Welcome to the board Steve! There are now at least 4 of us with 31's and 450 Cummins.
In regard to your cavitation problem, I had some minor vibration on my boat, but I was never sure if it was cavitation, or just a big prop pushing water against a big rudder. I think cavitation should cause some pitting in the prop, which I didn't have. At any rate, I made some modifications to my skeg, shown in this thread
viewtopic.php?t=230
in an attempt to get a little more speed. I didn't get more speed, but eliminated the vibration entirely.
According to this, our problem is probably the skeg preventing a clean water flow, and by streamlining the skeg, I cleaned up the water flow
What is cavitation ?
This is bubbles of partial vacuum caused by excessive propeller speed or loading. This is often caused by incorrect pitch, damaged blades, or bad water flow to the propeller. The tell tail sign of this is usually pitting or marking along the face of the blade. Cavitating propellers can still give lots of thrust but the small bubbles can implode causing vibration and pitting of the blade surface. A perfectly balanced but incorrectly matched propeller can under these circumstances give the impression of bad balance. The bubbles form and implode erratically causing uneven pressure on the blades and this is what causes the vibration. The cure for this is a correctly matched propeller with as clean a view of the water as possible.
One other thing I did, on advice from Terry Compton, was change the shaft to 2". My boat, Jan 1998 build, had a 1 3/4" shaft. Terry said they had upgraded to 2" to reduce vibration sometime after my boat was built.
On the subject of props, I'm running a 24x22. The boat was delivered with a 24x24, but I had the pitch reduced to allow me to turn up rated rpm, plus a little. If you are not able to turn 2,600 rpm in the heat of summer, fully loaded and a dirty bottom, you are harming the engine. The main mechanic at BoatDiesel.com goes on and on about this.
At any rate, welcome. Hope the others chime in.
My 31 TE with the 450Hp Cummins develops a cavitation periodically when underway at anything over 18 kts. It is less noticable if I keep the tabs all the way up. The local dealer has stated that he has not heard of this problem from others. The prop has been changed to 24X24 at the suggestion of Albin since this is the current spec. (my original was 24X23)