Ships can reduce underwater noise by improving propeller and hull design, controlling machinery vibration, keeping equipment and surfaces in good condition, and adjusting how they operate. The right combination depends on the vessel: speed changes that help one propulsion system may not work for another, and safety and operating requirements remain central.
What makes a ship noisy underwater?
Underwater noise can come from several parts of a vessel. The International Maritime Organization (IMO) identifies propeller cavitation, engine machinery and hull movement as important sources; hull design and onboard machinery also feature in its earlier guideline work. Their relative contribution varies with the ship and its operating conditions, so a measure aimed at one source may have little effect if another source dominates. IMO’s SDC 8 summary and its overview of noise-related publications describe these source areas.
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Cavitation occurs when pressure changes around a propeller cause bubbles to form and collapse. It can be a major source of underwater noise, but machinery vibration and the way vibration travels through a vessel also matter. A useful noise-reduction plan therefore starts by identifying the vessel’s likely sources and operating conditions rather than assuming that one universal fix will work.
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How can ships reduce noise through design and refit?
Match the propeller to the vessel
Propeller design, its interaction with the ship’s wake, and the propulsion system’s operating settings affect cavitation. Designers and owners assessing a new build or refit can consider how the propeller and hull work together, rather than treating the propeller as an isolated component. The IMO’s revised guidance discusses propeller and hull design as vessel-specific engineering considerations. Read the revised guidelines (MEPC.1/Circ.906).
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Limit machinery and vibration transmission
Machinery selection and installation can influence both the noise equipment generates and how vibration is transmitted through the hull into the water. Designers and shipbuilders can assess machinery arrangements and vibration isolation as part of the overall vessel design. The appropriate hardware solution depends on the ship; the IMO guidance does not prescribe one add-on as suitable for every vessel.
What operational changes can help?
Set speed and propulsion for the propeller type
For a fixed-pitch propeller, reducing vessel speed, shaft revolutions per minute (RPM) or engine output can reduce cavitation noise. The IMO recommends understanding the ship’s cavitation inception speed—the speed at which cavitation begins under specified conditions—and operating below it in designated protected areas when practicable.
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That advice does not transfer unchanged to controllable-pitch propellers. Simply reducing vessel speed may not reduce noise; shaft speed and propeller pitch may need to be optimized together. Operators should use vessel-specific knowledge or measurements where available, and account for safety, operational requirements and energy efficiency before changing settings. These qualifications are set out in MEPC.1/Circ.906.
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Trim and draught affect the power a vessel needs, which can in turn affect cavitation. Voyage planning and ship handling are also among the operating considerations in the IMO guidance. Operators can review these factors together with speed and propulsion settings for the actual route and vessel rather than making a single adjustment in isolation.
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Why do maintenance and hull condition matter?
Wear, damage or deterioration can undermine the intended performance of a propeller, hull, machinery or vibration-isolation system. The IMO identifies maintenance of moving parts and machinery, propeller and hull condition, and hull cleaning among relevant maintenance or operational considerations. A maintenance program should therefore include the components that affect the vessel’s noise performance, not just propulsion settings. The IMO’s SDC 8 summary discusses maintenance examples; further measures appear in the revised guidelines.
How should a vessel owner choose measures?
Compare potential measures against the ship’s actual noise sources and operating profile. The IMO material does not establish a universal best measure or a comparable cost-effectiveness ranking, so decisions should be evaluated for the particular vessel.
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- Expected noise effect: Is the measure aimed at a source that matters on this vessel, and can the result be assessed?
- Safety and operational feasibility: Can the vessel use the proposed speed, propulsion settings or route without compromising safe operation or its mission?
- Energy and fuel consequences: Could the measure improve efficiency, create a trade-off, or require a balance between the two? An IMO expert workshop considered possible co-benefits and trade-offs between energy efficiency and underwater noise, but it does not establish one outcome for every ship. Workshop summary.
- Implementation needs: What capital work, maintenance and operational changes would be required?
Where practical, record the vessel’s operating conditions alongside any noise assessment. That makes it easier to judge whether a change appears to help and to interpret results when speed, loading, route or propulsion settings differ.
What does IMO guidance require, and what is its purpose?
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 provide guidance to support designers, shipbuilders, owners and operators in taking action and developing noise-management plans. The reviewed IMO materials do not establish a universal, enforceable underwater-noise limit for every commercial ship. For current implementation information, consult the IMO’s Ship noise topic page, which may be updated as the organization’s work progresses.
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The reason to manage noise is environmental as well as operational. The IMO describes potential adverse effects on critical life functions in marine mammals, fish and invertebrates, and notes the relevance to coastal Indigenous communities that depend on marine life. The IMO’s background on the revised guidelines outlines that scope.
For a professional reference focused on practical vessel measures, the Institute of Chartered Shipbrokers lists an Underwater Radiated Noise Guide, 1st Edition. Its listed contents include speed, propeller improvements, maintenance, voyage planning, trim, ship handling and a sample noise-management plan.
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