cavitation plate height?????

[fishskull]

Hi everyone, Im new to the site and need some help. I have a 1979 valco aluminum boat with a 20" transom and I am running a 1974 25hp evinrude long shaft outboard motor. At wide open throttle the boat hesitates then grabs and the engine revs high. I was told it could be cavitation so I measured from the bottom of my boat to the cav plate and it is 3 inches above the bottom of the boat ( I was told it should be even). my question is where should the cav plate be at? also how do I lower it? do I have to cut my transom down 3 inches?? any help would be appreciated.
thanks

 

[JB]

Re: cavitation plate height?????

Howdy, fishskull.

Welcome to iboats.

What is happening is prop ventilation, and it is happening because the anti-ventilation plate (often incorrectly called the "cavitation" plate) is too high.

If you do, indeed have a 20" transom and your engine is, indeed a "long" shaft (20") there is something wrong with the rigging. The AV plate should be even to 1" higher than the bottom.

I suspect that your outboard is a short shaft (15") and is rigged as low as it can go or is a long shaft (20") rigged too high.

You will not get rid of the prop ventilation without fixing the rigging or mismatch.

Good luck, let us know what you find.

 

[fishskull]

Re: cavitation plate height?????

thanks for your response, I measured from where the motor mount rests on the transom to the anti-ventalation plate and it is 18". the model number is 25403m it said it was a long shaft motor. do I cut the transom down or is there a better way to fix this without buying a new motor. thanks

 

[ezmobee]

Re: cavitation plate height?????

Here's a homemade jack plate idea to put a long shaft motor on a short transom. I see no reason why it wouldn't work for the reverse http://forums.iboats.com/showthread.php?t=321432