sound suppresion

[gunsmith]

In my boat I plan on using a GM 671. This is a killer for noise. It will be in it's own room and I am hoping to use a wet exhaust. What is best to use for killing noise. The engin room is not large, about 14' wide by 6'deep and 8' long. It will have a hatch covers with the decking of 1.25" B.C. Fir. Any sugestions? The hull is a typical 1.25" carvel on 4" x 2" bent fir and a inner planking of 1" spuce from the gunnel to the engine bed. There is a "V" birth on one side behing a bulkhead and fuel and water tanks on the stern end. Can I just insulate the deck above the engine and the bulkhead adjacent to the vee birth?

[Donn]

Gunsmith...use the search thingy and search the building/repair forum for "soundproofing." There's a recent thread on the subject of soundproofing an engine compartment. You only need to use the stuff on surfaces that are adjacent to the interior of the hull. The combination of hull and water effectively muffles under-waterline sound.

[Peter Malcolm Jardine]

The principle of sound suppression is stopping the movement, or absorption of sound waves.
There are a lot of materials used in engine noise suppression, but not all of them are very effective. Older boats used tentest, or fibreboard on the inside of the engine room hatches and the forward engine room bulkhead.
It didn't work all that well.

I installed some soundproofing in my father's trawler (perkins diesel) and it is made up of three distinct parts. On the surface it has a shiny foil or mylar film, over about an inch of dense (but not ridgid) foam. It is followed by another inch of foam, but in between is a thin layer, perhaps 1/16th thick, of sheet lead. It was glued in place using a special contact cement. The important issue was to install the material in as large a piece as possible, without interrupting the lead sheet.After insulating the entire engine deck and bulkhead, approximately 65 to 70 percent sound reduction was realized. Not cheap tho.

There are other ways of reducing noise without spending as much money, but generally it takes more room, (ie tube traps for sound waves) and within marine engine rooms materials must be approved for fire resistance. Hope this helps.

[NormMessinger]

Just shutting the door (so to speak) makes a lot of difference, Yeah yeah, so all I know about is a GM-10. But a lot of sound leak out of little cracks.

[imported_Conrad]

Much of the noise is generated by the intake air pulse. Looking onto something more substantial than the simple stock air filter, such as an air-sept filter, or after-market truck intake air silencer will get rid of almost half of the "scream" 6-71's are famous for. Then you're left with the less objectionable lower frequency mix which can be handled with better mounts, insulation, etc.

[Lucky Luke]

Hello,
soundproofing the E.R. is one thing, but the major concern with these "old" 2-stroke GMs is the difficulty to do a good exhaust silencer, and the only fact that it may be "wet" is not enough.
These engines do not accept as much back pressure as the 4-stroke do, although back-pressure is prejudiciable for all engines.
Detroit Diesel sell silencers dedicated to their engines, which are good, but not that good. "We" (no advertising on this forum...) did work out more satisfactory ones, with water swirling inside, but a 6-71 will require about 3'height and 2'diameter, just for the silencer, plus the big pipes in and out. Is that space available in your E.R?

[Dale R. Hamilton]

a 14' long engine room, Damn what kind of a boat are you building?

Dial up www.soundown.com, (http://www.soundown.com,) ask for Gary Kissal. I think these guys wrote the book on noise suppression- they make the stuff mentioned above. I can personally vouch for its effectiveness, several boats, a Mack truck, and generator compartment. Don't skimp on it tho.

Dale

[Donn]

I'll vouch for http://www.soundown.com too. I have it in my engine compartment, with 2 little diesels, and it works great.

[Tom Lathrop]

Knocking down the air transmitted sound is good but, especially on a diesel, most of the bothersome noise is transmitted through mechanical connection to the hull.

I've seem a couple projects where the engine is decoupled from the shaft with CV joints and a remote thrust bearing. This allows soft mounting of the diesel just like a genset would be. It also allows the engine angle to be independent of the shaft angle. I would expect that this is, by far, the best noise reduction scheme available.

[ 10-31-2003, 12:08 PM: Message edited by: Tom Lathrop ]

[werner]

Hi,

Made that kind of foam/lead /foam insulation myself for my car at a fraction of the costs ,and it seems to work well ( Aston martin used this kind in the sixties)
the thin sheetlead I found not to expensive in a shop for flower"arranging"? (don'tknow the english word)
perhaps not a bad idea to look for fire retardant foam?

[gunsmith]

After the mention of lead I remembered a job we did on an industrial building for a chemical company. That is exactly what we had to do, cover the standby diesel room with lead. When the unit was test fired you hardly new it was there. I know what you mean with the addition of more advanced air intake equipment. This will also help. As for the wet muffler. I had not planed on such a large silencer . In fact I pictured it more like a rubber truck muffler. This is one I still have to check. We are lucky and have an advanced sea rescue station in our town for this coast . There they have a large rescue boat and I noticed the other day that it has a wet exhaust. I'm going to check and see what they are using and maybe I will have a better idea of what I will need. Thanks for all the help.