Engine Noise reduction
I am trying to reduce the noise of my engine and am considering fitting a flexible coupling between the shaft and engine flanges. I have flexible engine mounts and use a PPS Deep sea seal on the shaft with a Phenolic cutlass bearing.

The product I am considering is a R&D Marine flexible coupling www.randdmarine.com

My engineers say I should not do this as I will introduce too much flexibility in the drive shaft system and increase wear. Any thoughts or experiences?

John B
Swan 411 010

Dear John
If you have the original arrangement with a flexibly supported shaft seal the recommendation is to not put in a flexible coupling. The advice you got is correct.
Instead suggest you check if the engine has an induction silencer, and that the engine box has no slots or openings where noise can escape. The ventilation ducts need to have noise traps.
Believe your engine is a Perkins M65, this is not the smoothest one. Vibration can be reduced by increasing the weight of the engine bed, for example by filling it with lead shot or concrete.
Kind regards
Lars

Hi John,

I also plan some effort on reducing engine noise this year. I will start by adding noise insulation underneath the three floor boards in the aft cabin. Some posts on the forum suggest that this makes quite a difference. We have a Nanni, which is fairly big and caused the previous owner to remove all noise insulation from inside the engine box. I will add back in 10mm of material, and watch for noise leaks as Lars suggests, which should improve things considerably. The insulation inside the engine room is original and in bad shape, but hard to reach without removing the engine. I will see what I can do there.

Lars mentions intake insulation. Also interesting seems to be insulating the metal box covering the valve train with a heavy foil. This seems to be standard in car but not ship diesels. Then there is the exhaust system.

We have a PSS shaft seal in combination with a tailor made shaft coupling that shall reduce vibrations and therefore noise (and stop a water leak). Since the rest of the system is not insulated yet we did not hear much of a difference when we put this in place.

Christian IF 411/028

I hear for the first time of the 'induction silencer', has it to be fit on the air intake of the engine?
I have googled some images, some of the them are in the exhaust system, but it does not look ok with a water-air system.
I would appreciate to have more information regarding this topic
My engine is a Yanmar 4jh2e 62 hp.
Alternatively a water separator, like those of Vetus, would help mutch to reduce the noise?
On my boat the noise is quite low, anyway I would appreciate any system to reduce the noise.
Thank you
Matteo 47/016 Grampus

Dear Matteo
Another name is "air intake silencer", and your Yanmar engine has one at the aft end. This is the round pot, see the engine manual.
Exhausts using water injection are generally very quiet outside the boat because the water cools down the exhaust gases quickly and at the same time cuts the noise. Vetus refer to the outside.
Inside it should be checked that the exhaust hose and pot are flexibly supported so the vibrations are not transferred to the structure.
If the water that sloshes out through the exhaust is disturbing it would be possible to install a water separator in the exhaust, then only some gases will come out aft.
Kind regards
Lars

Dear Lars,
thank you very much, your advice is always unvaluable!
I can bear the low noice of the water escaping from the exhaust as long that it guarantees me the regular flow of the engine water pump....
Thank you again very much!
Kind regards,
Matteo/Grampus 47/016

Guys, thanks for the feedback particularly Lars.
I thought I knew my yacht fairly well after 16 years but!
1. Where should I find the ventilation ducts? The only ventilation comes from under the bulkhead near the shaft. See the picture.
2. What should I expect to see in a "flexibly supported shaft seal"? The shaft seal I have is the PPS as mentioned before. Is there was there anything else in the through hull shaft?
3. My exhaust pipework comes through the side of the engine compartment under the sink unit and along the side of the double bunk. It fits tightly through the holes made in various sections of wood. I have a basic GRP water injected exhaust. It sounds like this could be an area to look at.
4. When I replaced the engine some years back we replaced all insulation including fitting to the aft cockpit floor.
5. The engine is a Perkins M65 which is a large unit for the yacht, but it operates at low revs.

Dear Matteo
There are also other possibilities for keeping an eye on the flow of cooling water. A temperature alarm can be put on the exhaust hose near the elbow, or the siphon break loop water can be branched off to a visible position like the topside or the cockpit.
Kind regards
Lars

Dear John
1. Guess that not much attention was paid to engine space ventilation back in the 1970:ies.
As the engine space gets hot particularly after the engine has been stopped it would be advisable to arrange a vertical duct up along the bulkhead, opening to the bridge deck. The hot air will rise, and draw in cool air through the opening around the shaft.
If the air supply is through the lazarette there should be a duct or free openings all the way to the engine space, preferably with a noise trap at the forward end.
2. If the shaft seal is connected to the stern tube with a piece of rubber hose attached with hose clamps it is flexibly supported. The engine has flexible mounts allowing it to move slightly, and the shaft seal can then absorb the movements, the stern tube has some clearance for this.
3. It is unusual to have a GRP exhaust, usually rubber hose is used. As the water lock is likely to be to port of the engine it needs to be checked if there is a critical heeling angle to starboard when water can flow back into the engine.
Kind regards
Lars