making an engine quieter

[Guest]

my boat is a prout cat with a central diesel engine just behind the cockpit. the exhaust is water cooled exiting from the transom abaft the bridgedeck. limited space in the engine compartment so there is a simple vetus silencer. much of the engine bay has a foam / lead sheet noise insulation installed.

the whole setup is noisy, both with reflected exhaust noise and also with engine noise , plus some structure born noise since the drive leg is bolted direct to the transom.

have been thinking about fitting a dry exhaust so i could use a really effective silencer off a car. anybody tried this?

anybody know of a way of bolting the drive leg flange to the transom (its surface mounted) through noise reduceing bushes etc that are still watertight.

any other suggestions people can make to reduce the racket from either exhaust or engine?

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[Col]

What engine / drive are you running?


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[yoda]

The Dolphin petrol engine uses a dry exhaust as standard. Try contacting Mr Chapman at Dolphin engines and ask him where he gets them from. Tel number on his web site. I don't think you will find it much better than your water cooled exhaust with a decent muffler from the like of Vetus

yoda

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[Jcorstorphine]

You will need to lag the exhaust pipe if you change it to a dry system. Unlike a car, there will be no cooling effect from air as the exhaust will be in a sealed area.

You should look at a better wet silencer, softer engine mounts, sound insulation and some for of flexible drive.

John C

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[Avocet]

Are you sure it's the exhaust that's noisy? Hang over the transom at full throttle and see if it really is the main source of noise. Our engine is only small and it has a wet exhaust with a waterlock and plastic silences and to be honest, all you hear out of it is a hiss and gurgle. The bulk of our engine noise is just diesel knock and induction noise.

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[Guest]

Yanmar 2gm with silette catamaran drive. the drive is a bit like a very long outboard leg pivoting on a flange bolted to the transom.

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[Guest]

I couldnt say that the exhaust is the major source of noise but it certainly contributes. part of the problem is that the noise echoes between the two hulls. if you go forward when motoring, the exhaust noise comes quite loud over the bow.

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[vyv_cox]

I wouldn't recommend a dry exhaust on a boat. It's very difficult to arrange a flexible joint between the engine and pipe with the result that either the coiled flexible leaks gases into the engine room or the pipe is made rigid and it rattles against the hull. A hot exhaust pipe is difficult to insulate and can be quite dangerous. Installation is more problematical because the pipe cannot be laid in the most convenient places.

A wet exhaust with a trap and silencer should not be noisy. My engine transmits some noise despite many silencing measures but almost no noise emerges from the exhaust.

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[kgi]

I have noticed this on cats with a centrally mounted engine, the only thing i have seen done was a pipe was led from the outlet down into the water, the other things you might consider is actually insulating the exhaust hose against radiant noise and make sure its not touching the bridgedeck anywhere, mount the water trap on a rubber pad as well, i think some of the problem with a centre mounted engine is it turns the bridgedeck into a sounding board........keith

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[Col]

Send a PM to boatone, over on mobo. He recently got some fabbo sound deadening. I think this would benefit you.

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[snowleopard]

i drive my cat with 2x3gm30s and the noise is quite reasonable (though one man's reasonable is another's hell)

i have vetus waterlock, muffler and gooseneck and get no audible noise from the exhaust. the whole shooting match is rubber mounted (3 rubber engine mounts and saildrive legs passing through rubber gaskets. the compartments have quilted lead-lined sound insulation on the front and mattresses on top.

look for gaps in the insulation, gaps in the box and solid connections between engine and hull.

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[qsiv]

The exhaust noise is interesting. I would suggest that a really good waterlift silencer (from the likes of Halyard) would make a material difference. As well as that I would take every opportunity that you find to increase the bore of the outlet from the silencer - this will have the effect of reducing the velocity of the gasses leaving the system, and would be expected to reduce the penetrative nature of the sound.

I would also pay particular attention to the form of the final flange - a square cut will exacerbate the sound - I would experiment with different shapes, such that the exhaust gasses are channeled in such a way as they are NOT directed toward the resonant surfaces of the inboard faces of the hulls. You could do this b clamping on an offcut of the exhaust hose.

I transformed the noise of a previous boat by casuing the gasses to exit horizontally so that I avoided a resonance between the water and the underside of a davit slung dinghy. Experimentation can yield great results. Be careful of increasing the back pressure in the system as the exhaust valves will NOT thank you!

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[pvb]

In addition to everything else...

In addition to regular soundproofing materials, you need to seal up all the access hatches to the engine compartment with foam strip to keep the noise inside.

Also, there's a lot of induction roar from many diesels. You can minimise this by baffling the air intake system to the engine compartment. On my boat, the air intake ducting starts in a box in the cockpit. I built a baffle inside the box, and lined the inside surfaces with foam-backed carpet - this gave a significant reduction in perceived engine noise in the cockpit.
<P ID="edit"><FONT SIZE=-1>Edited by pvb on 21/02/2003 18:54 (server time).</FONT></P>

 

[philmarks]

Re: In addition to everything else...

Halyard Marine used to do an excellent little booklet on exhausts and sound deadening.

A lot of noise is conducted thro' structure, so good mountings are essential. Also any holes leak noise and hatch edges should have foam strips and hatches should be clamped down. I use standard foam with heavy polymer barrier from ASAP supplies. Good stuff. As one other post said - foam backed carpet is good (and I also put a heavy rug over where my genny is located 3 cyl 20hp Kubota). A load hum but otherwise OK. I do agree that resonance from exhaust outlet could be another item to consider.

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