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Old 01-17-2022, 06:04 PM   #1
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Wet Exhaust BIG NO-NO

After reading the install manual on my new genset I now understand why. Hydrolock. It happened twice last year.

My boat is a 1988 with twin CATS and an Onan Genset. The exhaust for the genset is piped through a wet muffler, to the exhaust of the port main engine. A big no-no it appears.

Why would the Onan be plumbed this way? The Onan install manual says the same thing!

Looks like I need a new hole in the stern of ASD!
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Old 01-17-2022, 06:35 PM   #2
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Let me know, I have no problem drilling holes in your boat…
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Old 01-17-2022, 06:52 PM   #3
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Let me know, I have no problem drilling holes in your boat…
Very funny. Be nice!!
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Old 01-17-2022, 06:55 PM   #4
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It was an honest offer. I sincerely don’t mind drilling on your boat. I have done it for several friends that were nervous about drilling through the hull. I would even try to get it in the right place…
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Old 01-17-2022, 07:00 PM   #5
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In theory if the main exhaust tubes are big enough and placed where they'll never become fully submerged, then as long as the rest of the rules of good exhaust design are followed, it should be safe. That said, I'd much prefer a separate thru hull.
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Old 01-17-2022, 07:21 PM   #6
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It was an honest offer. I sincerely don’t mind drilling on your boat. I have done it for several friends that were nervous about drilling through the hull. I would even try to get it in the right place…
Thanks.

Cook Engines in Portland is doing the week. They have a lift.
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Old 01-17-2022, 07:22 PM   #7
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In theory if the main exhaust tubes are big enough and placed where they'll never become fully submerged, then as long as the rest of the rules of good exhaust design are followed, it should be safe. That said, I'd much prefer a separate thru hull.
They did tell me I could keep the original set up, but would void the warranty.
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Old 01-17-2022, 07:29 PM   #8
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Big no-no for other reasons as well. Best to go with the separate thru-hull, but if you were really against that, what many big sportfish do is join the gen exh run to the main engine piping just before the outlet and do a ‘pipe-within-a-pipe’ detail all the way back to essentially have 2 ‘separate’ runs share the thru-hull outlet. I’m sure if you search the inter webs you can find a picture of what I’m talking about.
But best & probably easiest to add a new hole.
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Old 01-17-2022, 07:50 PM   #9
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It was an honest offer. I sincerely don’t mind drilling on your boat. I have done it for several friends that were nervous about drilling through the hull. I would even try to get it in the right place…
Reminds me of a funny story. I used the assistance of a looper moored at my pier for a day or two to drill a 3/8-inch hole in the bottom of my wooden hull to mount an Electro-Guard zinc. I was in the water with the gooped up bolt going up through the hull, and he was inside with a battery-operated drill. He was really nervous, but it went smoothly. Later he bragged, "I drilled a hole through Rich Gano's underwater hull with the boat in the water."

I probably would draw the line at anything larger.
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Old 01-17-2022, 07:52 PM   #10
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I have read about putting in a transducer through the hull in the water using a bowl on the outside. Not sure I would want to try it though.
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Old 01-17-2022, 08:23 PM   #11
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I like my combined exhaust. I installed the stern exhaust on my boat when doing the refit and repower. Now to be fair, I had the removed fiberglass exhaust pipe from the original motor. So the 4" exhaust from the John Deere and the 2" exhaust from the Onan combine in an 8" fiberglass exhaust pipe. One of the nice advantages of this setup is that with almost empty fuel tanks, the inside of the outlet is just below sea level at anchor. No water splash sounds while running the generator. Both the engine and generator have lift muffler systems with the generator reaching 30" above sea level and going through a silencer / water gas separator. If done correctly, they're very safe and extremely quiet.

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Old 01-17-2022, 08:55 PM   #12
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‘If not done correctly’ gives a bad name to so many otherwise reasonable ideas.
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Old 01-17-2022, 08:57 PM   #13
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Originally Posted by Alaskan Sea-Duction View Post
Thanks.

Cook Engines in Portland is doing the week. They have a lift.
they did a yanmar install for me in 2011.

their website shows 2013-2022. did they go through a change?

very good installers. i would take their recommendations.
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Old 01-17-2022, 09:24 PM   #14
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Tom. Is the lazerett a normal install location for the genny? I am asking due to location the current exhaust setup protects genny better for a wave surge going up the exhaust pipe as it has the length of the engine pipe. Guess a flap will be added to prevent that.
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Old 01-17-2022, 09:24 PM   #15
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Another reason to not dump generator exhaust into a main is that the big pipe can act like a megaphone and increases the volume of the generator exhaust.



Also, going back to my suggestion to install a separator, when installed correctly a separator makes backflooding the generator almost impossible.

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Old 01-17-2022, 11:57 PM   #16
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Tom. Is the lazerett a normal install location for the genny? I am asking due to location the current exhaust setup protects genny better for a wave surge going up the exhaust pipe as it has the length of the engine pipe. Guess a flap will be added to prevent that.
On a 42ft Camargue it is located in the engine room. On the 48ft they place it in the cockpit. Sound shielding too..

Two 50gal fuel tanks are also in the cockpit.
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Old 01-18-2022, 06:32 AM   #17
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Tom,

You probably thought about these already but some pre-caffinated ideas this morning:

Have the yard make the new hole sufficiently high so that the exhaust does not disturb the water. The blowing/bubbling/splashing could be the noisiest thing with your new set (it is with our NL 9kw). The elevated height will work nice with a Gen Sep, if you are going that route.

Take Care,
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Old 01-18-2022, 07:19 AM   #18
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If practical, I would add a gensep. It eliminates any splashing water outside, and the exhaust is no louder than a dryer vent.


Have you figured out why/how the old gen got water in it? Is it just because of the shared exhaust, and do you understand how that drove water into the gen?


Obviously you want to be certain that the new gen doesn't suffer the same fate.
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Old 01-18-2022, 07:42 AM   #19
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If you don't go for a gensep, try to put a small inline muffler after the waterlift, right before the thru hull. The extra volume will dampen the water pulsing and give closer to a steady stream of water coming out (which means less noise).


I also agree with getting the thru hull well above the water. Not too high if it's discharging water (noise), but high enough that it doesn't get submerged to avoid extra noises from moving water out of the way.
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Old 01-18-2022, 11:15 AM   #20
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Tom, I also removed an Onan, 6kw, and replaced it with a NL 5kw. Regarding the sound shield, the shop that installed it said that the NL without the shield would be quieter than the Onan with the shield. They were right. And my location is under the salon sole, between the two engines rather than your lazarette location which is farther removed from living spaces. My gen is quiet in the salon and sounds like a sewing machine. We hardly notice it. If you deleted the shield, your access and maintenance would be easier too. I'd spring for upgraded isolation mounts instead, to get rid of any vibration. I'd also go with a separate through-hull for the exhaust and make sure they give you a 1 foot loop above the waterline.
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