Operational Guide · Registers & Logbooks

# Underwater Radiated Noise (URN)

A still-voluntary topic that naturally converges with energy efficiency: the same measures that reduce emissions often also reduce underwater radiated noise.

URN underwater noise marine mammals MEPC

## Operational Explanation

Underwater Radiated Noise (URN) from commercial ships can have short- and long-term negative consequences on marine wildlife, particularly marine mammals that depend on hearing for communication, orientation and foraging: coastal waters used seasonally by populations of odontocete and mysticete cetaceans, including orcas, humpback whales and fin whales, are particularly exposed.

At its eightieth session, the MEPC approved the Revised guidelines for the reduction of underwater radiated noise from shipping, in force since 1 October 2023, covering ship design, construction, modification and operation, applicable to any type of ship. The IMO approved an Experience-Building Phase (EBP) to gather information on good practices and application experience, expected to run until 2026 with a possible two-year extension.

A dedicated workshop held at IMO headquarters on 6-7 November 2025 specifically explored the relationship between ship energy efficiency and underwater noise reduction: ships that adopt energy efficiency measures also reduce their URN level as a side effect, creating a direct synergy between decarbonization objectives and marine wildlife protection.

## Regulatory Reference

Revised Guidelines for the reduction of underwater radiated noise from shipping (MEPC 80, in force since 1 October 2023): voluntary in nature, applicable to the design, construction, modification and operation of any ship; Experience-Building Phase ongoing until 2026 (possible two-year extension).

## Scope of Application

Every ship, with particular attention to those operating in coastal areas identified as critical habitat for cetacean populations sensitive to underwater noise.

## Procedure / How to Complete It

1. Assess, at the design or retrofit stage, the technical measures available for reducing underwater noise (propeller design, isolation of vibrating sources, hull hydrodynamic optimization).
2. Apply low-cost operational measures to reduce URN (speed reduction in sensitive areas, avoiding propeller cavitation through optimal engine load management).
3. Consider adopting a voluntary underwater noise class notation, where available and commercially relevant for your market segment.
4. Participate, where possible, in data and experience collection under the Experience-Building Phase ongoing until 2026.
5. Monitor the synergy between the energy efficiency measures already adopted for the CII rating and their positive spillover on URN reduction.

## Practical Example

Application example: a ship that regularly transits a coastal area identified as a seasonal habitat for humpback whales voluntarily reduces speed on that stretch of route, achieving both a reduction in underwater radiated noise and a side benefit in fuel consumption.

## Real Cases

The November 2025 IMO workshop on the relationship between energy efficiency and URN formalized an observation already known to the most attentive operators: the same measures adopted to improve the CII rating (engine load optimization, speed reduction, propeller maintenance) almost always produce, as a side effect, a reduction in underwater radiated noise, offering an additional argument for their adoption.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
CII energy efficiency measures and URN reduction treated as completely separate programs | Failure to recognize the direct synergy between the two objectives, with possible duplication of effort | Integrate URN impact assessment into the same energy efficiency initiatives already under way
No consideration of coastal areas sensitive for marine wildlife during passage planning | Potentially avoidable impact on cetacean populations in areas known as critical habitat | Consider sensitive coastal areas identified in passage planning, assessing voluntary speed reductions
Propeller cavitation not monitored as a primary source of underwater noise | Failure to identify a cause easily correctable through engine load management | Monitor operating conditions that favor cavitation and manage engine load accordingly

## PSC Observations

The topic is not subject to PSC verification since the guidelines are voluntary in nature; any future adoption of mandatory requirements could change this scenario in upcoming regulatory cycles.

## Operational Tips

- Integrate URN impact assessment into the same initiatives already under way for energy efficiency and the CII rating.
- Consider coastal areas sensitive for marine wildlife during passage planning, assessing voluntary speed reductions.
- Monitor operating conditions that favor propeller cavitation, a primary and easily correctable source of underwater noise.

## Checklist

- Technical URN reduction measures assessed at design or retrofit stage
- Low-cost operational measures applied (speed, engine load management)
- Voluntary URN class notation considered where commercially relevant
- Participation in the Experience-Building Phase assessed where possible
- Synergy between CII energy efficiency and URN reduction monitored

## FAQ

Are the IMO Revised Guidelines on URN mandatory?
No, they remain voluntary; they have been in force since 1 October 2023 and cover ship design, construction, modification and operation, applicable to any type of ship.
What is the Experience-Building Phase for URN?
An experimental phase approved by the IMO to gather information on good practices and application experience of the guidelines, expected to run until 2026 with a possible two-year extension.
Why do energy efficiency measures also reduce URN?
Because factors such as engine load optimization, speed reduction and propeller maintenance, adopted to improve the CII rating, also reduce the vibration and cavitation that generate underwater noise, creating a direct synergy between the two objectives.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

Operational Energy Efficiency in the Engine Room CII (Carbon Intensity Indicator)

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