Noise transmission through aluminum hull

Discussion in 'Boatbuilding' started by Deering, Dec 12, 2018.

  1. Deering
    Joined: Feb 2005
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    Deering Senior Member

    As I’m wrapping up the hull modification and repowering project for my aluminum catamaran, one of the issues I’d like to work on is noise transmission. The aft deck above the engine compartments is really loud. I’m also getting a fair amount of noise going through the hull.

    The original build had the engine compartment lined with acoustic foam, probably SoundDown or similar. It worked pretty well and I plan to replace it. My engines are on soft mounts. I’m using an Aquadrive thrust bearing system to decouple the propeller shaft from the transmission. I have a water lift muffler that meets manufacturer’s recommendations, connected via silicone hose. In short, I think I’ve done everything I can do to isolate mechanical vibration from the hull.

    Thoughts on how to dampen the noise transmission through the metal hull?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    When you say loud...do you have a decibel reading for the actual value??

    It is most likely because you do not have ER fire protection, such as found on commercial vessels. Commercial vessel must comply with fire regulations and as such the ER is lagged with a minimum of A30 fire rating insulation (at least on engines over 750kW if I recall correctly). This does the job of fire protection and noise reduction too. But you can also source those nice matt damping batts that you can add to the hull plating between frames/stiffeners. These help significantly to reduce the noise inside the bear metal noise box of a hull.

    So, i would add fire insulation - it is also nice to have for safety if anything else.

    Then make sure your air-in and your air-out are not contributing to the issues. The air-in is often the culprit. Makes a lot of noise and vibration if poorly arranged.
     
  3. Barry
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    Barry Senior Member

    In order to get the most of any sound attenuating program you have to deal with all of the basics if you want an effective job.

    1) One of the main contributors to cockpit noise is the fact that the engine hatches are not sealed into this area. You should install a soft gasket around such openings. Even a small crack
    can permit the propagation of sound and this is a simple fix. As an experiment, turn on a radio in an adjacent room and shut the door put a towel across the lower opening, note the sound level, then crack the door about 1 inch. This small opening will increase the sound dramatically.

    2) Your should install a silencer to the intake and outlet openings for the engine ventilation and engine combustion air. On some of our aluminum boats, we would take .80 inch thick aluminum sheet and custom fab boxes at the intake openings. These boxes would have a larger cross section (about 30% ) to allow for internal losses. Into these boxes we would
    install stainless steel mesh, fine, similar to the ss pot scrubbers that you have to clean pans in the kitchen.

    3) In the engine compartment the obvious solution is an application of Sound Down or equivalent product. We have used as thick as 1 1/2 inches foam with the high density decoupler and mylar inner surface. This surface has to be exposed to the engine as it needs to vibrate to absorb the sound pressure waves. (one manufacturer that I had seen put this product into an engine hatch and then put a covering of thin sheet aluminum over the entire foam creating a sound reflective surface, not very effective)

    4) as an option to the above and we have not tried this product is a ceramic paint on coating that absorbs noise. We have never used it but there was an aluminum boat builder I think either on Vancouver Island or maybe around Vancouver itself that used it and they said it worked great. You just painted it on. It was a thin coating and the you tube video was rather surprising as to the amount of sound that the ceramic coating would reduce. The ceramic was suspended in the paint film. I have looked to see if I could locate this video and have not but if you google
    Ceramic Aluminum Coatings for Boats you might find it. Alternatively, contact Western Marine in Vancouver BC Canada and ask them about the coating.
    (note this is a different coating than a thick visco-elastic coating that can reduce sound)

    As Ah has stated, just ensure that the fire requirements are met

    I will check some hard files to see if I can dig out the name of the ceramic coating material that I referenced above. Not sure if I have it anymore.
     
    Last edited: Dec 12, 2018
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Yep...works a treat. Used it on CTVs.

    A typical one is here, as I can't remember the one previously used - it was a while ago:

    The key thing is, where is the location of the fwd ER wtb...is it inside the cabin/deckhouse?...or...now with the extended hull, this wtb is aft of the deckhouse?
    If inside....very hard to prevent the noise/vibration path, all you can do is mitigate as much as possible.
     
  5. Barry
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    Barry Senior Member

    You will have stringers and frames in the engine area and perhaps they are not always at right angles. So measuring the Sound Down to fit snugly in between these can be time consuming.
    The trick is to get some thin strips of wood or thin semi rigid cardboard (shoe box stiffness) about 3 feet long by 1 1/2 inch wide by say 1/16 to 1/8 inch thick.

    if the opening between the 4 sides are 10 inches by 9 inches by 8 inches by 7 inches, (exaggerated) cut 4 pieces of the strips about 1/2 an inch shorter.. Put the 9 1/2 strip flat on the hull against the 10 inch side, and take the 8 1/2 inch strip and push against the 9 inch side and glue the joint, continue around the perimeter. The glue sets in seconds, pull the glued four sided
    strip piece out, lay it on the Sound Down and mark and cut it.
    The easiest way that we have found to cut this material is a simple jig saw with a very fine blade, safer than utility knives or reciprocating twin blade knives.
     
  6. Deering
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    Deering Senior Member

    Barry, thanks a ton for all of the great advice! Engine hatches are gasketed. Should be easy to fit the Sound Down between structure - it’s all pretty symmetrical in that area. I’ll explore that ceramic paint further.

    The engine ventilation hoods are still from the original construction. Interesting idea on the mesh inside them. Maybe I can retrofit something into them. Because the new engines are considerably smaller than the original, both combustion and ventilation air requirements have decreased so I can add a bit of obstruction without penalty.

    Ad Hoc - the engines are still in their original compartments, which are in the cockpit just aft of the cabin. But they are pretty much open to the rest of the extended hull, which gives it a lot of space to reverberate in, like a resonance chamber.

    One possible culprit might be the exhaust. I’m porting the wet exhaust into the tunnel. I have water lift mufflers but I’m suspecting some of the remaining sound is bouncing around in the tunnel. Not much I can do there other than try to dampen the vibration in the hull plating. I wonder if that ceramic paint would be helpful in that area?

    Good suggestion on the dB measurement. I can do that.

    Greatly appreciate the advice.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Aaahh....then i woudl suggest adding an aft peak, and lag it.

    Oppss...most people think this is the best way, as there is no obvious exhaust fumes. But it it is the worst. The tunnel is like an echo chamber.
    We always exhaust outboard side, never inboard side.
     
  8. Deering
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    Deering Senior Member

    Hmmm. I believe you might have been the one who told me that the tunnel was a good option. Oh well. It’s where it is now, so I’ll need to mitigate the best I can. The other drawback to tunnel exhaust is that I can’t visually monitor the exhaust to see if cooling water is flowing and there aren’t other visual cues of problems.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Are you sure??....maybe i mistyped and wrote "..do.." instead of "..don't..."...? Sadly I do that too often when i type quickly..
    It is not what we recommend at all. We never do this on any of our catamarans...

    Thus, if i did mistype or other, i'm sorry for the bum steer. :(

    Exactly...that's one of the main reasons we tell out clients too...but they think exhaust fumes may be an issue for the passengers boarding...which it never is as the boarding locations are always sited farther away !


    EDIT Update:

    Aahh...i see here.

    Now I understand the reason for the 'bum' steer. I made an incorrect assumption, and didn't clarify with you - in hindsight I should have done.

    The exhaust I show in the thread, is located almost at the transom and the raft (deck structure) is absent between the hulls in this location. Thus there is no "tunnel" for the nose to reverberate around. (as I noted... if the exhst is far aft...). Like this:

    upload_2018-12-26_9-34-58.png

    If the exhst is more fwd, well "inside" the tunnel region....er....yes...it will be an issue. Thus my mistake in assuming the location of your engine exhst would be equally as far aft post extension. They aren't.....my bad. Sorry for not seeking further clarification from you.
    Lesson learnt - can never ask enough clarification questions!!
     
    Last edited: Dec 25, 2018
  10. Deering
    Joined: Feb 2005
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    Deering Senior Member

    The expectation was never that this boat was going to be silent - it’s a smallish, lightweight aluminum boat after all, and it’s designed to be a hybrid between a yacht and a working boat. Just want to keep noise to an acceptable level. I think there are a number of sources to deal with, you and Barry pointed me towards several, so it could well be that the exhaust noise is only a minor contributor.

    I extended the cabin roof aft about 3 meters over part of the aft deck which includes the hatches for the engine compartments. Not enclosed, just an aluminum roof. I suspect that’s a large part of the noise. Once the engine rooms get quieted down that may resolve it. My next chore is to order Sound Down and install. Are there other comparable products I should explore?
     

  11. Ad Hoc
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    Ad Hoc Naval Architect

    Aahh...just notice (hidden into my own text)...you're asking what is an aft peak:

    upload_2018-12-27_9-2-54.png

    Using the same image as previous post, but with additional info. Ive shown typical locations of the two aft most WTBs. The fwd engine room and aft engine room WTB.
    You'll notice the fwd ER WTB is not under the saloon...a major source of noise and vibration into the accommodation areas.
    Also the after most WTB, which is the aft ER WTB, is also called the aft peak if this is the last WTB in the hull, as it goes aft. This is reduce the volumetric size of the ER, for damage stability reasons. It also provides a barrier between the steering gear compartment too; which is often sited in the aft peak. Whether this is a waterjet or prop driven boat. Thus having this additional WTB will allow you to lag the whole ER much more easily too.
    I would recommend using fire rated insulation anyway, as it doubles up as a good sound barrier. Don't, place it directly onto it, if there is a small air gap, circa 50mm (check with each supplier) this improves the fire/thermal rating but also helps with the sound attenuation too. If you sprayed the ceramic paint onto the hull as well..that would help double up even more. More money I know....but improves the situation - if it is an issue you are unhappy with.
     
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