20 - Outline of theory of intermittently cavitating propellers

Summary

Cavitation on ship propellers has been the bane of naval architects and ship operators since its first discovery on the propellers of the British destroyer Daring in 1894. Primary interest in propeller-blade cavitation was, for many years, centered upon the attending blade damage and the degradation of thrust arising from extensive, steady cavitation. It was not until the advent of the rapid growth in the size of merchant ships in the past three decades (with concurrent marked increases in blade loading) that extensive, intermittent or unsteady cavitation appeared and was indicted as the cause of large forces exciting highly objectionable hull vibration. Efforts in the modeling of hull wakes in water tunnels date back to about 1955 (cf. van Manen (1957b)) when tests of propeller models in fabricated axially non-uniform flows were being conducted at Maritime Research Institute Netherlands (MARIN), National Physical Laboratory (NPL) and Hamburgische Schiffbau–Versuchsanstalt (HSVA). Non-stationary blade cavities were observed then but there seem to have been no notice or measurement of unsteady near-field pressures attending unsteady cavitation until the experimental work of Takahashi & Ueda (1969). They measured pressures at one point above a propeller in a water tunnel in uniform and non-uniform flow and gave a brief contribution to the 12th International Towing Tank Conference (ITTC) in Rome in 1969. Their principal results are shown in Figure 20.1, where it is seen that the pressure amplitudes increased dramatically with reduced cavitation number.