Ship Quieting Technologies2018-10-23T09:08:35+00:00

Ship Quieting Technologies

Image of a caveating propeller.

Cavitating propeller (in a water tunnel experiment). The formation and rapid collapse of bubbles produced by a rotating propeller is the main source of underwater sound produced by ships. Image credit: US Navy.

Ship propellers, motors, and gears create sound. The sounds produced depend on numerous factors including ship type, size, hull shape, propulsion system, ship speed, and transit conditions. Unless close to a vessel, most of the sounds that ships produce are low frequency (below 500 Hz) and contribute to ocean ambient noise.  Studies have shown that marine animals may alter their behavior in response to ship noise. Multiple activities have focused on reducing ship-related underwater noise. These activities include the publication of international acoustic standards, the European Union Marine Strategy Framework Directive, and the International Maritime Organization (IMO) non-mandatory guidelines.

Drawing of no skew vs high skew propellers

No skew (left) and high skew (right) propeller blades. Propellers with a “high skew” reduce the vibration of a vessel’s body as well as propeller noise. Adapted from Wikimedia (Tosaka).

Propeller-induced cavitation (the formation and rapid collapse of bubbles) is the main source of underwater sound produced by ships. The IMO recommends that propellers be designed to reduce cavitation by appropriate selection of propeller diameter, blade number, and pitch.

The shape of a ship’s hull and the resulting wake affect propeller performance and resultant noise production.  The IMO recommends hull and propeller design be considered together to achieve noise reduction.  For example, hulls can be designed and structures added to produce a uniform wake.

Machinery onboard a vessel also produce sounds that can propagate through the hull. The properties of underwater-radiated sounds will vary depending on the type of machinery, its location on the vessel, and other factors related to vessel design. Usually, machinery that is directly bolted to the hull produce the highest levels of underwater sound. Larger machinery such as engines, turbines, diesel generators, etc. also have a greater influence on underwater sound production.

Diagram showing how sound can be produced and travel in a ship system.

Simple diagram (cross-section of a vessel) showing propagation paths of underwater sounds produced by ship machinery. Underwater sound radiates through structural (blue and green) and airborne (red) paths. Adapted with permission from noise-control.com

Selecting equipment with inherently low noise and vibration levels is one of the most effective methods for reducing machinery noise. The transmission of machinery vibration into the water can be reduced by vibration isolators and/or isolation mounts. These use “soft” or “elastic” materials (usually steel coil springs) located between the machinery and the ship’s hull. Commonly-used thermal and fire insulation materials, such as fiberglass and mineral wool, also help to reduce radiated noise.

Operational modifications and maintenance also help reduce noise production for both new and existing ships. Reducing ship speed is one operational procedure that can reduce ship noise. Regular propeller cleaning removes marine fouling and reduces surface roughness, helping reduce propeller cavitation. Maintaining a smooth underwater hull surface and smooth paintwork helps reduce a ship’s resistance and may help reduce underwater noise produced, as well as increase energy efficiency for the ship.

Efforts to moderate the effects of ship noise include the use of underwater acoustic propagation models to assess the “noise footprint” of individual vessels and to produce “noise maps” that show the contributions of multiple vessels to the local ambient noise. For example, to help inform management decisions about vessel noise in Glacier Bay National Park, Alaska, scientists estimated the acoustic exposure of humpback whales to vessel noise under a variety of scenarios. Model simulations showed cruise ship speed to be the dominant factor affecting noise exposure in Glacier Bay, with fast ships producing the highest maximal sound pressure level and cumulative sound exposure levels over a 24-hour period. (For more information, please see the Advanced Topic on Sound Pressure Levels and Sound Exposure Levels) Cruise ships traveling at 13 knots produced cumulative sound exposure levels 3 times lower than those traveling at 20 knots; maximal sound pressure levels also decreased for the slower vessels. Therefore, rather than decreasing the allowable number of cruise ships in Glacier Bay, or synchronizing the timing of their arrivals, the scientists suggested that reducing the speeds of the cruise ships, and/or otherwise quieting the ships, offered the best potential to lessen potential impacts on humpback whales.

 

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