Underwater radiated noise, often shortened to URN, is one of the least visible outputs of shipping. It cannot be seen in a plume above the funnel or measured on a quay with a hand-held meter. Yet sound travels efficiently underwater, and commercial vessels can add persistent noise to coastal and ocean environments that many marine species use to communicate, navigate, find food and avoid danger.
For ports and shipping companies, underwater noise reduction is becoming a practical part of environmental performance. It is not an argument against maritime trade. It is an opportunity to improve how trade is delivered in sensitive waters, near marine mammals, fish habitats, migration routes and coastal communities. Many noise-reduction measures also improve propulsion efficiency, reduce fuel use or make operations more predictable. The objective is simple: turn the volume down without compromising navigational safety or service reliability.
What underwater radiated noise is and where it comes from
Underwater radiated noise is the sound emitted by a ship into the water. Its sources include propulsion machinery, generators, pumps, thrusters, hull vibration and, most significantly for many vessels, propeller cavitation. Cavitation occurs when pressure changes around a propeller create and collapse small vapour bubbles. It can be a major source of noise and can also signal that the propeller-hull-engine system is not operating at its most efficient condition.
Noise varies with vessel type, speed, loading condition, maintenance, hull roughness, propeller condition, water depth, manoeuvring activity and weather. A ship’s acoustic footprint is therefore not fixed. The same vessel may be quieter during one passage than another. This is good news for operators because it means that practical changes can make a difference.
The environmental concern is not merely that the sea becomes louder. Marine animals use sound differently from humans. For some species, sound is essential over distances where sight is limited. Elevated background noise can mask calls, alter behaviour, disrupt feeding or migration, and add stress during periods when animals are already vulnerable. The scale of effects depends on species, location, timing and cumulative activity. Ports should avoid simplistic claims while taking the risk seriously.
Why ports have a role in quieter shipping
Vessel design and maintenance decisions are often made by owners and operators, but ports influence the conditions in which ships arrive, manoeuvre, wait and depart. A port authority may set environmental incentives, share sensitive-area information, facilitate just-in-time arrival, coordinate approach speeds, improve berth availability and recognise performance through voluntary programmes. Ports can also bring together ship operators, pilots, tug companies, researchers, NGOs, regulators and coastal communities around a common outcome.
This role is especially valuable because underwater noise is a system issue. A ship that must rush to recover a berth slot may generate more noise than a ship able to optimise its passage. A vessel held at anchor because of uncertain port information may spend longer operating in a sensitive area. A maintenance decision that improves hull and propeller condition may reduce fuel use, emissions and noise together. The port cannot solve every element, but it can help align incentives and remove avoidable operational causes.
Start with location, species and season
An effective underwater noise reduction programme is geographically intelligent. The first task is to understand where noise matters most. Map harbour approaches, anchorages, fairways, pilot boarding areas, turning basins and nearby habitats. Combine this with available information on marine mammals, fish spawning areas, migration corridors, protected zones, seasonal patterns and local research.
The purpose is not to create an exclusion zone around every ecological interest. It is to target measures where they have the greatest benefit and least operational disruption. In one port, a seasonal voluntary speed reduction along an approach channel may be appropriate. In another, the priority may be reducing high-noise manoeuvring near a breeding area, providing vessels with earlier berth information, or designing a financial incentive for quieter ships.
Engage marine scientists, indigenous and local knowledge holders where relevant, fishers, conservation organisations and regulators early. They can help define what is known, what is uncertain and which monitoring methods are credible. This makes a programme more legitimate and avoids treating biodiversity as a communication exercise rather than an operational consideration.
Vessel-level measures: quieter can also mean more efficient
The International Maritime Organization’s revised guidance on underwater radiated noise identifies a range of voluntary approaches. At vessel level, the most useful measures typically focus on propellers, hull condition, machinery isolation and operating profile.
Propeller maintenance and design are central. Repairing damage, maintaining surface condition, addressing fouling and assessing cavitation behaviour can reduce noise as well as improve efficiency. For newbuildings or major retrofits, owners can consider propeller geometry, wake equalising devices, hull form and machinery arrangements that reduce vibration and cavitation risk. These choices must be evaluated for the vessel’s intended service, not chosen from a generic catalogue.
Hull cleaning and biofouling management are another important link. A rough or fouled hull increases resistance. To maintain speed, the propulsion system may need more power, which can intensify cavitation and noise. Responsible cleaning and coating strategies can therefore deliver a three-way benefit: lower resistance, lower fuel consumption and less noise. Any cleaning programme must of course comply with local biosecurity and discharge controls.
Operational measures can complement hardware. Avoiding unnecessary high-speed transit, using the lowest safe and commercially feasible speed in identified sensitive areas, improving trim where appropriate, and minimising harsh manoeuvring can reduce acoustic output. Safety always comes first. A speed recommendation must be developed with pilots, masters and authorities who understand currents, traffic density, weather and manoeuvring margins.
Port-level measures: make the quiet choice easier
Ports can create the conditions for quieter operations in several ways. The most important is predictability. Port call optimisation and just-in-time arrival help a ship plan its passage to meet a realistic service window instead of hurrying only to wait. This can reduce fuel use, greenhouse-gas emissions, anchorage time and unnecessary noise. The benefit depends on reliable data and a shared commitment across the port call chain.
Environmental incentive schemes are another useful lever. Reduced port dues, recognition programmes or differentiated fees can reward vessels that demonstrate quieter design, maintenance or operating performance. A well-designed incentive should be transparent, evidence-based and simple enough to administer. It should also avoid rewarding paperwork alone. Criteria need credible verification, periodic review and a clear link to the environmental outcome.
Ports can provide practical guidance through notices to mariners, voluntary programmes, pilots’ information packs and digital planning tools. The message should be specific: where the sensitive area is, during which period, what action is requested, how it interacts with safety requirements, and how vessels can report compliance or operational constraints. Broad requests to “be quieter” are less effective than a clear, navigationally sound protocol.
At berth, ports can examine auxiliary-engine use, shore power availability, tug operations, thruster practices and maintenance facilities. Shore power is primarily an air-emissions and climate measure, but it may reduce certain at-berth machinery noise where the local system and vessel configuration support it. The environmental case should be based on measurement and local conditions rather than assumed.
Measuring noise without creating a monitoring burden
Measurement is useful when it supports decisions. Ports do not necessarily need a permanent, complex hydrophone network before taking action. Start with the question that needs answering. Do we need to identify the noisiest part of an approach? Are we testing whether a voluntary speed programme changes sound levels? Do we need a baseline for an incentive scheme? Is a particular habitat or season a priority?
Depending on the question, monitoring may involve fixed hydrophones, temporary deployments, vessel-source measurements, modelled sound maps, automatic identification system data, maintenance records and ecological observations. Work with qualified acoustic specialists and ensure that results are interpreted with appropriate uncertainty. Sound levels depend on depth, seabed conditions, temperature, currents and background noise, so comparisons require consistent methods.
Publish the method as well as the headline result. Transparency builds trust with operators and communities, and it allows the programme to improve over time. A modest, repeatable monitoring plan is often more valuable than a one-off study that cannot be compared later.
The commercial case: lower noise, stronger licence to operate
Underwater noise reduction can improve a port’s relationship with regulators, communities and environmentally conscious cargo owners. In areas where marine biodiversity is a public concern, a credible programme demonstrates that the port understands its wider environmental footprint. It can support sustainability reporting, concession expectations and biodiversity commitments.
The strongest commercial case, however, is operational. Measures that improve hull and propeller condition, reduce unnecessary waiting, improve arrival predictability and avoid wasteful speed changes often have direct efficiency benefits. These benefits should be calculated for the actual service, because a noise programme will be sustained when crews, operators and commercial teams can see how it fits normal performance management.
Avoid presenting URN reduction as a guaranteed fuel-saving measure in every case. Some actions are primarily ecological, some have trade-offs and some require capital investment. The right message is more credible: identify measures that are safe, measurable and suited to the vessel and port, then capture co-benefits where they genuinely exist.
A practical quiet-shipping action plan
For a port beginning this work, a sensible sequence is to establish a cross-sector working group; map sensitive locations and seasonal priorities; select one achievable operational measure; define safety conditions and exemptions; set a baseline; communicate the programme clearly; and review results with participants after one season.
Possible first pilots include a voluntary speed-reduction corridor, an arrival-planning improvement in a sensitive approach, an environmental dues incentive for verified quiet-vessel performance, or a joint hull-and-propeller maintenance awareness campaign. Start with an intervention that can be explained in one page and measured with a manageable data set.
Scale only when the programme is trusted. Build feedback from masters, pilots, agents and terminal planners into the design. If a measure adds risk, unacceptable delay or confusion, adjust it. The aim is sustained adoption, not a short-lived announcement.
A quieter ocean is part of a modern port’s environmental leadership
Ports are accustomed to managing visible environmental pressures: air emissions, water quality, waste and energy. Underwater radiated noise deserves the same practical attention. It is a shared impact of shipping and a shared opportunity for ports, vessel operators and coastal stakeholders.
Turning the volume down means using better information, better maintenance, safer operating choices and smarter incentives. It means protecting marine life while improving the quality and predictability of maritime operations. In a sector built on movement, the next environmental gain may sometimes come from making that movement quieter.
Sources and further reading
– IMO: Ship noise and the Experience Building Phase
– IMO Revised Guidelines for the Reduction of Underwater Radiated Noise