Originally Posted by sailingmonica
So we know what it is. The question now is, what can be done to fix it: increase or decrease propeller
size and/or pitch
I suspect the correct response is: match the propeller's ability to transfer power to the water
to the engine's ability to transfer power to the propeller
. What you don't want is excess power in the prop that cannot be transferred to the water, as that energy will go somewhere and if the power cannot be used to push the boat forward then it will be used to boil water and you'll have cavitation.
Of the two suggestions you mention, and assuming that 'prop size' = 'increase propeller diameter', this is a double-edged sword. Increasing diameter increases blade tip speed which increases likelihood of cavitation depending upon blade twist/shape, and increasing diameter increases load the propeller can transfer from the engine
to the water per rotation, and therefore usually reduces max RPM
the engine can attain. It might work out that simply increasing diameter prevents cavitaion because you've slowed the engine down or increased the power the prop can transfer to the water...
Changing pitch also directly effects the ability of the prop to transfer energy, and a similar cycle ensues.
The upshot is to match the prop to the engine and boat such that you can achieve max RPM
(no less, no more) that the engine design calls for at hull speed
(for a displacement
hull), and then operate that engine/prop at no more than max RPM (you might typically operate a diesel
engine at 80% rated max RPM), and at those RPMs you should not have cavitation problems.
Anyone have a better approach/explanation?