Cavitation

[Westering]

Hi!
Sovereign 32 - new Yanmar 40HP replaces Mercedes 42HP (overkill I know but the engine was a bargain). New matched 2-bladed prop replaced the other 2 blader. Result: cavitaion with noise and vibration in ahead.
Local advice was to fair the leading-edge of the rudder, which I have done with some improvement but not much. (The keel is very long fin with skeg - almost long keeled really - small cut-out for prop.
An acquaintance with same boat though slightly different rudder had a 30HP engine but fitted a 3-blader - he has no problems. Should I do the same? - though I'm reluctant to accept the drag as I race a bit and have a very good centreing prop brake! The noise is annoying though and I worry about the vibration causing wear somewhere(?) and we are going to do some extended cruising this summer...
Suggestions wlecome - though not too technical please or you will lose me!

 

[Cruiser2B]

How does the pitch of the new prop compare to the old? If the engine will take it, maybe you should try the old prop.

 

[gardenshed]

if changing prop, try a feathering one, 2 or three blade. You get the performance of the fixed prop, less drag (for your racing) and you can alter the pitch to get a good match between the power characteristics of the engine and the drag from your boat.

I've tried different pitchs on my prop but not had cavitation. Too big an angle and the engine just runs out of puff at 2000 revs, too little and you rev to 3000 but the boat doesn't go anywhere fast, so the cavitiation may be either problem with the blades (imprefections on the leading edge) or another problem related to water flow around the cut out for the prop.
After changing the engine and gearbox is the prop in the same position or is it closer to the aft edge of the keel?
Lots of hassle every time you want to make a change! you have my sympathy

 

[Pye_End]

Need to look at the prop speed as well as the prop configuration.

Cavitation can be improved by increasing the surface area (or reducing the prop speed!). If you do not want to go for a 3 bladed then a bigger diameter will do this, even if it means reducing the pitch a tadge.

Rudder vibration is another issue though, and possibly harder to solve. How do the 2 props and 2 prop speeds compare?

 

[pampas]

I would imagine from your description that the old prop had a few degrees of lay-back with none on the new thus reducing the distance between the leading edge and the cut out? reuslting in poor laminar flow to the prop.