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Old 26 May 2004, 18:26   #11
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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.

HTH.
M
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Old 26 May 2004, 20:10   #12
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Here is the suggestion I reveiced from Humber:

"As the engine in a new product we have not actually tested this combination yet, however we have recently undertaken extensive tests with the 130hp Volvo in the Ocean Pro 6.3m and based on this we would anticipate that for full dive load a 3 blade 15 x 17 should be OK and a lighter load for all round performance 14.25 x 19 4 blade. We must point out that this is not guaranteed and is offered as a guide line only."

We now use the 14,25 x19" 14blade propeller provided by Volvo....
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Old 26 May 2004, 20:22   #13
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This test, which they are referring to, is documented in the latest RIB International magazine. It is quite interesting.

Michaels, is your boat an Ocean Pro or an Ocean Offshore. I just ask because an 8mtr Ocean Pro would be a long, narrow boat. What are you using it for?

I understand you fitted the engine yourself, is this correct?
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Old 26 May 2004, 20:48   #14
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The boat is a 8m Ocean Pro with 2,60m beam.
Humber took 680 for making doing the Hull modification for this engine. Since they had just done the 6,3 Volvo D3 installation we thought it was a good idea to let them make everything ready for putting in the engine. So the craft came ready with a engine well, engine supports and a finished hole in the transom. Everything sealed up with Fiberglass. We did the installation supervised by the local Volvo dealer that also checked the the installation during and before starting it up. They had no complaints at all.

We will try to measure the cavitation plate tomorrow and se where it is located compared to the keel. I do believe it is some few centimeters above.

The boat is rigged for both leisure (light load - totals 1500kg) and diving (heavy load - totals up to 2500kg). The consolle are located far back with a row of jockey seats behind it.
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Old 27 May 2004, 04:10   #15
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wow, very nice looking boat.

Hope you solve the cavitation problem. (I'm no pro at this but I would try a spacer to lower the drive)

Rene
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Old 27 May 2004, 16:49   #16
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cavitation

Hello

The cavitation plate on the leg, should always be at the bottom hull level - at the keel i mean -- the only place it 'should' be otherwise it is like running a short shaft outboard on a long shaft transom -- it goes ok, for a while then cavitates --
Question for you -- all outdrives, with the diesel inboard as far as i know - when trimmed right up, to the beaching, trailer position, whatever, never quite come above the level of the keel, as inboard owners should know, thats why i never anyway will beach the boats unless on sand/mud, as the leg will always be slightly lower, not a problem you get with an outboard, as you can raise it above the keel level.............

I would lift your leg right up, and if it comes completely clear of the keel level, then it is rightly to high, i cannot see that volvo have suddenly fixed this glitch, as with all outdrives, the other thing is of course is to put it on the trailer, and put the leg right down, the see the height of the cavitation plate?
i have seen a home job, with a new boat, and a mercruiser (fitted at home) and the smart owner decided he would raise it i bit, so he could beach as per an outboard, and not hit the leg -- all it did was cavitate --

prob no use - but there you go -- do not believe in all this 'bending' on the trailer mallarchy -- unless you are trailering a banana.

cheers
pete
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Old 27 May 2004, 17:30   #17
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Quote:
Originally Posted by seafariskye

prob no use - but there you go -- do not believe in all this 'bending' on the trailer mallarchy -- unless you are trailering a banana.

cheers
pete
Pete!
The "bending on the trailer malarkey" you are referring to may not be a problem you experienced, however that does not make it "malarkey". I was asking about the bending of the hull with a view to broadening my own horizons, not saying it was the problem. I think everyone is trying to help and probably could do quite well without your negative input. If you don't have something good to say, don't say anything at all.
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Old 27 May 2004, 17:38   #18
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Pictures from todays measures

Here are the pictures showing that there is a great distance from the keel to the cavitation plate. Aprox 43 millimeters. The extension kit are about one inch (25,4mm) There is also pictures from how the measuring was done and one picture showing the "bend" on the keel I was speaking of.

Any comments ????
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Old 27 May 2004, 18:40   #19
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Nice boat!

The leg is far too high - It looks like from the second picture that the tip of the propeller is almost at the surface.

The extension kit should make a fair bit of difference although it may not solve it completely. What are the choices if it needs to be lowered further? Can you stick two extension pieces on?
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Old 27 May 2004, 18:55   #20
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RIBase
43mm!

Time to have a look at the VP installation manual and then a discussion with whoever cut the hole for the sterndrive I think would be next.
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