A comparative study of underwater radiated noise from electric and conventional boats

Electric propulsion systems for marine vessels are becoming more common, particularly on small craft. While often assumed to be much quieter than their combustion counterparts, there are only a few studies that quantify underwater radiated noise (URN) levels, and the results paint a complex picture. In this work, the URN characteristics of two vessels are compared. The vessels are of the same design, but one is powered by a conventional outboard engine whereas the other has an electric outboard. The results show that the broadband source levels for the electric vessel are lower across the entire speed range tested (4-20 kn). The largest benefit is at low speed where cavitation is minimal. At 4 kn, the broadband source level for the electric vessel is 43 dB lower than for the conventional vessel. At higher speeds, cavitation becomes increasingly important, but the broadband source level remains lower for the electric boat even once cavitation is present. Furthermore, the high-frequency noise from the motor is found to be lower than in other studies, highlighting that not all electric propulsion systems should be treated equally from a URN perspective.