How to quiet noisy exhaust?

Discussion in 'Diesel Engines' started by Brands01, Jan 12, 2010.

  1. Brands01
    Joined: Nov 2006
    Posts: 102
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Sydney

    Brands01 Senior Member

    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
     
  2. Frosty

    Frosty Previous Member

    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!!
     
  3. kistinie
    Joined: Aug 2007
    Posts: 493
    Likes: 8, Points: 0, Legacy Rep: -74
    Location: france

    kistinie Hybrid corsair

    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
     
  4. Brands01
    Joined: Nov 2006
    Posts: 102
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Sydney

    Brands01 Senior Member

    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
     
  5. Brands01
    Joined: Nov 2006
    Posts: 102
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Sydney

    Brands01 Senior Member

    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.
     
  6. kistinie
    Joined: Aug 2007
    Posts: 493
    Likes: 8, Points: 0, Legacy Rep: -74
    Location: france

    kistinie Hybrid corsair

    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.
     
  7. apex1

    apex1 Guest

    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
     
  8. kistinie
    Joined: Aug 2007
    Posts: 493
    Likes: 8, Points: 0, Legacy Rep: -74
    Location: france

    kistinie Hybrid corsair

    1 person likes this.
  9. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    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
     
  10. Brands01
    Joined: Nov 2006
    Posts: 102
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Sydney

    Brands01 Senior Member

    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 :(
     
  11. apex1

    apex1 Guest

    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.
     
  12. Frosty

    Frosty Previous Member

    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
     
  13. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

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

    FF
     
  14. Brands01
    Joined: Nov 2006
    Posts: 102
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Sydney

    Brands01 Senior Member

    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
     

  15. Brands01
    Joined: Nov 2006
    Posts: 102
    Likes: 1, Points: 0, Legacy Rep: 20
    Location: Sydney

    Brands01 Senior Member

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

    Barney
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.