Taming engine noise, thoughts on quieting down an engine

Discussion in 'Boat Design' started by G4s1198, Feb 18, 2022.

  1. G4s1198
    Joined: Feb 2020
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    G4s1198 Junior Member

    Greetings all. I’m looking for ideas on quieting down the air cooled engine that powers my paddle boat. It is air cooled so this will be a challenge if possible at all. Any input or ideas welcome!
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Best you can do is a sound shield. Easier to change motors. You only get two ear drums.
    Kirk likes this.
  3. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    I once had an architect give me a tour of the engine room aboard his sailboat. He'd lined his engine room with lead, explaining that density deaden sound. Since sound travels faster in water than air and faster in steel than in water, I'm not sure density is the important point.

  4. G4s1198
    Joined: Feb 2020
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    G4s1198 Junior Member

    I think most sound proofing is foam so I would tend to agree with you about the density Will. I just placed an order for some sound dampening material they use in the bodies of cars and maybe that would help stop the vibrations moving through the hard surfaces creating the sound waves
  5. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Engine inside a sealed foam box would be the best as far as sound, vibration may require special shock mounts to reduce transmission, JMHO. Of course complete sealing is impractical, because of the heat buildup, so cooling fans would have to be added- with the exhaust directed away, or to a baffle on the water surface. Switching to a water-cooled engine may be the best long-term solution?
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Not sure on the number of cylinders etc
    Do the exhaust(s) enter into a plenum? The largest source of noise will be the exhaust side, so install a well designed muffler.
    An enclosed engine box would reduce noise but as mentioned earlier, you need to provide an air supply and hence it would not really be enclosed. Some air cooled motors, (picts would be nice) will have a built in cooling fan on the crank shaft output. Yours?
    So perhaps, depending on the engine location you could build a barrier. Ie say provide a silenced intake to an engine box with a front, assuming that you are ahead of it, two sides and a top and leave the back open to permit sound wave energy to
    reflect off the inside of the enclosure and out the back.

    To absorb some of the reflected sound within the enclosure/box and potential amplification of coincidental waves, I would recommend an enclosure lined with something like Sound Down though there are others.

    Say a 1 1/2 inch (example only) thick material. The will have proper sound foam say 7/8 inch, with a high density decoupler (used to be lead) maybe 1/4 inch then another 3/8 of foam. These sound mats will normally come with a mylar
    film, like aluminum foil, to resist the foam capturing dirt or oil. Do not put a hard surface on the inside of the box as the sound actually can become louder.

    We used to build intake silencers using a 4 to 6 inch tube say 2 feet long, stuffed with coarse stainless wool. Similar to stainless pot scrubbers, as there is always reflected noise that can exit out of the intake.

    Silenced, intake, into the enclosure in the front ( possibly one on each side at the front, to provide the start of the cooling air path. A damped enclosure, and an open back to allow the hot air a non restricted egress as well as an opening to permit reflected sound waves to exit to the rear and an exhaust muffler to deal with the exhaust noise.

    I have a 2000 watt Honda air cooled gen set, not used on a boat, and if it is even a short distance away, there is very little noise.

    Pulled from the net
    Pure constructive interference occurs when two identical waves arrive at the same point exactly in phase. ... Because the disturbances add, the pure constructive interference of two waves with the same amplitude produces a wave that has twice the amplitude of the two individual waves, but has the same wavelength.
    Last edited: Feb 19, 2022
  7. SolGato
    Joined: May 2019
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    SolGato Senior Member

    If you want to get serious about this, you might want to think about taking more of an acoustical treatment approach rather than “sound proofing”.

    Sound travels differently depending on frequency. Highs are more directional than lows, and high frequencies use and transfer more energy into surfaces than low frequencies which can travel much farther since they lose less energy.

    To control sound, you can either absorb or deflect it.

    Enclosing a motor may quiet the high frequencies which have a hard time passing through a barrier, but the low frequencies will still escape.

    One of the best ways to contain low frequencies is to create what is known as dead air space. Kinda like the radiant barrier of a roof, the sound wave passes through the first barrier into a pocket of air, then hits the second barrier and is deflected and trapped until it dissipates.

    Higher frequencies are much easier to deal with as they can be absorbed with dense materials or deflected with hard surfaces.

    So, the first step would be to try to determine what frequencies are most offensive and where they emanate from.

    It’s possible you might be able to simply absorb and deflect some of the sound to reduce it to a more tolerable level without having to fully enclose the motor, cutting off its air supply.

    Being an Aircooled motor, I’m guessing a combo of both high mechanical and lows.

    Also my experience with small engines has taught me that certain parts of them like to sing.

    For example, the pull start clutch housings which are usually made of thin perforated metal. They sometimes emanate a ring that drives me nuts. A spray can of Plasti-dip or coating of spray bedliner on stuff like that can do wonders.

    Anyway, just a few things to think about. Good luck.
  8. waterbear
    Joined: Mar 2016
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    waterbear Senior Member

    Part of my job involves acoustics, so I know a little bit about noise isolation (probably just enough to be dangerous).

    I saw the picture of your boat here (Paddle wheel help https://www.boatdesign.net/threads/paddle-wheel-help.66392/page-2). It looks like the motor is inside, and there is a lot of thin aluminum on the cabin. It also looks like expensive treatments are not in the budget.

    If this were my boat I would do the following, in no particular order:

    -Add "rubber fin dampers" to the cooling fins of the motor. This is worth about 3db, which is meaningful improvement. -3db is half the acoustic energy, although you would need a reduction of -6db for it to be perceived as half as loud. (Edit: note this is not -3db off of the total sound, just -3db off of the component radiated from the motor.)

    - As others suggested, isolate the motor using vibration isolation mounts. Think of your boat like a violin. The motor is the vibrating string, and the hull, which is thin and stiff, is the body of the instrument which amplifies the sound. You must also isolate the paddlewheel bearings from the hull and any exhaust mounts, which are mechanically connected to the motor. You want the areas of the hull where the mounts attach to be as stiff as possible. Vibration isolation mounts can be as little as a few bucks a piece in the smaller sizes.

    - Enclose the motor if possible. If you do this make sure there are no air gaps - use gasketing, spray foam, caulk, etc. Sound likes to go through gaps.

    - Absorb the sound at the source. Fiberglass house insulation is the best absorber and it's cheap. I don't see it used in marine applications, maybe it absorbs water and grows mold? I would use the yellow corning rigid insulation if possible, but the soft pink stuff works just as well or maybe better. Sounddown probably has some practical advantages, but I bet it's expensive and not as effective as plain fiberglass. The lowest wavelength you can absorb is 2x the thickness of the fiberglass (when the glass is mounted on a wall), so 2 inch would absorb down to 3300 hz and 4 inch would be 1650 hz. The thicker the better.

    -Add vibration damping material to the thin panels on the boat. Ideally you would make the panels as stiff as possible to push the resonances into the higher frequency ranges, where they can be effectively absorbed by damping material. I would cover 100% of the engine compartment in dynamat type material, and maybe 25 percent of the aluminum cabin. I would buy something like this generic aluminum faced butyl mat for about $1/square foot, these use the same "constrained layer" damping technique as the sundown tiles.

    Amazon.com: Siless 50 mil (1.3mm) 52 sqft Car Sound Deadening mat - Butyl Automotive Sound Deadener - Noise Insulation and Vibration Dampening Material ( 52 sqft) : Automotive

    Hope this helps
    Last edited: Feb 18, 2022
    Will Gilmore likes this.
  9. G4s1198
    Joined: Feb 2020
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    G4s1198 Junior Member

    All great suggestions! I appreciate them all To answer the questions the exhaust exits into the stock engine muffler and from there goes through a stainless steel exhaust pipe attached to the muffler. It exits out of the roof into another muffler before final exit. The cabin is thin metal, I have insulating foam board on the walls of the cabin and cloth covering the insulation (like the cabin of a car) although in the area the engine is mounted the fiberglass hull is exposed which some of you hinted at seems to be a point at which the noise of the engine can reflect. I stuffed some foam under the plywood floor as it was hollow under the engine and that seemed to help. I like Waterbear’s suggestion of some type of fin dampener I’m going to try that and putting the aluminum sound dampener material I ordered on the exposed metal engine parts like the gas tank and the pull start housing like Solgato suggests. Waterbear’a astute observation is correct this is an ex spouse support budget build so not looking for anything too expensive. Don’t consort with wicked women boys, and if you do for goodness sake don’t marry them! Lol.
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Another material that may work in your application is a painted product called Silent Running. Marketed as a Marine product, you merely paint the product on. An advantage would be is to be able to paint areas that cannot realistically
    be covered with a thick sound absorbing material. Ie foam, etc.

    There are quite a few mastic type coatings but you would have to check to see if they qualify for marine use. ie fire retardant, toxic fumes when burning, enclosed spaces etc

    During a boat show, we ran across an aluminum boat builders from Vancouver Island who coated the inside of their boats with either this product or a similar product. They raved about it but I have never been in a boat with this coating.

    There seems to be three types of paintable (maybe more) products. Ceramic, Aluminum and Mastic.
    Google Sound proofing paint for Boat/Marine ----videos----- and you will find many vids that illustrate their effectiveness. Of note is that the coatings appear to reduce with high and low frequencies effectively
    Last edited: Feb 23, 2022
  11. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    Maybe you should consider a steam power plant. Almost silent, on my steam sidewheeler the paddles make all the noise, and the engine is silent, also no noticable vibration, because the engine only turns 70 RPM. My electric boat is also nearly silent, but not practical to go more than a day trip of a few miles.
  12. G4s1198
    Joined: Feb 2020
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    Location: Nj

    G4s1198 Junior Member

    Barry, spray on dampener sounds
    Like a helpful product! I’ll have yo check into that. Fredrosse your paddle wheeler is awesome and I love the steam power but not sure if I have the technical skill for building a steam engine. I’m sure they arent the kind of thing you can find commonly around although they are very neat!
  13. waterbear
    Joined: Mar 2016
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    waterbear Senior Member

    My understanding is this type of damping material is less effective than the foil faced constrained layer type materials on a per pound basis. That said, I would imagine spraying on the material is less labor intensive than hand applying and cutting sheets. Also, if I am a boat manufacturer I might not want to trust mystery mat from Amazon to stay adhered to the inside of my boat.

    Constrained layer materials can be made out of two sheets of anything rigid with a layer of damping material in between. There is a product called green glue that is designed to bond two layers of sheetrock into a damped assembly. A wall with two layers of sheetrock on each side plus green glue has 10db (90%) lower sound transmission than a wall with two layers of sheetrock on both sides and no glue. The glue layer itself is only half a millimeter thick or so.

    Another thing you (OP) could do is unbolt the engine and place it on a rubber mat, then run the engine and see what kind of improvement there is. You could also find a sturdy box that fits over the engine and briefly try running it with the box stuffed with insulation. I would compare three setups:

    isolated motor + box
    isolated motor with no box
    bolted motor + box

    This will tell you how much sound is coming directly from the motor vs from the motor exciting the hull and how offensive their spectra are. You can get a free spectrum analyzer (eg spectroid for android) app for your phone to compare the different arrangements. Note the phone's mic may not have enough dynamic range in the cabin (the graph may go off the chart) so the phone may need to be placed outside the door. For each measurement the phone should be in the exact same location.

    Sorry to hear about your divorce troubles. I knew a guy who got divorced in the 1960s and sadly had to pay alimony for over 50 years until he died.
  14. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    barium clay is a high end solution. It comes in small sheets about 8x5" on waxy crepe paper and is use to wrap sewer (toilet) pipes.

    I once quieted and insulated the engine box of a mid-engine 1968 Dodge van with thin foil backed pink fiberglass held in place with chicken wire and tiny through bolts. It worked so good I had to fix the heater core for cold weather!

    You could probably build a box out of thin plywood and insulate it and with a bit of care not hurt the air cooling, and the box exterior would be a handy place to store and attach do-dads and drink holders etc.

  15. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    I assume the engine is in open air. There are three places where the sound is emitted. The engine head the exhaust pipe and the sound coming out of the tubes.

    Nothing you can do with the head. It is air cooled. The exhaust pipe, you can wrap it with motorcycle/car high temp exhaust wrap. The exhaust noise, well, you can try a muffler.
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