMO materials do not establish a universal, enforceable underwater-noise limit for every commercial ship. Check the IMO’s ship-noise overview for the latest implementation status: it describes an experience-building phase running until MEPC 85 in 2026, with a possible two-year extension.
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The IMO’s action plan emphasizes implementation, learning from experience, awareness, and training, along with further work on targets and policy. This makes a ship-specific management plan more useful than treating any single operational change as a complete solution. For additional professional reading, the Institute of Chartered Shipbrokers lists its Underwater Radiated Noise Guide, 1st Edition, which covers measures including speed, propeller improvements, maintenance, voyage planning, trim, ship handling, and a sample noise-management plan.
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Why reducing ship noise matters
The IMO describes potential adverse effects of underwater noise on critical life functions in marine mammals, fish, and invertebrates. Noise can also matter to coastal Indigenous communities whose livelihoods and cultures depend on marine life. The IMO outlines this environmental scope in its background on revised guidelines.
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