Monday, May 3, 2010

Erosive Effect of Cavitation on Propeller

Cavitation of marine propeller creates negative effect on the blades of a propeller. Besides reducing the thrust power, it also erodes or wears the surface of the blades. Not all of the propellers in cavitation condition experience this erosive effect.
Experts in marine propellers and ship propulsion explain that cavitation erosion is caused by the traveling of bubbles around the surface of blade aerofoil. When these bubbles collapse, they generate pressures that are harmful to the surface of the blades and at the same time these bubble turn into smaller bullets usually called microjets whose speed is very very high. When the microjets hit or travel along the surface of the propeller blades in very high pressure, they cause erosion on the blade surface. Continuous microjets impacts (or also called (bombardment) of the bubbles on the surface of the blades of the propellers is the cause of fatique failure of the blade surface that triggers the beginning of propeller erosion.
To prevent erosion of the propeller, naval architects and marine engineers perform cavitation testing of propeller which they have designed before they are manufactured. When the tested propeller model shows the phenomenon of cavitation, usually they will reanalysis the propeller to find the solution. Propeller designer might increase the blade area ratio of the propeller and adjust the pitch ratio. Another alternative is changing the RPM which will eventually effect the selection of main engine of the ship whose rpm greatly influences the RPM of the propeller. If the RPM of the engine is too high, a reduction gear can be installed. But naval architects will prefer to find another engine whose RPM is lower without reducing the specified power which has been determined previously from resistance calculation or ship resistance model test.
In recent years, the need for high speed marine vehicle is increasing. This is answered by ship designers by the introduction of catamaran. This type of twin hull ship still uses marine diesel engine as its driving power. To fully support the high speed, propeller designers work hand in hand with marine engineers and naval architects to design an integrated propulsion unit that has high propulsion efficiency with low risk of cavitation on  the propellers. by Charles Roring