Squealing engine/drive train noise

[Christian W.]

I have a 20hp Bukh in my ship, with a PSS seal. Although the engine is at least 20 years old, service and upgrading of parts is up to date. Hitherto running like a sewing machine, but on going back into the water after a couple of weeks on the hard, at mid-revs a penetrating squeal emerged, only to disappear as I dropped the revs.

I have tried:

<replacing the alternator belt, from which I've had a similar noise in the past caused by slippage

<Running under load without alternator belt - same noise

<replacing transmission oil

<checking and cleaning bellows and mating faces on PSS, no problems with dry running

<Diving down and checking on propeller shaft anode - no problems, not loose.

<running engine unloaded through entire rev range - no problem, no noise.

My boat is steel and resonates any such noise so that it's impossible to pinpoint the location of the sound source, but I suspect it is from the gearbox aft. Would it be possible that it could come from the shaft bearing aft of the seal?

The noise creeps in as I increase revs, and likewise dies down as I reduce. It is very loud and piercing, not especially high pitched, and most like the sound of a slipping alternator belt.

At all other times I have loved and understood the engine.

Any clues would be much appreciated - Thanks!

[Jim Cate]

Could possibly be something (a bit of fishing line...) jammed into your cutless bearing. Perhaps this is what you are calling the shaft bearing... and it could indeed be from that. You might try rotating the shaft manually, see if there is unusual drag or if you can detect any hint of the screeching noise.

Good luck,

Jim

[shakey doug]

Have you replaced or had the Prop repitched? Could be a Singing prop.

[gamayun]

Interesting. The same thing happened to my engine for the first time last Saturday. I had run her hard (but not excessive) in the morning towing a Cal20 to get us to a race start on time (long story....cops had shut down the Bay Bridge after shooting at a guy -- right in front of the marina -- who had stolen one of their vans, and so my crew was stuck in traffic for over an hour). The engine was fine on the way over, then started this sound on the way home that night when wind had died. I would rev down and it would be fine for about 10 minutes or so after I had throttled it up again. That went on for 5 or 6 times before we got back to the slip. It seems to be emanating from the PSS shaft seal and was wondering if it might be the transmission. I was planning to try to replicate it at the dock and try to get my ear close enough to isolate. Was also going to check belts, change transmission fluid, basically everything on your list, too, but haven't been back to the boat to do that yet. Curious to hear what others think and what you figure out, Christian. I'm planning to dive the boat next Saturday, but don't think there's anything stuck in the prop or something wrong with the zincs.

[a64pilot]

You have eliminated the belt, and since it doesn't do it in neutral, most likely have eliminated the engine I believe. That leaves transmission, unlikely I believe, but if it was some kind of bearing in that I think it would be getting hot.
Kinda leaves the cutlass and the shaft seal, or I guess prop

[Stu Jackson]

I got a call from a friend in San Diego some time ago.

"I've got this weird sound coming from the back of the boat. Sounds like a squealing sound. What is it?"

"Have you had any work done on the boat recently?"

"Yes, they put in a new cutlass bearing."

"Oh, then you must have gone a ground."

"How'd you know..."

CB was my first guess for your issue, too.

Good luck.

[Christian W.]

Originally Posted by shakey doug

Have you replaced or had the Prop repitched? Could be a Singing prop.

Shakey Doug - I had just had the prop recoated with PropGold - would that have such an effect?

I forgot to mention that the problem doesn't happen in reverse gear.

[Christian W.]

Gamayun I read an article in one of the Brit. magazines, by a dripless seal mechanic, that if you don't reflood the seal after being on the hard it will squeal.

Good luck too.

[gamayun]

Originally Posted by Christian W.

Gamayun I read an article in one of the Brit. magazines, by a dripless seal mechanic, that if you don't reflood the seal after being on the hard it will squeal.

Good luck too.

Yeah, I had heard this was called "burbing" the seal. When the sound was happening, I had checked for bubbles in there, thinking it was some kind of cavitation going on. Nothing had changed with the boat -- I have not had it on the hard and have not grounded it.

Actually, now that I say that, I wasn't able to replicate the sound after I burped the seal. I'll be running it again next Saturday and see what happens. My plan is to haul the boat soon and replace the seal and cutlass bearing, which might just be 15 years old or older. Maybe it has sprung an air leak and bubbles are building up inside the seal....? I sincerely hope it's not a transmission issue!

Let me know if you figure it out. I am interested in hearing what you find.

[shakey doug]

Most PSS & rubber bellows type seal require ''Burping'' after dry dock or long periods of hard astern work, generally it will be a squeal with PSS type or a drone with the rubber bellows type.
The new prop coating should not cause the problem. The blade edges should be finished to a wedge shaped profile try to avoid a square edge

[Rapanui]

You did not mention if you replaced the cutless bearing or pulled the shaft on the hard. If so, that could be the cause. Had to trouble shoot an extremely loud squeal on a 52m patrol boat. Turned out that the lubricant used to install the shaft (Fairy washing up liquid) had affected the cutless surface similar to rosin on a violin bow string with a similar result. It did eventually die away.

[Christian W.]

Problem solved!

On a hunch as well as out of desperation I dived down and removed the cone-type propellor anode, and bingo, no noise!

The female zinc fits over and is attached to the male part by means of an Allen screw. I had tightened the screw but on removing the zinc found some electrolysis corrosion on the inside of the zinc cone, which suggests vibration set up between the two (lucky them!).

I also more accurately tracked the loudest level of noise by means of a piece of pipe touching parts of the transmission box and cutless bearing at one end and my ear on the other - very effective.

Many thanks to all and I hope this helps.

[gamayun]

Yay! An easy fix, too

I don't have this type of zinz, but I will try the pipe trick to track the source of the sound, if it comes back.

[shakey doug]

Same piece of hose you use for tuning the Carby? LOL.
Great find, well done.