Salish Sea Ecosystem Conference

Reducing ship-generated noise

Naval architects and shipbuilders have many tools for reducing the underwater noise produced by ships. These tools are mature and well-understood. They have been used for decades for the creation of ocean science research vessels, and silent warships. The creation of a quieter ship does, however, require a customer commitment - the reduction in noise is not free. In the case of research vessels and warships there is a clear willingness to pay for silence. But the cost of quiet is not as high as some may think. Some of the tools of ship-silencing are simple to implement in the design phase. Indeed, since underwater noise represents wasted energy, some of the silencing treatments are also fuel-saving treatments. In this presentation the author wishes to provide the audience with an improved understanding of the range of options that a ship designer has for ship quieting. The presentation will present an overview of the various avenues that are available. These include: Reduce noise generation: Ship-generated underwater noise starts as a vibratory input in some piece of ship machinery. The main actors are usually the main engines and the ship’s propellers. In some cases there may be other important sources, including auxiliary machinery or even hydrodynamic noise. In all cases one means of intervention is to reduce the original vibratory input. Simplistically this means finding machines that vibrate less. Reduce noise transmission: Once the vibration is created, it is transmitted through the ship’s structure toward the sea. The transmission path may be short, as in the case of propeller cavitation, or it may be long, as in the case of vibration from an auxiliary diesel engine. One tool for reducing the noise that reaches the sea is to impede the propagation of the vibration from the source. This can be accomplished through features like resilient mountings, or even by the choice of an alternative structural material. Reduce noise radiation: Once the vibratory energy reaches the shell of the ship, this shell acts like a drum head to radiate the energy into the sea. But, like a drum head, it can be tuned to be a good, or a bad, radiator. In a realistic ship design the naval architects will employ a mix of interventions of each sort. As an extreme example, selecting high-frequency machinery on high-impedance mounts in a sandwich-composite hull would yield a vessel that is extremely quiet. But even less-extreme mixes of techniques have the potential to significantly reduce ship-generated underwater noise. We ship designers, at least the better ones, have the tools to produce quiet ships. We just need customers to ask for them.