Abstract
The underwater sounds from the boat driven by hydrojet were compared with those from boats driven by propeller in order to verify whether the hydrojet propulsion system is acoustically suitable for fishing boats. A measurement of underwater and airborne sounds was made in Negishi Bay, Kanagawa Prefecture. It was observed that the overall sound level in the water from the hydrojet boat was about 20 dB lower than that from the propeller boat at an engine speed of 3, 000 rpm and the spectra of the hydrojet boat had little energy at higher frequencies than 1, 000cps in contrast to the spectra of the propeller boat. Considering the experimental results and studies on the auditory sense of fish which are reported in several literature, the hydrojet propulsion system will be acoustically advantageous to fishing boats, although the problem of the difference of the underwater sound between the tested boat and fishing boats in practical service remains to be solved. This propulsion system may be effective when low underwater sound levels are required.
