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Old 03-05-2014, 10:37 PM   #1
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Combining genset and main engine exhaust

I'm in the final stages of completing a genset re-location from the machinery space below the galley to a new hatch in the veranda. In doing the swap, I will be moving both the main engine and genset exhaust through the port side transom. I thought I'd do two different holes, but then heard of someone who routed the genset exhaust into the main engine exhaust just before the transom exit. Both are water mufflers. Has anyone done this that can comment? Thanks in advance.
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Old 03-05-2014, 10:44 PM   #2
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I'm in the final stages of completing a genset re-location from the machinery space below the galley to a new hatch in the veranda. In doing the swap, I will be moving both the main engine and genset exhaust through the port side transom. I thought I'd do two different holes, but then heard of someone who routed the genset exhaust into the main engine exhaust just before the transom exit. Both are water mufflers. Has anyone done this that can comment? Thanks in advance.
I've seen it done on sport fishers done like that.
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Old 03-05-2014, 11:01 PM   #3
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Not a good idea unless the two pipes exit right at the transom. As in genny exh tube inserts ME exhaust tube, but both exit together. If tied in any further up the ME pipe, there will be backpressure trying to force exh/water into genny pipe. No big deal if both are running, but if genny is off it can get filled with water.

Worse yet if sea or loading conditions have exhaust underwater, you could force water into the main.

On all the installs I do, gen gets its pipe, ME get its pipe. Why take the risk. Gennies with water in them paid for the construction of my boat. Anything you can do to minimize that risk is worth it. Unless you want me to build another boat!!
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Old 03-05-2014, 11:11 PM   #4
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I know a person who had a late _90s 54Searay that has the Genny exiting the main exhaust. Perhaps you can research how they did it,

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Old 03-06-2014, 01:52 AM   #5
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I think you could do a co-axial exhaust pipe where they both exit at the transom, where the exhaust only mixes outside the boat.
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Old 03-06-2014, 07:40 AM   #6
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Totally contrary to ABYC standards for good reason. Even a run of the mill half brain dead surveyor will put a giant red flag on your next insurance survey. running either one without the other could push water back up into the non-running engine. Depending on layout the siphoning potential is high.

Pretty sure it would also void your generator warranty.

If that isn't enough, the exhaust would have to exit well above the waterline on the side of the hull (not transom) otherwise even more red flags go up.
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Old 03-06-2014, 08:08 AM   #7
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My boat has both combined. I wouldn't attempt it if they both didn't have water lift muflers with a significantly high hose loop above the normal water level. In sportfish and other planing vessels, the engines are generally run at a much higher percentage of maximum RPM. Even though are trawlers are set up to run at maximum RPM, we generally run a much smaller percentage. As a result, IMO back pressure is a non issue. On my boat the exhaust exits on the side at the stern. The generator exhaust enters the horizontal engine exhaust tube vertically from the top less than a foot from where it exits the hull. The tube protrudes a short way into the big tube, and I wonder if it may actually act like a venturi instead of back pressure to the generator.

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Old 03-06-2014, 08:09 AM   #8
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Look closely at the sport fishers and you'll see it is two different pipes. Sometimes it is the genset, sometimes AC or even all three.
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Old 03-06-2014, 08:18 AM   #9
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Totally contrary to ABYC standards for good reason. Even a run of the mill half brain dead surveyor will put a giant red flag on your next insurance survey. running either one without the other could push water back up into the non-running engine. Depending on layout the siphoning potential is high.

Pretty sure it would also void your generator warranty.

If that isn't enough, the exhaust would have to exit well above the waterline on the side of the hull (not transom) otherwise even more red flags go up.
I think you're mistaken here. This is a very common setup on sport fish boats over 45'. See a lot of the Carolina and Florida built boats where the main engine, the generator, and the raw water discharge from the AC units all exit the one exhaust pipe. They have been doing it that way for quite a while.

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Old 03-06-2014, 08:22 AM   #10
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Originally Posted by O C Diver View Post
I think you're mistaken here. This is a very common setup on sport fish boats over 45'. See a lot of the Carolina and Florida built boats where the main engine, the generator, and the raw water discharge from the AC units all exit the one exhaust pipe. They have been doing it that way for quite a while. Ted
Yep, the way I was talking about had a sea cocks right behind the transom where the ac, genny, and other things drained.
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Old 03-06-2014, 09:32 AM   #11
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It is pretty rare - can't even think of one - to see a generator exhaust well above the waterline. Why would anyone want to do such a thing anyway? Nearly all the boats I deal with can select generator exhaust discharge on the transom at the waterline or over the side at the waterline near the generator location.

With regard to sharing exhaust pipes, it is simply bad practice to do so on a wet exhaust system. If the generator/or main engine is running and a wave blocks the outlet momentarily, damp gases can flow back through open valves of the non running engine and may lead to corrosion at least.

Even sharing a dry exhaust is poor practice for many of the same reasons. About a pound of water is created for every pound burned and that water goes out the exhaust. You really don't want any of that moisture going into the non running engine where it instantly condenses.
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Old 03-06-2014, 10:48 AM   #12
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Originally Posted by mbevins View Post
I know a person who had a late _90s 54Searay that has the Genny exiting the main exhaust. Perhaps you can research how they did it,

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I had 3 sterndrive Searays and the exhausts were separate. Guess one would expect that though
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Old 03-06-2014, 10:59 AM   #13
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I was attempting to avoid making two new holes in my transom. With the genset now below the veranda, it's really no issue to make another hole for the gen exhaust as well, but with both exhausts on the port transom, it'll probably look pretty messy.
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Old 03-06-2014, 11:00 AM   #14
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Quote:
Originally Posted by O C Diver View Post
I think you're mistaken here. This is a very common setup on sport fish boats over 45'. See a lot of the Carolina and Florida built boats where the main engine, the generator, and the raw water discharge from the AC units all exit the one exhaust pipe. They have been doing it that way for quite a while.

Ted
ABYC P-1 1.5.7 A separate exhaust system and terminus shall be provided for each engine and generator installation

P-1 also requires that back pressure shall be in accordance with manufacturer specification. Unless you have the ability to measure this on a joint system for both engine and generator you have another problem and will void your warranties.

Adding an a/c discharge to the mix endangers both genny and propullsion engine and is also prohibited by P-1
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Old 03-06-2014, 11:06 AM   #15
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And both side are generally correct...even though collocated..their teminous is at the transom...both of them...usuallyy you see the ends of all the drains/exhausts...it's not like one just dumps into the other 3 or more feet into the boat and trickles out just one large opening.

I have seen one large "drain" that many things including the A/C discharge go into on Sea Rays...but the exhausts didn't.
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Old 03-06-2014, 11:10 AM   #16
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Quote:
Originally Posted by O C Diver View Post
I think you're mistaken here. This is a very common setup on sport fish boats over 45'. See a lot of the Carolina and Florida built boats where the main engine, the generator, and the raw water discharge from the AC units all exit the one exhaust pipe. They have been doing it that way for quite a while.

Ted
You need to look closer. They have separate pipes, terminating at the transom inside the larger main engine pipe, they do not share the same pipe.
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Old 03-06-2014, 11:11 AM   #17
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Well, what the heck...I may as well tie in the head and macerator discharge. With a well-timed dump and a rev of the engine, I could spray those pesky jet-skis crossing my wake on the ICW.
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Old 03-06-2014, 11:16 AM   #18
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You need to look closer. They have separate pipes, terminating at the transom inside the larger main engine pipe, they do not share the same pipe.
No not all, but I know what you mean.
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Old 03-06-2014, 11:37 AM   #19
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You need to look closer. They have separate pipes, terminating at the transom inside the larger main engine pipe, they do not share the same pipe.
Think that's pretty much how I explained it being done in post #7. You can see all the pipes if you look in the exhaust, but they all exit the transom in one large exhaust pipe. At least the ones I've seen combined do it that way.

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Old 03-06-2014, 11:41 AM   #20
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My boat's prebuilt hull originally had two exhaust ports: one starboard and one port. Since I didn't order a genset, the completed boat came with only one exhaust port (port side, for the engine).

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