Sound is how the ocean sees. Whales, dolphins, fish, and even lobsters rely on hearing to communicate, find food, avoid predators, navigate, and reproduce — and into that acoustic world, commercial shipping pours a rising flood of low-frequency noise. Scientists measuring ocean noise have found that shipping noise is doubling roughly every decade, as the global fleet's carrying capacity nearly quadrupled between 1996 and 2020. When ship noise masks natural sounds, marine mammals have been observed abandoning preferred habitats, ceasing to sing to mates, breaking off foraging, and showing elevated stress hormones. For endangered populations such as the Southern Resident killer whales, acoustic disturbance is formally recognised by both Canadian and U.S. governments as a key threat to recovery. Yet underwater radiated noise, or URN, remains the rare marine environmental issue that is not regulated — the IMO's revised guidelines are recommendatory, and the industry is currently inside an experience-building phase running to 2026 that will shape whatever comes next. What makes URN unusually tractable is the physics: the dominant source is propeller cavitation, the dominant control is speed, and the reduction measures overlap almost perfectly with fuel efficiency. Slow a ship one knot and you drop roughly one decibel; slow the global fleet by ten percent and total sound energy from shipping falls by around forty percent. This guide covers the noise sources, the marine-life impact, the IMO framework, the design and operational measures that work, the measurement standards and class notations, and the port programmes already proving the results. To manage hull and propeller condition, speed data, and environmental compliance in one system, book a Marine Inspection demo.
Green shipping · underwater radiated noise
Underwater Noise from Ships: IMO Guidelines & Noise Reduction
A technical guide to underwater radiated noise — propeller cavitation, the IMO framework and experience-building phase, design and operational reduction measures, measurement standards, class notations, and the port programmes proving what works.
1 kn = 1 dB
Rule of thumb: each knot of speed reduction cuts about a decibel
10% slower
A 10% fleet slowdown could cut total shipping sound energy ~40%
Doubling
Ocean shipping noise is doubling roughly every decade
Where Ship Noise Comes From
A ship radiates noise into the water from several sources, but they are not equal. Understanding the hierarchy is what makes URN reduction tractable, because effort concentrated on the dominant source pays disproportionately. See condition tracking in a demo.
Dominant
Propeller cavitation
Vapour cavities form in low-pressure regions on and behind the propeller blades, then collapse violently as pressure recovers. Each collapse radiates a broadband acoustic pulse. Cavitation is by far the largest contributor to a merchant ship's underwater noise.
Secondary
Machinery noise
Main engines, auxiliary generators, pumps, and gearboxes generate vibration that transmits through mountings and structure into the hull, and from the hull into the water as structure-borne noise.
Secondary
Propeller-induced vibration
Pressure pulses from the blades passing through an uneven wake excite the hull plating above the propeller, which then radiates sound even without cavitation collapse noise.
Minor
Hydrodynamic & equipment
Flow noise over the hull and appendages, plus discretionary equipment such as echosounders and thrusters, which add noise when operated unnecessarily.
The practical consequence is that cavitation control dominates URN strategy. And cavitation is speed-dependent: below a threshold known as the cavitation inception speed, the propeller does not cavitate meaningfully at all. Push past it, and noise rises steeply. Almost every design measure in the field is, at bottom, an attempt to raise the cavitation inception speed so the ship can operate quietly across more of its speed range.
Why It Matters: the Impact on Marine Life
The biological case is not abstract. Decades of research have quantified how vessel noise degrades the acoustic environment that marine mammals depend on for survival.
Masking
Ship noise overlaps the frequencies whales use to communicate and echolocate, drowning out calls. Modelling of right whale mother-calf up-calls near container vessels found the call detectable only when the receiving whale was very close to the caller.
Behavioural disruption
Animals have been observed leaving preferred habitats and interrupting foraging, nursing, resting, and mating behaviours in the presence of vessel noise.
Chronic stress
Analyses of North Atlantic right whale samples suggest noise from large commercial vessels elevates stress hormones, lowering resilience in populations already under pressure.
Forced adaptation
Some whales shift the frequency, duration, and rate of their calls to compete with low-frequency noise — the Lombard effect — an energetic cost imposed by shipping.
Acoustic looming
A rapidly rising received level from a fast-approaching vessel can be perceived as an imminent threat, provoking stronger flight responses than the peak level alone would predict.
Crucially, mitigation compounds. Research on cargo vessels shows that increasing the closest-approach distance from 300 to 3,000 metres substantially cuts the maximum level a whale receives and nearly eliminates acoustic looming — and that slowing the vessel as well produces a stronger reduction across every measure. Distance and speed together work synergistically, which is precisely the logic behind the seasonal slowdown zones now operating in whale habitat.
The IMO Framework: Guidelines, Not Regulation
URN occupies unusual regulatory territory. It is not covered by any binding IMO instrument, yet a structured international framework is taking shape around voluntary guidelines and a formal learning process. See compliance tracking in a demo.
1
Revised guidelines, 2023
MEPC 80 approved the Revised Guidelines for the Reduction of Underwater Radiated Noise from Shipping (MEPC.1/Circ.906), adopted in July 2023 and in effect from 1 August 2023, alongside MEPC.1/Circ.907.
2
Experience-building phase
An experience-building phase (EBP) collects best practices and lessons from applying the guidelines. It runs to a target completion of 2026, with a possible two-year extension depending on outcomes.
3
Early uptake encouraged
MEPC 81 encouraged early implementation of the guidelines as part of the EBP and invited submissions of experience — making voluntary adoption today the input to tomorrow's framework.
4
No revisions until complete
No further revision of the URN guidelines will be undertaken until the EBP concludes, so the current text is the operative reference for owners and designers now.
The guidelines recommend that owners and designers plan URN management and, as far as practicable, establish a ship's baseline URN condition. That baseline may be predicted through computation, empirical methods, or model tests, but is preferably measured — and it should be established under the ship's normal operating conditions, at typical operational speed and draught, with standard operating equipment running. Everything else is measured against that baseline.
The measures overlap with efficiency
Track Hull, Propeller, and Speed — the Same Data That Cuts Noise
Nearly every URN reduction measure — clean hull, polished propeller, optimised speed — also cuts fuel. Marine Inspection tracks hull and propeller condition, schedules cleaning and polishing, and logs speed and fuel performance per voyage, so noise reduction and efficiency are managed from one record. Book a 30-minute demo to see condition and performance tracking, or start a free trial today.
Design Measures: Raising the Cavitation Inception Speed
For a naval architect, the goal is clear: delay cavitation to higher speeds and reduce it when it does occur. The guidelines note that hull and propeller design should be adapted to each other, because the propeller only performs as well as the wake the hull delivers to it. Scroll the table on mobile.
Design Measures for URN Reduction
Two points deserve emphasis. First, wake improvement devices such as the Schneekluth duct are marketed on efficiency grounds — reported fuel savings of up to 12% and propeller-induced vibration reductions of up to 50% — meaning the quieting benefit often arrives inside a business case that already pays for itself. Second, large two-stroke main engines, which power most merchant ships, are generally unsuitable for resilient mounting; the machinery-isolation route belongs mainly to four-stroke and diesel-electric arrangements.
Operational Measures: What Any Ship Can Do Tomorrow
Design changes need a yard. Operational measures need only a decision — and because cavitation scales so sharply with speed and propeller condition, they deliver the fastest URN reductions available to an existing fleet. See maintenance scheduling in a demo.
Slow down
The single most powerful lever. As a rule of thumb, each knot of speed reduction cuts noise by roughly one decibel; a 10% fleet-wide slowdown could reduce total shipping sound energy by around 40%.
Polish the propeller
Blade imperfections, roughness, and erosion encourage cavitation. Polishing between dry-dockings restores a smooth surface, reducing cavitation and its noise.
Clean the hull
Biofouling raises resistance, forcing higher machinery load and propeller RPM for the same speed — which directly increases URN. Hull maintenance is noise maintenance.
Route away from habitat
Rerouting lanes away from important habitat has proven effective at reducing noise in the frequencies whales use for calls and echolocation.
Increase separation
Greater closest-approach distance sharply cuts the received level and near-eliminates acoustic looming — and combines synergistically with slowing down.
Switch off what you can
Avoid operating noise-emitting equipment such as echosounders when they are not needed, and manage controllable-pitch propeller settings for quieter operating points.
The alignment with commercial interest is the quiet argument here. Slower steaming, a clean hull, and a polished propeller are the same measures that cut fuel burn, CO2, and CII rating pressure. URN reduction is, for most operators, a co-benefit of things they already have reason to do — which is why the IMO has explicitly discussed how noise-reduction measures may contribute to greenhouse-gas targets.
Measurement, Standards and Class Notations
What gets measured gets managed, and URN has a mature measurement architecture even without binding regulation. Owners pursuing a quiet-ship credential work through class. See survey records in a demo.
ISO 17208 series
The reference standards for describing and measuring underwater sound from ships, including requirements for precision deep-water measurements used for comparison.
Hydrophone accuracy
Measurement instrumentation is expected to hold uncertainty within about ±2 dB, and not more than ±3 dB, across a frequency range from roughly 10 Hz to 20 kHz.
Key definitions
The acoustic centre is taken midway between the engine room centre and the propeller, at 0.7 of draught; levels are expressed in decibels relative to 1 micropascal.
Class notations
Classification societies assign URN notations to vessels on successful completion of measurement trials, with major societies each publishing their own URN rules and guidance.
A practical caution from the class rules: if modifications are later made to the hull form, main machinery, propulsion, large auxiliaries, or the propeller — anything that alters the vessel's underwater noise performance — confirmatory URN measurements may be required, and class must be notified in advance. A quiet-ship notation is a condition to be maintained, not a certificate to be filed.
Port Programmes: the Proof at Scale
The most compelling evidence that URN reduction works comes not from models but from ports that asked ships to slow down and then listened. The results are unusually clear for an environmental intervention.
The ECHO programme
Launched by the Vancouver Fraser Port Authority in 2014, ECHO runs seasonal voluntary slowdowns in Southern Resident killer whale habitat in the Salish Sea, with underwater listening stations measuring the result.
Measured reductions
2022 trials reported median broadband noise reductions of about 2.7 dB in Haro Strait and 2.8 dB in Boundary Pass — roughly 46% and 48% cuts in sound intensity — and 3.1 dB at Swiftsure Bank, around a 51% cut.
High participation
A 2022 trial saw 93% pilot-reported participation, and 2024 slowdowns cut underwater sound intensity by up to 34% in Boundary Pass. Whales were detected on 26% more days when the slowdown was active.
Quiet Sound, Puget Sound
Washington's voluntary large-vessel slowdown saw over 70% of roughly 800 transiting ships slow down, halving the underwater noise reaching killer whales — including at the frequencies they use to communicate and hunt.
Financial incentives
Ports have begun rewarding quiet vessels; the Port of Prince Rupert was among the first to offer reduced harbour dues for quieter ships, and proposals exist to vary port fees by noise performance.
There is a further dividend. Modelling indicates that a 30% speed reduction shrinks a vessel's acoustic footprint by around 95% while simultaneously cutting the ship-strike rate by roughly half — so slowdown zones protect whales twice over, from noise and from collision. For an owner, participation costs schedule flexibility and earns fuel savings, reputational credit, and in a growing number of ports, lower dues.
Building a URN Management Approach
Pulling the threads together, a practical URN programme follows the guidelines' logic: understand where you stand, act where the physics pays, and verify what changed.
Establish a baseline
Predict or, preferably, measure the ship's URN under normal operating speed, draught, and equipment configuration, as the guidelines recommend.
Target cavitation first
Because cavitation dominates, prioritise propeller condition, wake quality, and operating speed relative to the cavitation inception speed.
Maintain relentlessly
Hull cleaning and propeller polishing between dry-dockings preserve both the acoustic and the fuel performance the ship was designed for.
Respect the zones
Participate in seasonal slowdowns and habitat routing, where the marginal noise reduction per knot is worth the most biologically.
Verify after changes
Re-measure after modifications to hull, machinery, or propeller, as class rules require, and record the outcome against the baseline.
Feed the EBP
Submit experience from applying the guidelines; early adoption today shapes the framework that emerges once the experience-building phase completes.
The strategic point for owners is that URN is one of the few environmental issues where the compliance path and the commercial path run in the same direction. A clean hull, a polished propeller, a well-matched wake, and a moderated speed reduce noise, fuel, carbon, and CII pressure at once. The fleets that keep good condition and performance records are, without additional effort, building exactly the evidence base a future URN framework will ask for. Book a demo to see condition and performance tracking.
Frequently Asked Questions
What is underwater radiated noise from ships?
Underwater radiated noise (URN) is the sound a vessel emits into the water, dominated by propeller cavitation and supplemented by machinery vibration transmitted through the hull, propeller-induced pressure pulses, and flow noise. It is measured in decibels relative to 1 micropascal and is a recognised form of ocean pollution affecting marine life.
How does ship noise affect whales and marine mammals?
It masks the sounds animals rely on to communicate, echolocate, forage, navigate, and breed. Whales have been observed leaving habitats, interrupting foraging and nursing, and showing elevated stress hormones. Some shift call frequency and rate to compensate, and a rapidly approaching vessel can trigger a flight response through acoustic looming.
What are the IMO guidelines on underwater noise?
MEPC.1/Circ.906, the Revised Guidelines for the Reduction of Underwater Radiated Noise from Shipping, was adopted in July 2023 and took effect on 1 August 2023. URN is not covered by binding regulation; instead an experience-building phase runs to a target of 2026, possibly extended by two years, and no further revisions will be made until it concludes.
Why is propeller cavitation the main noise source?
Vapour cavities form in low-pressure regions around the propeller blades and collapse violently as pressure recovers, each collapse radiating a broadband acoustic pulse. Below the cavitation inception speed a propeller barely cavitates; above it, noise rises steeply. Most design measures aim to raise that inception speed so the ship runs quietly across more of its speed range.
How much does slowing down reduce ship noise?
As a rule of thumb, each knot of speed reduction cuts noise by around one decibel, and slowing the global fleet by 10% could reduce total shipping sound energy by roughly 40%. Real programmes bear this out: Vancouver's ECHO slowdown trials measured broadband reductions of 2.7 to 3.1 dB, equivalent to cutting sound intensity by roughly half.
What design measures reduce underwater radiated noise?
Increasing blade skew, reducing tip loading, fitting blade end plates and propeller boss cap fins, using wake improvement devices such as ducts and stern flaps to deliver a uniform inflow, careful selection of propeller diameter, blade number, pitch and sections, plus resilient machinery mounting and structural damping to cut structure-borne noise.
Are there class notations for quiet ships?
Yes. Classification societies assign URN notations after successful measurement trials, each publishing its own URN rules and guidance, with measurement referenced to the ISO 17208 series. If the hull, machinery, propulsion, or propeller is later modified in ways affecting noise performance, class must be informed and confirmatory measurements may be required.
Quieter ships, cleaner ships
The Same Records That Cut Noise Cut Fuel
Track hull and propeller condition, schedule cleaning and polishing, log speed and fuel performance per voyage, record slowdown-zone participation, and keep survey and measurement history against each vessel — so URN management and efficiency management are the same discipline. Marine Inspection keeps both on the record. Book a tailored walkthrough or start a free trial today.
