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.

healhustler said:

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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I've seen it done on sport fishers done like that.

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!!

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,

Sent from my Nexus 7 using Trawler

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.

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.

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.

Ted

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.

boatpoker said:

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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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

O C Diver said:

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

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Yep, the way I was talking about had a sea cocks right behind the transom where the ac, genny, and other things drained.

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.

mbevins said:

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,

Sent from my Nexus 7 using Trawler

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I had 3 sterndrive Searays and the exhausts were separate. Guess one would expect that though

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.

O C Diver said:

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

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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

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.

O C Diver said:

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

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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.

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.

caltexflanc said:

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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No not all, but I know what you mean.

caltexflanc said:

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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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.

Ted

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).

healhustler said:

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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This is exactly how the Vinette holding tank is plumbed.

boatpoker said:

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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Don't know what to tell you. There are hundreds of boats that have it that way. There is a 100'+ yatch that visits Sunset Marina every summer. The marina doen't have big enough shore power for them, so they just run their generator all the time. Everything comes out this one pipe in the transom. Then again, maybe they have a generator that requires an 18' exhaust pipe.

Regarding water intrusion, both my generator and my engine have water lifts and the loops look to be 24"+ above the exhaust which exits above the water line. My generator has an exhaust water separator. The water is separated out and goes over board through a seacock in the bottom of the hull (to eliminate the splashing noise I've been told) and only the gas exits through the exhaust pipe.

While I've not done it yet, I will need to measure back plessure on the exhaust system for warranty purposes when we install the new motor.
While I haven't talked to the dealer (manufacturers representative regarding verification of all criteria for warranty), I imagine measuring back pressure on this system is not a big deal as they have to do it on all installations that have a water lift. Who knows, maybe they just put a gauge on the drain plug on the lift.


Ted

Greetings,
Mr. Poker. "...is also prohibited by P-1". ABYC standards are NOT laws! They are what the boating industry in North America have agreed are probably good ideas to be applied to new builds.
AYBC P-1 1.7.2 states "An indicator shall be provided that is effective at all helm positions to indicate loss of exhaust system cooling water supply. This indicator shall be independent of the engine block temperature indicator(s)."
How many used boats have a loss of coolant water indicator? How many "giant red flags" has this raised on any of YOUR surveys?
I think the point has been made that shared exhausts are common while not, as Mr. RickB pointed out, necessarily a good idea.

I don't recall saying anything about law.

My boats have always had raw water flow detector alarms at the helm.

Greetings,

prohibited

Use Prohibited in a sentence
pro·hib·it

[proh-hib-it] Show IPA
verb (used with object) 1. to forbid (an action, activity, etc.) by authority or law: Smoking is prohibited here.

2. to forbid the action of (a person).

3. to prevent; hinder.

Didn't mean to yell. Just the way it copied. Good for you that you have a raw water flow detector. My question was how many vessels that you have surveyed failed the survey by not having one? I know several members have such a warning device but I suspect I'm pretty safe in saying that they are in the minority.

O C Diver said:

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.

Ted

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Perhaps we are confusing semantics but saying the same thing!
I have had the opportunity to look at literally hundreds of these boats in and out (for hurricanes) of the water here in eastern NC and in FL. I may have some photos on another computer, will look next time I have a chance.

RT Firefly said:

Greetings,

prohibited

Use Prohibited in a sentence
pro·hib·it

[proh-hib-it] Show IPA
verb (used with object) 1. to forbid (an action, activity, etc.) by authority or law: Smoking is prohibited here.

2. to forbid the action of (a person).

3. to prevent; hinder.

Didn't mean to yell. Just the way it copied. Good for you that you have a raw water flow detector. My question was how many vessels that you have surveyed failed the survey by not having one? I know several members have such a warning device but I suspect I'm pretty safe in saying that they are in the minority.

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In context "prohibited" ... if you wish to meet ABYC Standards.
Surveyors do not pass or fail boats, they simply point out issues.
I always recommend (in my reports) raw water flow alarms where there is no engine temperature gauge (common on sailboats) or plastic mufflers.

PS. Sharing exhaust for any purpose is illegal in Canada (TP1332E) and European RCD,

Ted: I don't think I've got that 24" above the water line for the lifts.....more like 16-18", limited by the veranda sole. Still, that's a lot more than I had for the genset when it was mounted under the galley, which needed an anti-siphon that went through the galley sole and looped under the fridge. For all practical purposes, the joint exhaust should work, and I envisioned the genset exhaust tubed into the top of the main exhaust, just as you described.

I've gone this far and to great expense for this swap. I'll do the two separate exhausts as recommended, but since both the main and the genset exhaust exit on the port side, it only makes sense to exit them easily to the port transom. Many boat owners complain about genset exhaust exits on one side or another, especially when they are rafting. I really dislike the idea of two separate exhausts on one side of the stern, but....

Another plus with separate exhaust is you can look over the transom and see that both are pumping water as they should. Combined, the genny water flow could be reduced and it would be hard to tell.

When I installed the genny in my personal boat, I sent the exhaust out the port side a few inches above WL. When running hull speed, the exhaust is buried. In hindsight, I'd rather have run it straight out back to the transom.

On a few boats around here with genny and water lift in ER, and a long straight FG tube out to the transom, the long tube seems to get rid of the annoying whoosh-whoosh. The water flow seems to have a chance to even out in the pipe and just spill out the exit (mostly). Provided the tube is large enought, like 2" for 5-6kW.

Greetings,
Mr. bp. RE: your PS....TP 13332E 9.1.2.7 does indeed state "No additional discharges other than cooling water shall share the exhaust gas passage." but does not mention sharing exhaust inputs to the same cooling passage that I could find.
TP 13332E as far as I can see is a LAW as are USCG requirements unlike AYBC which are recommendations. Not meaning to pick on you at all but the general consensus seems to be that AYBC is the law. I fully understand the need for guidelines and the guidelines DO provide direction and uniformity.