A new type of boat, the Plug-in Hybrid Electric Boat (PHEB), was developed by electrifying a boat, and it is hoped that this will contribute to the environmental issues and reduce the use of oil. The system of the PHEB, the fuel consumption, the magnitude of vibrations and the generated noise are described in this paper. Boats which run by battery have already existed for some while. However, the smallness of energy-density in a battery, as well as the unreliability due to the evaluation of the residual battery energy, has been made it difficult to be applied for a larger ship. Accordingly, a system, which can be applied to both engine-powered and battery-powered sail- ing, depending on the situation, has been developed and the system was named a Plug-in Hybrid Electric Boat (PHEB). A 22 ft-length prototype boat was manufactured, and the system has proved to have both high reliabil- ity—which is the essential advantage of an internal-combustion engine—and the quietness and cleanness which are typical features of electric boats. The results of the tests are also described in this paper.