Air cooled engine with wet exhaust? - Page 2 - SailNet Community
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post #11 of 41 Old 01-17-2012
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Originally Posted by SloopJonB View Post
I must be missing something here. How does an air-cooled engine get a wet exhaust? Are you talking about pumping raw water from overside into some sort of injection elbow just to cool the exhaust gases?

If that's the case, why bother? I've seen plenty of dry stack exhausts on V8 engine fishboats. Double tubing them and lots of lagging where there might be any contact with anything works just fine - why bother with pumps, through hulls etc?
You don't wan't a smoke stack on a sailboat. It's much easier to make a dry smokestack going stright through the roof than through the side of the hull.
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post #12 of 41 Old 01-17-2012
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If I'm reading your post correctly, it's not the noise you're concerned about, it's that the dry exhaust system shakes apart until it develops exhaust leaks, which are hazardous due to the heat and noxious gasses.

The engine was designed to operate with a dry exhaust, so I don't know if I'd change that. I served on submarines where sound and vibration isolation were absolutely critical to achieve the silence during operation that a submarine requires.

If I were you, I'd investigate installing a vibration dampening system at all of the exhaust system joints, and reducing the number of joints to the absolute minimum. Basically, you're looking at wrapping the tubing in heat wrap to reduce the temperature, and then connecting a circular bracket around it. Then, connect a rubber isolation mount to that, and connect it to a mounting point in the engine compartment somewhere.

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post #13 of 41 Old 01-17-2012
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If your engine is jumping around so much that it causes the exhaust to crack, before I re-engineer the exhaust system I'd look at replacing the engine mounts.
Do you have any pictures of your current installation?

It's 5 o'clock somewhere:


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post #14 of 41 Old 01-17-2012
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GREETINGS EARTHLINGS you say exsaused is to long have you tryed useing one-way flapping valves (this will split the unit up) to exsasepate the back pressure biuld up GO SAFE
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post #15 of 41 Old 01-17-2012
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Try this for starters:

Good Old Boat - Cool and Quiet and trouble-free article

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post #16 of 41 Old 01-17-2012
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could also consider a water jacketed system. Water will cool the exhaust run and dampen the vibrations a bit. But this would be pricey fabbing.

Example of jacketing butnote it is without consideration of transom exit and need to preven back flow: http://www.glen-l.com/weblettr/weble...nb-plate54.gif

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post #17 of 41 Old 01-17-2012
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Caterpiller has a nice discussion of exhausts for their big diesels. Towards the end are some design ideas and specs for wet exhausts: http://www.gregorypoole.com/products...%20SYSTEMS.pdf

Here is the AByC design criteria committee report of marine exhaust system design:
http://www.abycinc.org/committees/P-01.pdf

Silence marine has some figures for aapplications oftheir products.
http://www.silencermarine.com/inglese/silenziatori.htm

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Last edited by ewoden; 01-17-2012 at 10:57 AM.
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post #18 of 41 Old 01-17-2012
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ewoden, you are missing one big point- the engine is AIR COOLED, not water cooled.

It's 5 o'clock somewhere:


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post #19 of 41 Old 01-17-2012 Thread Starter
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The system does have some vibration dampening set up. I will try and get some photos later. Basically the system runs horizontally from the engine to the stern of the boat. It goes:

1. exhaust elbow
2. flex pipe
3. just some plain metal tubing
4. muffler
5. more plain metal tubing
6. flex pipe
7. what looks like plumbing tubing that creates the necessary 18 inch above waterline bend - which then exits the stern.

The whole system has a stainless steel shield running along the top wall of the exhaust. From here there are a system of hanging chains which you can connect the exhaust system to. At two points (one before the first flex pipe, and a second before the second flex pipe), there are rubber connection points to dampen the shake more. All of this is wrapped in fibreglass cloth.

2 things that I think are contributing to the leaks:
1. The use of flex pipe.
2. Too many sections.

If I was to try and make the current dry exhaust work, I would need to replace a number of the sections in the exhaust. Right now, it mainly seems to be the flex pipe that begins to crack and leak. We originally got a broken section of it replaced, and it seems to have already developed new leaks. So a big questions is - how necessary is flex pipe in a dry exhaust system? Right now, our funds are not as good as they used to, so any kind of replacement we do will need to somehow be made with off the shelf parts. Not much room for custom welding. Most likely auto exhaust parts. I realize long term, this wont survive the rigors of the ocean air, but it it may get us through the next few months at least. The rubber dampening devices set up seem to work well too - perhaps more of those.


Any opinions on auto exhaust parts in a dry exhaust system?
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post #20 of 41 Old 01-17-2012
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Quote:
Originally Posted by bljones View Post
ewoden, you are missing one big point- the engine is AIR COOLED, not water cooled.
How so am I missing anything? Wet exhaust with water pump (belt or electric) would still apply yes? We are not talking about cooling the engine in this case rather cooling the exhaust to allow for a flexible coupling aft or running hot exhaust insulated aft).

You can run a dedicated open loop through sleeved pipe to cool exhaust without mixing and without a lift muffler. (AKA semi wet system). It gets done lots of times, even with air cooled engines.

How does that differ for a water cooled versus air cooled engine? The only issue I can see is running a water pump. That's no big deal, belt feed it like an altenaor just like any water cooled engine or run a dedicated electric pump. Many see advantages to a sleeaved exhaust versus a lift exhaust. Corrosion avoidance being one of them. Such a system would also achieve some measure of vibration damping, and the cost can be kept down by using internal pipe in stainless where the heat is but mild steel on the outside to allow for a more predictable corrosion resistence than one gets with stainless. In the last illustration: http://www.glen-l.com/weblettr/weble...nb-plate54.gif eliminate vertical stack, put the shebang on an angle to exit 22 inches above water line, replace engine source water with open loop from through hull to pump, to stern throug hull. incorporte other poser's vibration dampers and you are good to go. heck you get downstream to a point where exhaust may be cool enough to use exhaust hose for the final run to the transom. Oh and one avoids the power losses associated with lift mufflers.

A ship in harbor is safe, but that is not what ships are built for.
William Shedd

Last edited by ewoden; 01-17-2012 at 02:38 PM.
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