This paper describes a project in which the objective was to reduce the underwater sound levels produced by marine outboard engines while simultaneously maintaining acceptable above-water sound levels. The exhaust systems of different marine propulsion systems were studied, with emphasis placed on those of two-cycle outboard engines.

The acoustic performance of a typical 140 horsepower (104 kW) two-cycle outboard engine was measured and recorded. Modifications were then made to the engine’s exhaust system in an attempt to reduce the engine’s operational underwater sound levels. These modifications are discussed in detail. With the modifications complete, the engine’s acoustic performance was re-examined.

The modifications to the outboard’s exhaust system were found to reduce the engine’s overall underwater sound pressure levels by 77% at 1000 rpm. Above-water sound levels were found to be slightly increased at low engine speeds and decreased at high engine speeds.

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