Different types of Cavitation
Cavitation is generated when the pressure in the water becomes lower than the waters vapour pressure. The water is then broken up into cavities, cavitation bubbles.
- The picture show a propeller of a typical RoPax at medium speed. A tip vortex is seen coming off the propeller tips, this is the same kind of vortex that could be seen from airplane wings at take-off. The vortex is generated by the pressure difference between the suction and pressure side of the blades. The water is coming over the tip from one side to the other and generates a vortex. When the strength of the vortex increases with increasing propeller loading the pressure drops and when it reach the vapour pressure the vortex starts to cavitate and also to generate more noise.
- On the blade at the top position a sheet cavitation is seen which is caused by the load variation on the blade during the propeller rotation. A variation caused by the variation of water velocities coming into the propeller disc due to disturbances from the hull and shaft line. The propeller blade have during its revolution a variation in loading that could be compared to an aeroplane flying in turbulence, all the time! The sheet cavitation is increasing the pressure pulses towards the hull.