Exhaust Routing Questions

[Bob Perkins]

Hi Everyone,

I'm at the point where I'm installing the exhaust system in my runabout (new construction). My problem is I'm not 100% sure of how to route the exhaust plumbing.

The constants are: Engine can't move, mufflers are in optimal spot, exhaust trumpets are in desired spot.

From the muffler back - there was an unavoidable mismatch in the positioning of the pipe.
http://www.imagestation.com/picture/sraid162/pa1b31da302c38190efaaea764a179372/f4ab7bd2.jpg

My thought here is to get someone to fabricate 4" SS pipe with a gentle "S" shape to get the exhaust out.

Between the mufflers and the engine I have this:
http://www.imagestation.com/picture/sraid162/p7da3748f3c4121a97e641462036ca0c7/f4ab7be5.jpg

Which fits - everything is in place w/o unacceptable stresses, etc.. But it seems inefficient to me. In the pic - the *compartment* aft of the engine hatches is where the legs of the passengers would go for the aft cockpit. The exhaust hose won't really be in their way (too much) - but making a firewall will be a pain, etc..

Is it acceptable to put a pair of 45 deg elbows up there (2 on each side) to route the pipe down and to the side, then back? My initial thought is too many bends and connections can't be good overall - but my second thought is I'm not doing this correct either.. I don't have a boat to compare too so I'm hoping to draw on some of you out there.

I haven't found any rules of thumb on marine exhaust design and layout.. I called Mercury marine and got almost no help, tried a few vendows too...

Hopefully this makes sense.

Particulars: 300HP Mercruiser using 4" exhaust.

Regards,
Bob Perkins

My Current project is at:
http://www.imagestation.com/album/?id=4291051329

[Dan McCosh]

There shouldn't be any real problem in terms of back presure with adding a couple of bends to this system to locate the exhaust tips. I would think an exhaust shop could fabricate the appropriate tubing. You are already downstream from the mufflers, where most of the back pressure is going to happen. If there is a serious concern, make an oversized bend--even a half ins. in inside diameter would be considerable.

[mmd]

Hi, Bob;

I don't forsee any problem in adding a few 45's to the system to make it align properly. Your exhaust line isn't all that long nor convoluted.

Rather than stainless steel, you might want to look at fibreglass elbows. They are available at 'most any marine chandlery, and with a wet exhaust system they are plenty heat-resistant. Do make sure that you get ones rated for gasoline engine marine exhausts, tho' - they have special heat-resistant resins in 'em. These would be a boon to that quick zig-zag aft of your fuel tank 'cause you can laminate them together to make the 'S' as short as possible.

[plyboat]

I often make custom bends from fiberglass tubing. Get super glue in the bottle with a spray bottle of excelerator (model supply stores). It is quick and easy to cut the angles and super glue them together to get the shape you want. Then sand it and glass it, but keep the glass at least 2" from the ends so the piece does not get to much larger than the hose ID. I use a strip of 12 oz biax on the joints and cover it all with a couple of layers of 6 oz. tooling cloth. This method allows you to custom make some real elaborate shapes that are sometimes needed.
---Joel---
---Joel---

[JimConlin]

I know nothing about powerboats, but if that engine were in its original home (some kind of Chevy?), it would have, at most two 2" exhaust pipes. I wouldn't worry about the constriction of a few 45 ells.

Until i was sure about the heat properties of my resin, i'd avoid glassing ells together.

[plyboat]

If it is a wet exhaust, which I assume it is, then heat is not an issue.
---Joel---

[brad9798]

The water will be rather warm on exit ... but not hot enough to damage.

Yea, definitely, especially south of muffles, a slight, or slightly more than slight bend is not going to be an issue at all.

I have seen wet exhaust routed in a some interesting ways when old yachts have been refit (aft-cabin powerboats, specifically) to accomodate a reconfigured aft-cabin ... addition of AC, etc.

It will not be a problem.*

Brad

*Brad is not responsible for anything if it doesn't work! ;) smile.gif

[Bob Perkins]

Thanks for the help everyone.. I re-looked at it tonight and a couple of 45 deg elbows up front should work fine.

I'd like to use stainless at the back - but I've heard a few chuckles on how near impossible that will be to get fabricated w/o breaking the bank. If I can put together glass ones at the rear and feel confident it will hold (thanks MMD) that may be a good route too...

Thanks again,
Bob

[nedL]

Another alternative is to use DWV copper waste pipe & fittings. It sweats (solders) together just like copper tubing. Real easy to do yourself.

[Bob Perkins]

MIke,

Thanks for the info -

I read on the Centek Site that their fibreglass elbows should be no closer that 24" to the mixing elbow (exhaust riser I assume). I think heat could be an issue close to the engine. I'll give them a call..

Back near the exhaust port - I like your method. I haven't looked up the numbers - but I assume standary epoxies (MAS, SYS III) can take the heat w/o a problem. The exhaust fitting will certaily be less than two 45deg pipes put together. The pic doesn't show it clearly - but I have enough room to keep it all pretty straight.

Thanks for the help everyone.
Bob

ps. They need to make a show like American Hotrod or Biker Build Off for wooden boats.. Hmmm.. I'll have to give the Discovery Channel a call..

[ohiomike]

use copper ells and sweat them together..just make sure your risers are above your waterline. dont want that water coming back to bite you were its hurts the most your wallet lol

ohiomike

[AHall]

Have you spoken with the folks at Soundown in Salem? They sold me a small piece of fiberglass exhaust tubing for joining a couple lenghts of hose. I believe it is laminated up with vinylester resin. Something to think about when making up a custom offset. (vinylester vs epoxy vs heat)

short story long, the guys at Soundown were super-helpful, and they can make up any thing you need exhaust related.

good luck,
Andy Hall
satisfied Soundown customer

[Bob Perkins]

Thanks for the new info - I'll try them.

I also found a metal 45 deg elbow in the buck algonquin catalog.. They are powder coated grey iron.. I'm not sure if that means they can be used in a wet exhaust or not..

The metal one would be nice to use close to the engine.

More research ... smile.gif
Thanks,
Bob