Leading Edge Roughness-A Way to Improve Propeller Tip Vortex Cavitation

Abstract

Cavitation has detrimental effects on propeller performance causing high levels of radiated noise for Naval Ships and the effects on the internal environment from vibrations and noise could be very disturbing for Commercial Ships. Tip vortex cavitation, being the first type of cavitation to appear on most propellers, can be delayed by disturbing the vortex shedding at the blade tips. In this paper a practical method to delay the tip vortex cavitation is presented together with results from cavitation tests giving the background and some highlights from the development. The study had two objectives: 1) To find out if the inception of tip vortex cavitation could be delayed by applying leading-edge roughness or a bulb. 2)To explore the influence of Reynolds' number and gas content of the water on the inception of propeller tip vortex cavitation. Particularly when investigating the first problem the use of a full scale test object is advantageous. Accordingly, the idea of using a hydrofoil corresponding to the outer part of a full scale propeller blade was adopted. The hydrofoil was tested in the high speed test section of the SSPA Cavitation Tunnel. By varying the water speed over a wide range, full scale, as well as typical model test Reynolds' numbers, were covered. Also the gas content of the water was varied. The results confirm the relation between Reynolds' number and inception found in the literature, provided the gas content of the water is not too high. Different bulbs and roughness patterns were investigated. The bulbs were not very promising but by the most efficient roughness patterns, an increase of the cavitation-free speed of 10-15 percent was obtained. The accompanying increase of the drag was 1-10 percent dependent on the loading. This corresponds to a maximum reduction of the efficiency of the equivalent propeller of about 2 percent. The most efficient roughness configuration will be applied to a high-speed propeller in full scale.