I've been debating what to write here or not since I'd rather not post than to talk shite. (Especially with the prospect of a red blob if I get any facts wrong
Cavitation, in it's truest sense is caused by the pressure dropping below the vapourisation pressure of water at that temperature.
Now since it's the pressure differential between the front and back of the blades that drives the boat forward, you HAVE to have a pressure differential.
2nd, almost all props cavitate. On all the underwater shots of boats and you see the twirling bubbles behind the prop - That's the tips cavitating.
Where it goes wrong is when effectively the pressure differential is too high, or when the change in direction is to sharp. These are 2 different modes of cavitation, I'm afraid I can't remember the correct names.
The 3rd factor is running the prop higher as your boat can mean that air is being drawn into the prop by the low pressure areas on the blades. (This is like air being sucked down a plug hole in the bath even though there's still some depth of water left). This isn't true cavitation though, even though people usually call it that. Still following?
So, there's a number of ways of correcting it. At the design stage, mounting the drive lower would have been a good way of doing it. But, neglecting temporarily the option of installing a drive spacer, what you might want to do is reduce the low pressure areas on the prop, or at least make sure they stay under control. It's possible even one of those add-on doel fin things might help simply to prevent air being drawn in, although IMHO this would be a real band aid solution and I think they must be a liability in offshore conditions too. Might be worth an experiment though as they are cheap.
So, as a starting point, what prop do you current have fitted and what are it's characteristics.
If it's a 3 blade aluminium prop then there's a fair chance changing prop will be able to fix it. Increasing the blade area, either through bigger diameter, increased blade area ratio, or number of blades might all help. Thinner prop blades might help. Staying with the same pitch but increasing the amount of cup and rake may also help.
I've probably got some of my points a bit mixed up cos I didn't really pay attention in those lectures, in fact I think I just copied someone elses notes. More fool me.
Mercury's pages are pretty good at explaining all the theory. You can get to them somehow through mercury's site, or go to my site (http://www.mattyorke.info
), choose Reference|Propeller theory and that takes you straight to the 4 articles by mercury.