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Deadrise Inboard Motor Soundproofing

Old 03-01-2021, 06:16 PM
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Question Deadrise Inboard Motor Soundproofing

I bought a robbins 29 a few months ago and its in great shape for her age. Its a rare boat, a Chesapeake deadrise style boat with an inboard 350 gas volvo. Anyway, I'll get to the point. Its loud as hell. Does that surprise me? No. Its a 5.7 v8 thats straight piped and ive spent plenty of times on similar boats. What im wondering is does anyone have any tips or recommendations for ways or products to soundproof the boat. The engine is probably about half above the deck and half below so as a result it has a huge engine cowling. The engine cowling has no insulation or soundproofing lining inside of it whatsoever and im thinking about putting something like that in it. Im wondering if people think that will actually make a difference? I dont want to spend the time and money if people think it will make no difference. Obviously not the end of the world as i knew what i was buying and love the boat but just wanted some input from people who have gone through something similar. And advice apreciated. Thanks.
Old 03-01-2021, 06:40 PM
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I've set up a few inboard boats, some diesel, some gassers. There are three sources of noise on these things: 1. Exhaust noise. If no mufflers, tailpipes will be loud. Mufflers do help. Really. 2. Engine noise from the engine itself. On a gasser, usually pretty quiet unless at high power setting. If running it hard, air intake noise can be very loud, only then will you get much advantage of engine box insulation. Actually better to seal any gaps in engine box, as sound "leaks" cause more noise than lack of insulation. 3. Prop tip noise. On plane many boats as the prop tips pass the hull, a pressure wave causes hull to vibrate. Worse if tips are close to hull, big keel in front of prop, or running fast.

You really need to figure out where the noise is coming from. Then you can make sensible fixes.
Old 03-01-2021, 06:50 PM
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Thanks a lot for the input. From what i can tell its noise from the engine itself. Especially since it gets pretty loud before you hit plane and at not super fast speeds. And recommendations for noise specifically from the engine? Thanks.
Old 03-01-2021, 07:05 PM
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If there are gaps around engine box, take a tote full of towels and get them wet. Shove towels in any gaps and drape them over and deck hatch gaps. Quick dirty and cheap way to see where the sound is coming from.

Also be sure sound is not coming from right above prop on hull. Might sound like engine, but it could be prop. Bring a helper to drive boat while you sniff around.
Old 03-01-2021, 07:24 PM
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ok will do. Thanks for your help!
Old 03-01-2021, 07:26 PM
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Soundown makes several products for sound deadening. Product is about 1 1/2” thick so make sure that you have room. I’ve used 3/4” fiberglass with foil covering that they use for ac ducts that works surprisingly well for sound deadening
Old 03-01-2021, 07:28 PM
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Ok great. Did you put it only in the engine cowling or in other spots as well? and did you have any issues with heat from the engine?
Old 03-02-2021, 05:36 AM
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I know these boats well and coincidentally, the designer/builders daughter (who built boats with him) actually works in our office.
Considering that the boat is a gasser, I'm pretty confident you're hearing exhaust noise and not sound from the mill itself. Her bottom and driveline geometry (and assuming she's propped properly) give me little cause to believe you're getting prop feedback.
Another thing that makes me believe it's exhaust noise is that it gets worse right before she planes off. My thoughts are you're getting sound reverberation off the wake she makes at the stern at that point of speed. Our old gasser did the same thing.
Getting mufflers in will be tight, and a pain in the butt to do. You'll probably need a smaller, ski boat style muffler to fit, but dollars to donuts, it's exhaust you're hearing..
Old 03-02-2021, 05:41 AM
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Yeah that makes sense. Probably just exhaust noise going throughout the hull thats getting pretty loud? ill look into some mufflers and see what i can find.
Old 03-02-2021, 06:04 AM
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An easy way to see if sound deadening will make much difference on the engine compartment is to put a comforter over the outside of the engine housing/deck and see if it make any difference.
Old 03-02-2021, 06:16 AM
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I purchased 1" Vinyl Foam Composite with the adhesive from Soundown. I got 1 sheet for each engine box and had some leftover to double up on the sides, etc. The adhesive works great on sides but anything suspended you will need to use the aluminum hanger pins that they sell or some other screw option. The sheets are heavier than you think. They call it mass loaded vinyl but it weighs like lead.

It helps a lot but you won't be reading or whispering to each other. But in my case me and the Queen are each sitting on a 5.7 300HP gas engine with exhausts 12' away so you need to manage expectations.

Picture of the boxes for reference


Old 03-03-2021, 08:09 AM
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Originally Posted by Luke459 View Post
Yeah that makes sense. Probably just exhaust noise going throughout the hull thats getting pretty loud? ill look into some mufflers and see what i can find.
Corndog was spot on in pointing to the lack of a muffler and gaps around the engine box as major contributors to the noise. You'd be surprised how much difference just sealing the engine box up tight can make. I thought mine was pretty good but when I replaced the old gasket with slightly thicker and wider neoprene that made for a very tight seal, it had a very dramatic impact on the sound level. Using the heaviest thickness of the Soundown sheet to line the box would also make a very big difference. Another thing to check out is the engine air intake. If there is no baffle, that would allow lots of noise to leak out.

Don't know how much space you have aft of the engine but the waterlift style mufflers do more to quiet the exhaust and may be easier to install if you cannot get to the exhaust pipe under the deck.

It is really worthwhile to quiet your boat because loud noise causes lots of fatigue even if you are not completely aware of it.
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Old 03-03-2021, 08:13 AM
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Thanks! Im looking into all three of those things. I think im gonna start with some thick soundown in the box and a good seal around it. If its still annoyingly loud ill move on with some mufflers. I wouldn't have room for waterlift as the pipes are straight from the engine out the back but could do some in line mufflers. Thanks for your help.
Old 03-03-2021, 08:17 AM
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Originally Posted by Luke459 View Post
Thanks! Im looking into all three of those things. I think im gonna start with some thick soundown in the box and a good seal around it. If its still annoyingly loud ill move on with some mufflers. I wouldn't have room for waterlift as the pipes are straight from the engine out the back but could do some in line mufflers. Thanks for your help.
Don't forget to check the air intake. You may need to build a baffle for it and line that with some of the Soundown sheet. That could be a major source of engine noise too.
Old 03-03-2021, 08:21 AM
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Ok great will do. Thanks again for your input.
Old 03-03-2021, 08:29 AM
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If you are running straight pipes with a v8 gasser, it will be loud out the transom. If you can fit in line mufflers, that will help a LOT. Another thing that can be done is to combine both exhausts with a Y-pipe and use a single larger muffler and one tailpipe. The way a v-8 fires unevenly from bank to bank makes this arrangement easier to muffle.

I would not put a lot of effort into the sound-down. Not much sound actually goes through solid wood or FG. Most sound comes out through LEAKS. I have done the sound down, and it made a little difference. Closed up the leaks and it made a BIG difference.
Old 06-11-2021, 05:46 AM
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Originally Posted by Corndog38 View Post
If you are running straight pipes with a v8 gasser, it will be loud out the transom. If you can fit in line mufflers, that will help a LOT. Another thing that can be done is to combine both exhausts with a Y-pipe and use a single larger muffler and one tailpipe. The way a v-8 fires unevenly from bank to bank makes this arrangement easier to muffle.

I would not put a lot of effort into the sound-down. Not much sound actually goes through solid wood or FG. Most sound comes out through LEAKS. I have done the sound down, and it made a little difference. Closed up the leaks and it made a BIG difference.
Hate to jump in here but I have kinda the same problem only with a older Isuzu diesel it sits in a FG box mostly above deck, straight 4” wet exhaust. It’s way too loud. There are four vent holes in the box and no form of intake baffle. I think it’s mainly engine noise I hear but can’t really tell. It gets crazy loud at 3k rpm.
Old 06-11-2021, 06:15 AM
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Originally Posted by 25Jefferson View Post
Hate to jump in here but I have kinda the same problem only with a older Isuzu diesel it sits in a FG box mostly above deck, straight 4” wet exhaust. It’s way too loud. There are four vent holes in the box and no form of intake baffle. I think it’s mainly engine noise I hear but can’t really tell. It gets crazy loud at 3k rpm.
With that small diesel, I'm anticipating alot of your noise being from the engine itself and not exhaust.
Any boat inboard without decent sound down and gasket around the engine box can benefit greatly from incorporating these, diesels in particular.
Old 06-11-2021, 08:00 AM
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What do you think about applying the soundown on the outside of the engine box and then putting a protective layer over it? I have zero clearance on the front of the engine and maybe an inch on the port side.

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