Control volume determination for submerged waterjet system in self-propulsion

Abstract

The submerged waterjet is a novel propulsor designed for surface ships which is highly integrated with the aft-hull bottom. As the difficulty in measuring net thrust directly, the momentum flux method is suggested as the analysis procedure for ships propelled by submerged waterjets. This paper numerically investigates a high-speed displacement ship propelled by two submerged waterjets at 0.12 <Fr < 0.41 in model-scale. The control volume of the submerged waterjet system is firstly defined according to its working-principles. For the typical case here, the capture area of suction streamtube is appropriate to be located around 2.0–4.0 duct inlet diameter away from the duct inlet, while the capture area of jet streamtube is recommended to be 0.50 duct outlet diameter away from the duct outlet. The suction streamtube shows obvious contraction along inlet-direction, while the jet streamtube is much closer to an equal-diameter tube along jet-direction and its area keeps almost constant with Froude number. At 0.20 <Fr < 0.41, the jet system thrust deduction fraction hangs around a small negative-value for an appropriate control volume, showing small difference between gross thrust and net thrust of the submerged waterjet system in this paper.