Model scale investigation of the effect of different speed reduction strategies on cavitating propeller radiated noise

The continuous development of design tools and analysis procedures has allowed propeller performances to increase progressively during years following more and more stringent design requirements. High propeller efficiency and avoidance of erosive cavitation are considered nowadays basic requests for a well designed propeller. However this may not be enough to meet the requirements of high added value ships such as passengers ships, mega yachts, research vessels, naval vessels etc. These kinds of ship need a propulsion system not only with good efficiency but also with low levels of vibrations and noise transmitted inside the ship and with low underwater radiated noise. Moreover, the above mentioned ships often work in a rather wide range of operational condition. Actually, the possibility to operate at reduced speed regards almost all kinds of ships, due to the increasing attention to fuel consumption and exhaust gas emissions, and the related tendency towards slow steaming. Speed reduction may be obtained in different ways. In the case of ships equipped with CPPs, in principle it may be achieved reducing RPM at constant pitch or reducing pitch at constant RPM; the latter is usually preferred when a power take off is present. This results in working conditions far from design ones for the propeller; Cavitation and related radiated noise may increase significantly in similar situations, despite the speed reduction is felt to be generally beneficial for the purpose. In this work, the effect of different speed reduction strategies on cavitation and radiated noise is studied by means of cavitation tunnel tests of four CPPs including both single screw and twin screw configurations.