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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?


James M. Cleary
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.

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 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.


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. )


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.


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?


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

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.


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.
Thanks all for the insight, and apologies for disappearing for a while. Work got busy, and then we took a wonderful 10 day trip on the boat around the Delmarva Peninsula.

At this point I am more convinced than ever it is simply cavitation. We have gotten to the point where we almost don't even notice it. I want to try a 16/11 (maybe just use the old one we had with the Perkins) and see what that does. Too bad it requires multiple short hauls to test these things!

In other news, while on our trip, I noticed black smoke coming from the exhaust, and the engine started sputtering. Shut it off and sailed into a nearby anchorage (luckily we were back in the Chesapeake by this point not out in the Atlantic). Opened up the engine room to a cloud of smoke. Turns out there was a big crack in the (brand new of course) exhaust riser, and it had spewed exhaust into the engine room until it overwhelmed the air filter and started starving the engine. Kind of remarkable that our engine room sound insulation appears to have kept it completely contained to the point where we did not notice any smell or other evidence in the cockpit or interior.

Anyway we were still about 60 miles from home, so I got some high temp gasket maker and used it to plug up the crack as best I could, and we limped the rest of the way back using the engine as little as possible. Thank goodness we had some decent wind!

Needless to say the Beta guy fixed that one right away...


Stephen Ruell
We were out again this weekend with our Beta 38 and I can't say we hear any cavitation. We tow a fairly heavy RIB dinghy which makes a lot of noise behind us and we haven't actually been out without it. Would the cavitation be loud or subtle?
I had thought our old propeller was 16RH11 but I found paperwork from when the engine repower was done by the boatyard They sent it to a prop shop that actually measured it and I was not correct. We have a 16RH12 bronze propeller.
Anyway it seems that 2000 rpm is about 5 knots and fairly quiet, 6 knots is about 2400 rpm and a little noisy. We can push it faster but there doesn't seem much point due to the noise level getting irritating. We don't have any sound insulation yet. The performance would be better if we didn't have the tow. Penoscot Bay has significant tidal current so it also effects the numbers and I am only reading the GPS over the ground numbers.
We think that the Beta 38 is fine for the Morgan 382, totally acceptable performance for us. The Beta 43 I am sure would be fine also and maybe it would be a little quieter.
For me the cavitation is quite distinct from regular engine noise. The 43 with sound insulation is (subjectively) pretty quiet on its own at 2000 rpm, and with the 17/12.5 prop anything beyond that is just pushing water and wasting fuel. The cavitation adds a bunch of decibels but I haven't tested it so can't really say anything scientific.


Jerzy Borzym
I'm ready to re power with Beta 38.
I have question for Stephen: What ratio i your transmission ?
It looks like they have two choices : 2 and 2.45 ratios.
I may have original prop 16X11


Stephen Ruell
Hi Yurek:
I think that this transmission is the TMC 60 with the gear ratio of 2.04 but will check at the boat this weekend. The Yanmar we took out was 2.03 so I figured it would match fairly well.


Jerzy Borzym
Thanks Steve:
I place order for new Beta today I give them prop size 16X11, Farron from Beta suggest 2.45 for this prop diameter.
I also ask for 6" High Rise Exhaust Elbow. My existing transmission on Perkins is 1.9
If anybody is interested I put my Perkins for sale.
Engine is in Southern Massachusetts.
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Terence Thatcher
Yurek, please keep uis informed about your repower progress. A Beta 38 looks like a good size for the Morgan, but it is a high reeving engine, compared to the 43. But it uses far less fuel art 2000 to 2200 rpm, which should be decent cruising speed. With my Perkins at 2000, I get 6 knots.but I am getting more hp at that rpm than from the Beta 38. But then, with a Beta 43, you get 30% more hp at 2000 rpm than with the 38. I admit, I don't know if hp matters, as long as one can turn the prop at the right speed. Another member said he only got 5 knots at 2000 rpm with a Beta 38. Oh, and one other thing, if you go with a 2.45 to 1 reduction gear, won't that require you to run the engine at an even higher rpm? I know they always say diesels like to run at 80% of top rpm. But Beta 38 tops out at 3600. A diesel running at 2800 rpm is going to be loud, no matter what. My old Perkins has a cruising rpm of 2000--55% of its max rpms, and it runs like a top after 42 years. I am only thinking if repowering because I don't want to rely on something that age given the type of cruising we do,
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jose santin

Running with a 2:1 gear and a 17x10 three bladed Michigan Wheel prop, my boat reaches 6 knts at 1800 RPM. ( volvo d2-40). If I had it to do again, I would go with the BETA 38.


Jerzy Borzym
I'm living on my boat. Perkins quit at the end of August, in next few weeks I swap engines. I made 80 hrs on Beta. Both engines behave similar:
Perkins 6.5 knots at 1900 rpm, Beta 6.5 at 2400 RPM. Above mention rpm's I hear cavitation noiseon both engines . In future I will try different propellers. I can reach 3600 RPM, so propeller is not over pitched.
Beta needs electrical fuel pump, engine was choking with rapid changes of RPM's.
Perkins used 0.73 gallons/hr Beta 0.83 I hope fuel consumption will go down after more engine hrs.
I think Beta Is a little quieter.
My suggestion: replace cutlass bearing when engine is out, you can pull the shaft toward the cabin.
It will be interested if we compare speeds people are reaching on their boats with shaft rpm's (not engine RPM's) and propeller diameter and pitch, maybe we find golden propeller.
My data: Beta Engine, shaft speed 980 rpm, boat speed 6.5 (clean bottom), propeller Michigan 16X11


Terence Thatcher
Thanks for the info Yurek. The fuel consumption is pretty high, given what beta claims in their tech sheets. So you must have chosen the 2.45 to 1 reduction gear. I guess I was wrong to question it.