It is important for a fisheries research vessel to reduce the underwater-radiated noise. In 1995, the International Council for the Exploration of the Sea (ICES) firstly recommended the maximum noise level for research vessels to ensure the acoustic research works and to prevent the fish reaction against the noise. Since then, it has been the world standard for the design of fisheries research vessels. For a small size vessel, however, it is very difficult to achieve the recommendations because of not enough space or capacity to make sound insulations. Moreover, the Froude number (Fn) of such small vessels tends to be higher than that of the larger vessels. Since the higher Froude number of the operating condition causes the large wave-making resistance, both propeller thrust and occurrence of cavitation increase, which tends to result in the higher level of underwater-radiated noise. Meanwhile, the fisheries research-cost can be reduced by using a small vessel, because not only the construction and operation cost, but also maintenance cost become lower than those of a large vessel. It would be desirable and efficient if a small vessel could satisfy the ICES's recommendation in place of a large research vessel. In this paper, the authors propose a design method for a small research vessel to reduce the underwater-radiated noise level. Utilizing this method, a small fisheries research vessel whose capacity is 290 GT and length is 33.5 m has been designed. From the sea trials after the construction, it has been proved successfully that the underwater-radiated noise level is the world lowest class at 10 knots (Fn=0.28) of researching speed and still satisfies the ICES's recommendation even in 11.6 knots (Fn=0.33) of the high-speed region.
In the field of acoustic intensity measurement, the two microphone measurement technique by using the FFT method is widely used for obtaining time averaged sound intensity. However this method can not be applied for measuring time-dependent process of energy transfer in an acoustic field of flow noise due to its nonstationary characteristics. In this article, we propose the instantaneous intensity analysis method by using the harmonic wavelet transform. The analysis result of the data obtained by the measurement on ship reveals that the direction of flow induced sonar noise shows strong fluctuations but most of them are arrived from both directions of up and down quarters of the sonar array at high speeds.