How to quiet noisy exhaust?

[Brands01]

Hello all,

I have a Bukh DV36 diesel in my boat. It throbs at a very low frequency, and makes you feel like you're sitting inside a subwoofer when you're in the saloon. Part of the problem originally was poor engine enclosure design, which I have rectified. However, the very deep throbbing remains. My suspicion is that it is exhaust noise.

I have a 2" wet exhaust system: the water is injected in the mainifold, it then drops into a small plastic wet box/water trap. The water trap outputs into a flexible rubber hose which runs the length of the boat to the transom.

My two solutions are to run the exhaust out the side of the hull, which would give it less length to reverberate; and to install a waterlift muffler.

Very interested in people's thoughts on these two options, and any other ideas people may have to cut this noise out.

Barney

[Frosty]

Hello are you engine mounts good, not too soft and not too hard as baby bear would say?

Just ram a rag or t shirt up the pipe for a quick test, see if that stops the boom.

Could also be air intake. Put a t shirt over that too, temporary!!

[kistinie]

Be careful, by example, 7 Hz is your cell frequency, so very low freq can be extremely dangerous for long term.

To create this low end noise you need distance to get enough the space for the wave length.
So the resonance you meet travel a long distance so that you can feel it

If the engine bay air tight for the inside of the boat ?


To get the source :

As frosty said look for I/O source, Detune the intake and the exhaust by putting another tube over. If infra sound goes down or up, you have it

If nothing, use you hand, and press hard on large vertical surface panels.
You can shake one and turn it into a subwoofer as you say. That panel could oscillates, becomes a speaker

Look for, not to rigid, big surface panel that shakes by mechanical noise conduction

[Brands01]

Thanks for those suggestions. So Frosty, you mean that I should stick a rag up the tailpipe? Won't the engine stop?

And Kistinie, could you clarify what you mean by putting another tube over the exhaust / intake?

Barney

[Brands01]

One other thing, Frosty, you mentioned engine mounts. Well I suspect that my beds might be a little bit soft - 60yrs old. I'll double check them as well.

[kistinie]

You temporally length or shorten or damp with a cloth like frosty said air intake and exhaust, if problem changes a lot, you found from where.

Originally Posted by Brands01

One other thing, Frosty, you mentioned engine mounts. Well I suspect that my beds might be a little bit soft - 60yrs old. I'll double check them as well.

60 years old engine mounts are sure to replace.

Your thoughts about shortening the exhaust length go into the right direction.

The trick with a rag works well, you just have to cover inlet or outlet for a second or two, to notice if your problematic frequency disappears. When your rpm change (due to clogged outlet) your result is false, of course the frequency changes with rpm.

But I am pretty sure, any change in the exhaust duct will solve the problem. (if not the engine mounts are the responsible part)

Regards
Richard

[kistinie]

Yes Richard
Either or both:
- Engine mounts turn a walls surfaces of your boat, into a monstrous big membrane woofer. energy is no more a speaker coil, it is your engine. Getting a vibration this way means the bush degeneration of silent blocks turned into a positive solution, in the intersection, of two vibrational transfer functions. Resonance's function of your walls gets a positive peak with the Resonance's function of your motor. It seems to me you rather need 0 in this intersection.
- Exhaust tunes, at very low frequency, with your boat closed volumes, horn amplify exhaust sound, create a loop, a resonance, a tune instrument. this is passive amplification of sound. Very smart effect, not with engine, but with all audio speakers, to reduce, 10 factor, W needed on electric amplification for the same SPL (Sound Pressure Level).


This answer is -71 rep certified today.
This antiresonance seems to given by one or a minority of contributors. I have to guess, as the baroque comments always join to the -Rep are anonymous ...

Many honest thanks to them
Antiresonance always return as good free energy gift.

[FAST FRED]

First toss the small water lift device and install a new unit that is about 1 ft in diameter.

Then use one of the Vetus top pieces high enough so the exhaust drains overboard. Out the side and the ONLY noise you will hear is the exhaust water dropping into the sea .

FF

[Brands01]

So I tried the t-shirt trick with the exhaust, and no dice - the noise remains. So now I begin to think more along the lines of the engine beds not being up to the task. I've got some fun ahead of me

Still the exhaust can be the reason. But you´ll have to replace the engine mounts anyway, so go ahead.

Good luck and keep us updated.

[Frosty]

Diesels have an unrestricted intake ( no butterfly) consequently they can be noisy. Try the T shirt around the intake. Singles can be difficult to quieten

[FAST FRED]

Good thinking , there are silencers that are sort of air filters (see any small old Volvo MD1 MF2 Md3 MD7 ) for a look.

FF

[Brands01]

Well, great news. I replaced the old waterlock with a new vetus waterlift muffler, and I tried the T-shirt over the air intake. Induction noise was it! I was completely shocked at the volume caused by the intake. All I needed was to put a 10cm length of hose over the end to get rid of the really low frequency. I suppose there is just as much air going in to an engine as going out, so there is no reason why it should be any quieter.

I will try and pick up an old volvo or truck intake silencer to do the job properly.

Thanks for your help everyone!

Barney

[Brands01]

Regarding intake silencing, would attaching a length (10 - 20") of rubber exhaust hose to the air filter do the trick? Any other suggestions?

Barney