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Old 21-01-2011, 02:24   #1
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Problem with Engine Fumes


I'm a newbie to this forum, and a newbie to boat maintenance. My name is Rick and I work for a scuba diving operator in Thailand who has just purchased a hand-built Phinisi Schooner to use for diving. Our mechanic had a change of life-plan recently and decided to quit to become a yoga instructor. Which leaves us without a good mechanic and me, a relatively inexperienced boat guy, to figure things out.

Our boat is a 23m long Phinisi Schooner with 7 cabins capable of holding 12 customers and 3 dive staff. The wheel house is midships and 5 cabins are fore of this area, and 2 are aft. The 2 aft cabins are right at the back of the boat. The engine is also midships, directly below the wheelhouse. The exhaust from the main engine (Mitsubishi 6D16 6-cyl diesel) vents on the port side above the waterline. The problem is that the aft port cabin gets all the fumes from the engine. I've searched the forums here and found it's due to the design of the boat and the 'station wagon effect'. What I need to know is how to get rid of it.

The engine is new as of 3 months ago and its tuned up and running exactly as it should (not too rich/lean).

I have 2 ideas and would like to see what people think.

1) If I ran the exhaust downward after it exited the boat, so that it exited below the water line, would that filter the exhaust at all? Does this put added strain on the engine?
2) The engine uses a pre-cooler which means that the exhaust pipe not only has fumes coming out, but a significant amount of water too. This means I can not run a smokestack style exhaust up high, UNLESS I do so but make a smaller exit for the water going down, and a bigger pipe for the fumes, going up. Do you think this would work, or would all the fumes go out the bottom anyway?

Is there any sort of device that can be put on the exhaust to cut down on the fumes? Some sort of muffler?

Thanks in advance for any advice.

Khao Lak, Thailand

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Old 21-01-2011, 04:22   #2
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I have owned a number of small speedboats with sterndrive V-8 gas engines that vented the exhaust under water, actually through the propeller hub and the fumes were not much reduced.

I see two options. One is to run the exhaust all the way to the stern. Since the exhaust has the cooling water injected you should be able to use flexible exhaust hose for the long run but of course you still have to deal with a long, hot, exhaust hose.

To run the exhaust up I think you will have to go with what is called a dry stack. Separate the water and the exhaust and send the exhaust gas up. This will give you very hot exhaust gas to deal with so proper insulation to avoid fire is required. I know of sailboats that exhaust up a mast but am not familiar with the details of how it is done.

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Old 21-01-2011, 05:01   #3
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Thanks for the reply. I'll look in to the stack.

We tried running the exhaust out the back, unfortunately it was at a slightly higher level than the engine, thus a lot of water built up in the pipe, hence why we bought a new engine 3 months ago.

The boat doesnt go to drydock for another 3 months, so if we can figure out a temporary solution until then AND something we can do when its out, that'd be best. Its possible running it out the back at a lower level when its on drydock might work. But I'd be worried that the 'station wagon effect' would still bring the fumes in the boat.
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Old 21-01-2011, 05:14   #4
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Our mechanic had a change of life-plan recently and decided to quit to become a yoga instructor.
I hate when that happens. Mine quit to become a massage therapist.
The next best thing to playing and winning
is playing and losing ...
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Old 21-01-2011, 06:01   #5
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use an exahust water seperator to run the water out the side and then run the cool exh up a fiberglass pipe out the top. Centek make the seperator that you would just run after the water lift muffler.
no muffler will change the odor of the exh.
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Old 21-01-2011, 07:19   #6
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How about a different approach? What if you create positive pressure in the cabin while the engine is running? If you blow clean air into the cabin fumes should not enter. It might be simpler to duct clean air to the cabin than to deal with the hot water/exhaust. Assuming you have an adequate alternator on the engine it should not hurt you too badly. You just need to figure out where you can get some clean air and where to run your duct. You don't have to worry about leaks either. If your duct gets a small hole in it, it's not going to sink your boat.
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Old 21-01-2011, 07:22   #7
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Rick, There are several exhaust systems that direct the flow of the exhaust under water and the raw water cooling discharge above the waterline and some that discharge both water and exhaust under the waterline. But it must be done correctly. Several of the major boat builders in the US now have this type of system. I believe Carver is one, not sure exactly which others. You can't just point the exhaust underwater though, it will take some modification. Chuck
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Old 22-01-2011, 23:25   #8
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Thanks to everyone for the ideas. Based on something I read in another post, I had thought about some sort of dorade positioned upwind of the exhaust. The only trick is getting it routed through the boat.

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