HomeUnderwater Radiated Noise (URN) Tech Made Simple: 2025 Update

Underwater Radiated Noise (URN) Tech Made Simple: 2025 Update

October 2, 2025

In 2023 the IMO updated guidance on how to reduce URN, and several class societies now offer “quiet ship” notations. In practice, owners cut URN by tackling cavitation at the propeller, isolating machinery from the hull, managing speed in sensitive areas, and proving results with standardized measurements.

What is it and Keep it Simple...

Underwater Radiated Noise (URN) is the sound your ship puts into the sea, mostly from propeller cavitation and machinery vibration carried through the hull. High URN can disturb marine life, so regulators and ports now expect practical noise-reduction steps.

  • The current rulebook: The IMO issued Revised Guidelines for the Reduction of URN from Shipping in 2023, think design, maintenance, and operations tips owners can adopt now. Class societies offer voluntary “quiet ship” notations. {index=0}
  • How it’s measured: Trials use the ISO 17208 series to measure radiated sound and derive source levels, your proof that a new propeller or retrofit actually lowered noise.
  • What actually works: Low-cavitation propellers (e.g., highly skewed/Kappel/CLT), wake-conditioning devices (ducts, boss-cap fins, twisted rudders), resilient machinery mounts and flexible couplings, smoother drives, and clean/low-roughness propeller surfaces. Slowing below the cavitation-inception speed in sensitive zones is often the biggest immediate win.
  • Operations matter: Seasonal slowdowns and “quiet corridors” around whale hotspots are being promoted by authorities; 10-knot advisories are common in North America and elsewhere.
  • Why owners care: Lower URN helps with port access and ESG goals, and often comes with fuel benefits (better propeller efficiency, smoother flow). A class notation (e.g., DNV SILENT, BV NR614, RINA DOLPHIN, ABS Underwater Noise) is a simple way to document performance for charters and stakeholders.
Underwater Radiated Noise (URN) Compliance — Advantages and Disadvantages
Solution Advantages Disadvantages Notes / Considerations
Low-cavitation propeller (highly-skewed / Kappel / CLT) Largest single URN reduction; improves efficiency and lowers fuel burn; durable benefit High CAPEX; needs accurate wake field data and model tests; lead time for manufacture Target cavitation inception at service speeds; pair with prop polishing for best result
Propeller Boss Cap Fins (PBCF) Cuts hub vortex cavitation; modest fuel savings; quick retrofit Effect size depends on wake quality; limited impact if other cavitation dominates Low downtime upgrade; verify clearance and hub geometry beforehand
Wake equalizing / pre-swirl duct (e.g., Mewis-type) Conditions inflow → smoother blade loading; URN and fuel benefits together Adds appendage drag; benefit is hull-specific; requires drydock fit and class approval Best on vessels with uneven stern inflow; CFD or basin test recommended
Twisted/flap rudder & rudder bulb (Costa bulb) Reduces rudder cavitation and pressure pulses; better steering at low speed Structural modifications; interaction with prop must be tuned Combine with prop upgrade for coherent wake/pressure field
Propeller surface finish & periodic polishing Delays cavitation onset; low cost; immediate URN and efficiency gains Benefit fades with fouling/damage; needs recurring service Schedule diver polishing aligned with biofouling management plan
Resilient machinery foundations (double-elastic/rafts) Blocks structure-borne noise from engines, gearboxes, pumps Weight/space and alignment constraints; engineering and class review needed Retune after major overhauls; check cooling/pipe flex connections
Flexible couplings & shaft alignment upgrade Cuts tonal noise from torsional/gear mesh; reduces vibration peaks Requires precision alignment; component costs and downtime Verify bearing loads and critical speeds post-modification
Low-noise pumps, HVAC, and VFD filtering Removes hotel/aux tonal noise, especially at low ship speed Incremental retrofit; savings are distributed across many units Specify quiet impellers, isolation mounts, and harmonic filters on VFDs
Electric direct drive / POD propulsion Eliminates gear noise; smoother torque delivery; potential URN and efficiency gains Major redesign; high CAPEX; not feasible for many retrofits Best aligned with newbuilds or deep refits; evaluate power quality and EMC
Stern fairings & flow-smoothing fillets Reduces eddies that trigger cavitation; helps prop/rudder acoustics Design-specific effect; requires hull modifications and testing Validate with CFD/basin model; combine with propeller redesign
Air lubrication / micro-bubble system Drag reduction; potential masking of broadband noise on some hulls Compressor power draw; effect on URN is case-by-case; maintenance Request vendor trials/guarantees focused on URN, not just fuel
Clean-hull & prop biofouling program Lower roughness → later cavitation; fuel and URN gains together Requires regular inspections and cleaning slots; coating selection matters Use ISO 19030 baselines to trigger cleanings by performance drift
Speed management below cavitation-inception speed (CIS) Immediate noise reduction with zero CAPEX; protects sensitive habitats May increase voyage time; schedule and charter impacts Define “quiet corridor” presets in passage plans; monitor CIS by draft/sea state
Seasonal routing / exclusion zones Targets peak wildlife seasons; operationally simple once planned Longer routes or waiting can add cost; requires port coordination Publish a company policy with charts and automatic ECDIS overlays
On-board URN monitoring (hydrophone + accelerometers) Provides evidence of improvements; catches regressions early Hardware fit and data handling; need crew procedures Log spectra by speed/load; tie alarms to cavitation onset markers
Class “quiet ship” notation path Clear target spectra; recognized by ports/charterers; supports ESG claims Trial costs and documentation; not all routes/loads covered by one rating Select notation matching ship type/speeds; plan trials during post-refit shakedown
Summary: Start with the propeller and stern-flow fixes for the biggest URN drop per dollar, add machinery isolation to kill structure-borne noise, and lock results in with clean-hull routines and speed management. Prove it with standardized trials and, if useful commercially, a class “quiet ship” notation.

2025 URN (Underwater Radiated Noise) — What’s Really Working

  • Propeller redesign first: Highly-skewed / Kappel / CLT blades cut cavitation and usually trim fuel too. Biggest single lever when paired with accurate wake data.
  • Hub & wake fixes pay: PBCF on the hub and wake-conditioning ducts smooth inflow → fewer bubbles, less low-frequency noise. Quick ROI on many hulls.
  • Keep props mirror-clean: Polished, low-roughness propeller surfaces delay cavitation onset. Diver polish after foul-release cleanings keeps results stable.
  • Machinery isolation: Double-elastic foundations, flexible couplings, and quiet pumps/HVAC remove tonal “hum” that carries into the sea at low speeds.
  • Speed below CIS: Operating just under the cavitation-inception speed in sensitive areas gives instant, low-cost reductions with predictable schedules.
  • Quiet routing presets: Seasonal slowdowns and “quiet corridor” routes around hotspots are being adopted, simple to implement and easy to brief on the bridge.
  • Prove it with trials: Standardized sea trials with hydrophone arrays (and onboard accelerometers) create credible before/after spectra for charters and ports.
  • Class notations help: Quiet-ship notations give a target spectrum and a badge customers recognize. Useful for tenders and ESG reporting.
  • Contract guarantees: Vendors increasingly offer URN clauses alongside fuel-savings guarantees, tie payment to measured sound levels at defined speeds.
  • Package approach wins: Best results come from a combined upgrade: propeller + wake device + machinery isolation + clean-hull program, locked in with bridge SOPs.
🧮 URN Upgrade Payback & ROI (Fuel + Slowdown Avoidance)
Operations & Economics
Expected Efficiency Gain (edit if you have vendor data)
Measures (toggle) & Costs
How URN helps avoid slowdowns

If quieter operation and smoother flow let you follow a less restrictive speed profile (where permitted), use the fields at left to value those saved hours. Set to zero if not applicable to your trades.

Annual fuel savings
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Annual time-value savings (slowdown avoidance)
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Net annual benefit (after added OPEX)
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Payback (discounted)
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NPV / IRR
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Notes: Results are indicative. URN measures are bought for noise reduction; fuel/time benefits vary by hull, route and port policy. Always replace defaults with vendor quotes, model tests (CFD/basin), and your actual operations data.
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