Identification of ventilation regimes of a marine propeller by means of dynamic-loads analysis

Abstract

In the present work a relation is established between the mechanisms leading to ventilation and the corresponding dynamic loads a marine propeller's blade is subject to during its rotation. Ventilation occurs on thruster propellers operating at high loadings and heavy sea states, experiencing continuous cycles in and out-of water. This leads to sudden thrust losses and violent impact loads, which can damage shaft bearings and gears of azimuth and tunnel thrusters. Open-water tests were performed in order to better understand the dynamic forces due to ventilation, and ultimately predict the corresponding losses. They were carried out in calm water with varying propeller submergence and loading. Two main ventilation mechanisms, depending on the propeller submergence, loading and advance ratio, were identified: (i) at deeper submergence through a free-surface vortex and (ii) at moderate submergences through the tip-vortex and ultimately the blade itself piercing the free surface. These two mechanisms can exist separately or at the same time, identifying three distinctive ventilation regimes for the dynamic loads. Unlike ventilation of surface-piercing propellers with super-cavitating profile, it was found that the tip vortex plays an important role in ventilation of conventional propellers, which is the object of the present study.