Development of Marine Propellers with Better Cavitation Performance

Abstract

This paper deals with pressure distribution which becomes effective when the propeller works in a severe condition resulting in the generation of harmful cloud cavitation. In such a case, it is considered that a way to stabilize sheet cavity and suppress cloud cavitation is more promising than the usual way to reduce the amount of cavity. Since pressure fluctuations induced by a cavitating propeller are mainly due to time variation of cavity volume, such an approach has a probability of not only suppressing erosion but also reducing pressure fluctuations.Using a propeller design method developed in previous reports, two propellers were designed and tested at a condition where the original MAU type propeller generated cloud cavitation and erosion. One propeller had flat pressure distribution which was proved effective at a condition of no cloud cavitation in the previous reports. The other had triangular pressure distribution with a negative peak at the leading edge and a gentle pressure recovery towards the trailing edge to stabilize sheet cavitation and reduce its volume variation. The experimental results showed that the triangular pressure distribution propeller succeeded in suppressing the cloud cavitation completely while the flat one failed. However, neither cavity volume variation nor pressure fluctuations were lessened by the triangular pressure distribution although almost the same levels were kept in both the pressure fluctuations and noise.Thus two-dimensional and quasi-steady study was performed to obtain a prospect for the pressure distribution which stabilizes sheet cavitation and also reduces its volume variation due to the change of the angle of attack. Two foil sections with a negative pressure peak at the leading edge and a region of constant pressure which is lower than the vapor pressure were designed with theoretical predictions by boundary layer and nonlinear cavity flow calculations. Comparative experiments to an existing MAU type section result showed that the new foil sections much lessen the cavity volume variation due to the angle of attack. They have a possibility of reducing the fluctuating pressure amplitude to less than half of that of the MAU section in addition to suppressing cloud cavitation.