High underwater tonal noise was detected from ocean survey ship. The source of the concerned noise was the propulsion motor excitation and its frequency range was 90~180 Hz with shaft rpm. Because low underwater self-noise was crucial for the ship's sonar operation, a variety of measurements and analyses were carried out for countermeasures. Most vibration energy from the propulsion motor was found to be concentrated on the lower part of hull from the measurement of shell structure-borne noise. Numerical simulation was based on FEM-BEM coupled analysis. Practical countermeasure was made up for reducing the underwater radiated noise through this numerical analysis. It was confirmed that reinforcement on the hull plate such as enlarging ring frame can increase the noise radiation, but increasing mass can help attenuate them. Based on the results of numerical analysis, multi-layered steel plates constrained with the sticky damping paint were installed over the shell plates and then considerable reduction of underwater radiated noise from the vessel has been attained.
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