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Beta 43 repower - prop cavitation?

I have posted at length elsewhere about my repower with a Beta 43, but starting a new thread to solicit some advice.

Quick recap: Beta originally suggested an 18"/12" fixed three blade prop. Which we installed, with resulting very small clearance to the bottom of the skeg aperture. It was very noisy during the sea trial. So we had the prop cut down to 17"/12.5". Didn't make any noticeable difference in the noise (or performance under power).

My own research and consultation on here led to the conclusion that the noise is cavitation, which should be solved or at least mitigated by reducing prop size again. But the Beta mechanic (and he also claims his contact at Beta itself) are convinced that the noise is actually being caused by water in the torpedo tube. His theory is that the Beta is getting more horsepower to the prop and causing the stern to sit down lower than it did with the Perkins, submerging the tubes. To test this he asked me to plug up the tube and sea trial again. I tried stuffing towels in each side, and heard no difference in the noise. He thinks that wasn't good enough, and I need to get a true airtight seal on it.

The scupper drains are directly under my feet at the helm and drain directly into the tube. I can hear water sloshing around in the tube at times through those even at cruising speed. The "cavitation" noise to me sounds distinct from that, and is tied directly to engine RPM. Faster spin on the prop leads directly to more noise, even before the boat picks up speed. His explanation seems improbable to me, but I am no hydraulic or mechanical engineer so I don't want to get ahead of myself.

Is there some factor I'm missing here?
 

jimcleary

James M. Cleary
Keefer
How about you stretch duct tape over the torpedo tube openings after you stuff them with towels. Enough tape should seal the openings long enough to do a sea trial then come off easily.

What if you went to a prop shop and "borrowed" a smaller prop, say a 16 x 11, and give that a sea trial. Even if that is too small a size, it will tell you if the prop size is the cause of your symptoms.

Jim
 
I glassed over the torpedo tubes 30 years ago. That's not the problem.
I'm running a 17 inch 4 blade Variprop that cavitates so bad (like having 2-2blade props) that I never run above 1900 rpm. It was suggested a couple of haul outs ago that the rounded square shape of the prop aperture lends itself to the cavitation. I've been living with it for 18 years.
 

dave_a

Dave Ahlers
Morgan fit a 16" diameter prop with the similarly powered Perkins. They could have used a 17" prop or a 15" prop. They were pretty smart guys.
Your problem is that your engine installer doesn't want to buy you another new prop. They guessed wrong and they want you to go away.
Do not go away. Make them replace it. Jim's idea of trying a 16" wheel is spot on.
 

terry_thatcher

Terence Thatcher
I agree. At least try a 16 x11, which is what we all run. The Beta 43 is quite similar in power. And I check recently and my reduction gear (ZF 15) is 1.875.) I cannot believe it is the torpedo tube. And I feel bad for you. All that money and now this. Damn. My Perkins starts cavitating at 2100 or so, however. A higher pitched sound than the rest of the engine noise. (As I vaguely recall; I have not run the engine in 8 months. Hope it still works. )
 

struell

Stephen Ruell
We kept the old 16x11 prop after our repower with a Beta 38 and don't notice cavitation noise. The engine gets louder as you increase rpm of course but not a noticeably distinct noise like you are reporting. Of course I don't hear a lot of things anymore.

Beta told our mechanic a 17x11 would be optimum but that it wasn't going to perform that different. However I just looked at their website and they give a prop size for many of the engines, and the Beta 43 is 18x12 and the Beta 38 is 16x8 . Only 5 hp differnt wouldn't think it would be such a difference in the propeller. For the Beta 35 they say 16x11, so those sizes are wrong or must depend on something else somewhere.

I talked to Stanley Feigenbaum from Beta Marine US at a boat show after our repower about propellers and he seemed very knowledgeable about the Morgan 38 skew aperture and propellers in general and maybe you should talk to him. His contact info is listed on their website.
 

terry_thatcher

Terence Thatcher
Stephen, how do you like the Beta 38? It seems like an awfully high revving engine, for our boats. At what rpm can you get 6 knots? Is it noisy at that rpm? I have thought the slower turning 43 might be preferable, but Keefer's experience gives me pause. Mark Pearson, tho, put in a big Yanmar with success. Mark, if you read this, what size prop are you running with you new diesel?
 

terry_thatcher

Terence Thatcher
Keefer, one other thing: I recall you can't slow down, either. That is a pitch issue. Cavitation can also be a pitch issue, I have recently read. If you are running 12.5 pitch, as I said, try 11.
 

jose santin

Member
I installed a 17X10 when re-powering with a Volvo D2-40. the transmission is a ZF MA-15 .
The ratio is 1.96:1 . I do get some cavitaion at wide open throtle and 2750 RPM, but I can run at 2000 rpm without problems. Cavitation apart, the engine manual advises to select a prop size that will allow the engine to operate at WOT at 2800 to 3200 RPM.
 

mpearson

Mark Pearson
Staff member
Mark Pearson, tho, put in a big Yanmar with success. Mark, if you read this, what size prop are you running with you new diesel?
Hi Terry - Indeed, we repowered with a Yanmar 4JH45, and we replaced the prop with a feathering Max Prop, 3 blade, 16". You can set different pitches and we are set at 14". Everything runs great, quiet, almost zero vibrations.

Those poor torpedo tubes always get blamed for things ;) seems unlikely to me the tubes are the culprit.

Our old Perkins & fixed blade 16 x 11 prop used to cavitate above 2500 rpm. A very distinct sound. Reminded me of driving an old car, with poor sound insulation down a gravel road, fast. A kind of clattering noise with the gravel flying up, etc.
 
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