Small Engine Dry Exhaust

I have a 26' trawler with a 27 hp diesel and dry exhaust. The exhaust comes straight up from a small elbow on the manifold through a sheet metal housing inside the cabin on the bulkhead.




The exhaust exits through the roof with a rain guard. The PO put a muffler on the end of the exhaust. Inside the sheetmetal housing there is what I suppose is a original (35 year old) glasspack which is only 3" or 4" diameter or so.

Anyway the boat is loud and I want to do something about it. Adding a silencer inside the sheet metal housing would be best I suppose but there is limited room. The housing is 8" X 9" with 1" of insulation inside so that only leaves 6" X 7".

A larger housing could be made but there would be complications with the engine box and other gear so while possible I would rather not do that.

I have been looking at cowl silencers and my idea is to put one on the outside immediately above the rainguard. They are available in stainless.



There is one that has a diameter about that of my existing rainguard. It might look odd but I am not too concerned about that. Does anyone have any other suggestions. I have communicated with some experts in the field but their responses have been very limited or non existent. I think I am too small a fish.

Mount a muffler horizontally on brackets on roof of house. Feed the muffler with a 90deg elbow from the stack pipe. If there is motion in the stack pipe, muffler may need to be on flex mounts. Also on outlet of muffler, put at least a foot of tailpipe on as that will greatly increase the effectiveness of the muffler.

No wonder that thing is loud. Not sure what that muffler thing is on the end of that pipe, but is sure as heck is cantilevered!!

If you can mount the muffler close to the engine it will be more effective. It will also be out of the weather and will not be as unsightly. You can wrap it for heat and the higher temperature inside the muffler will eliminate the water by-product of combustion better and the muffler should last longer and resist corrosion from the inside. You might even find a resonator will help make the engine exhaust virtually silent.

No wonder that thing is loud. Not sure what that muffler thing is on the end of that pipe, but is sure as heck is cantilevered!![/QUOTE]

It is called a Super Trapp:
https://www.jackssmallengines.com/diy/super-trapp-quiet-muffler-selection-guide/

That's a lawnmower muffler. Crap, you're not running a friggin' lawnmower!!!

XSB is right, best for muffler to be in the chase, but you may not have room for it as I think you mentioned. Nothing wrong with putting on the roof.

You can try it cheap by putting a car muffler on the roof. It won't last forever, but heck cars drive on salted roads and mufflers last for years. For 50 bucks it will tell you if you are on the right track.

Another solution might be to extend the exhaust pipe through the bulkhead and run it up the outside of the boat, then you have plenty of room for stacked mufflers, it might not have to live in a chase although my stack also vents the engine room - does yours? Any more pictures?

When I bought my boat it had a straight pipe up through the stack with no muffler at all. It was so damn noisy you couldn't stand outside while running. My new, wrapped Donaldson muffler is very quiet, doesn't sound like a road grader any more. No idea how long it will last.

Most commercial boats have dry exhaust. Put fireproof insulation inside the sheet metal and sheet foam on the outside. But leave room for an airflow. That will help noise considerably. They make long round mufflers that probably fit inside. Check Ebay.
Foam and lead sheeting are the best noise stoppers I have found.

Google "hospital critical muffler" and see what is offered for your sized engine.

You might not like the price , but you will like the quiet.

Perhaps a local land gen shop will have a used muffler?

I assume but it was not said, is the noise from the exhaust pipe end outside? A bark?

The "glaspack" inside the shroud , is it part of the problem? Can you detect noise from it through the shroud?

Can you hear noise from the engine box?
I see foam insulation on the box interior. Are there gaps at the bottom, at the back, anywhere? It's amazing how much racket can get through ANY gaps or openings, even tiny ones. Where the lift up part of the box meets the floor.

Sometimes a labyrinth arrangement can block a lot of noise since sound often cannot negotiate sharp corners such as a curb built around the opening that the lift up part of the box exposes. Especially if that curb is also lined with your sound absorbent foam.


It looks like you may not have any room for an inside muffler next to or behind the engine except for what you have, the "glaspack". I also wonder if the "glaspack" has failed leaving too much noise to the final muffler for it to be able to do its job.

I have not seen the SuperTrap muffler that you now have. I have seen other mufflers similar to the unit shown later. Known in my area as a MAC muffler. Lots of commercial boats with those and they are effective. So that similar muffler, to the MAC, should work if sized right.

Thanks for all the information. I have searched online extensively and come up with this:
https://www.asap-supplies.com/us/ex...ncers/dry-exhaust-noise-silencer-sn114-417107
It is rated Residential. There is a "High attenuation" model the same diameter and longer but it has offset inlet and outlet which would complicate things. Anyway they are both only 4.75" diameter and I think I could fit one of them in the shroud just above the engine. One minor annoyance is they are BSP thread.

https://www.donaldson.com/content/d...America/F110028-ENG/Exhaust-Product-Guide.pdf

Scroll way down, Style 1, second product.

That one you've posted will be doable but will cost as much in fittings as the muffler. Good choice, though.

What size is the pipe now?

There should be a version out there with NPT threads. Or even use muffler clamps.

I am thinking about two mufflers end to end. The muffler I mentioned above is 4.75 diameter and 18" long and I would have room for two. This is sometimes done I have found. I know backpressure can be an issue but I don't know how to calculate that.

Why not a wet exhaust and a lift muffler?

Nomad Willy said:

Why not a wet exhaust and a lift muffler?

Click to expand...

I have seriously considered that. It would take quite a bit of work to do that as it is currently set up with keel cooling. Also to get the outlet thru-hull high enough I would have to cut through the aft deck and make some kind of waterproof arrangement for a goose neck. Also the bulkhead behind the engine where most of the brand new battery and misc wiring is would have to be changed for risers etc. The engine a Yanmar 3hm is designed for raw water cooling. I have considered going to raw water cooling with a water lift muffler.

Still vote for trying a car muffler on the roof. You will likely be pleased by the results.

Ski,
I think your'e right. It's simple ... that's probably why the fishermen do it.

And as to noise little cars are quiet.


ak-guy,
Your boat is a Norwester and your engine's a Yanmar 3MJH30F right? Good boat for Gustavus.

ak-guy,
Your boat is a Norwester and your engine's a Yanmar 3MJH30F right? Good boat for Gustavus.[/QUOTE]

It's an Allweather with a Yanmar 3HM. As to Gustavus, there is no boat harbor. The only place to keep a boat is in the river where it grounds out at low tide. I haven't tried it yet, being a full keel boat it would lean over quit a bit but would probably be OK.

ak-guy,
Be careful. Grounding could possibly sink your boat. water could come in via air vents, loose fitting doors ect. Fine if your bilge pump can keep up but not so fine otherwise.

I was thinking of hand trolling there keeping the boat tied to an anchored float between Pleasant Is and the townside. I was working on the ferries and never did go fishing.

"Why not a wet exhaust and a lift muffler?"

This requires winterizing and takes up room more holes in the boat and is a big expense , and requires ongoing maint.

UGH!

Although it IS probably the quietest system .

Yes FF,
Just gentle splashing water.
I love the quiet.