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Old 05-07-2021, 03:57 PM   #1
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dry vs wet exhaust

I'm curious...especially from those of you that have had comparable boats with both.

daydreaming and gawking at some of these boats, it seems they could be ordered with either type of exhaust system. Some Nordhavn's for example...

Assuming both are properly designed for the boat, quality components, etc...which would you choose? why?
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Old 05-07-2021, 04:30 PM   #2
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Wet exhausts are quieter and easier to fit inside an engine room and the boat itself. The raw water used for exchanger cooling is used to cool the hot exhaust so it can be handled by an exhaust hose.

Dry exhausts send soot upwards to fall back on your boat, are noisy particularly on a fly bridge near the end and require an insulated chase for the exhaust stack usually through the galley/main salon. Dry exhausts either require a raw water pump for exchanger cooling or an external keel cooler.

All in all I think I prefer a wet exhaust. But if you want to give me a Nordhavn with one, I won't turn it down.

David
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Old 05-07-2021, 04:52 PM   #3
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Wet exhausts are quieter and easier to fit inside an engine room and the boat itself. The raw water used for exchanger cooling is used to cool the hot exhaust so it can be handled by an exhaust hose.

Dry exhausts send soot upwards to fall back on your boat, are noisy particularly on a fly bridge near the end and require an insulated chase for the exhaust stack usually through the galley/main salon. Dry exhausts either require a raw water pump for exchanger cooling or an external keel cooler.

All in all I think I prefer a wet exhaust. But if you want to give me a Nordhavn with one, I won't turn it down.

David
+1. The soot issue is a big one for me. We had a previous boat with a diesel fireplace and the soot was a PITA.
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Old 05-07-2021, 05:10 PM   #4
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the key is to have boat designed around the dry exhaust system. With proper muffler the noise can be acceptable. With a keel cooler the raw water pump is eliminated making for a simpler system. I would vote for a dry exhaust if I had a choice and could afford the boat. I does take up a fair amount of room running the exhaust line so that is a negative on the living area.
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Old 05-07-2021, 05:33 PM   #5
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Although I gave serious consideration to a Nordhavn, and had been persuaded by most of the "features" that make a Nordhavn unique and presumably ideally suited to their mission of long range voyaging, I ultimately chose a semi-custom boat that was more suited to fishing. One of those features was dry exhaust. (And having had a wet exhaust trailer boat in which catastrophic damage was done to the engine because an exhaust manifold had corroded, their points about the reliability of dry exhaust resonated with me.) Because the boat I chose was semi-custom, I explored, but quickly abandoned the idea of dry exhaust -- routing the exhaust stack through the salon would be disruptive, and having the exhaust above the flybridge would interfere with my tower -- and a tower is very important to fishing. Maybe I am living in a state of ignorant bliss, but my decision was made easier because I was persuaded that risks of a well-designed wet exhaust system are minimal. In choosing between wet and dry turbos for my engines, I opted for dry (perhaps as some sort of self-compromise on the dry exhaust issue). In retrospect the wet turbos would have been a much better choice.
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Old 05-07-2021, 05:43 PM   #6
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My N46 was dry exhaust. WOT the temp would climb to the point where I had to throttle down every now and then. Each time I hauled I sent the keel cooler to a shop to be boiled out and pressure tested. The keel cooler would fit into slight cavity for protection. I might have been in 'over-kill' mode, send the cooler to a shop but, ...... maybe I should not have run it a WOT for long periods of time plus the water temp in FL is a bit higher than other parts of the world too.

Now, my AT34 has a wet exhaust. No complaints so far.
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Old 05-07-2021, 05:57 PM   #7
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Speaking of soot I am wondering if any boat is using some kind of catalytic device to reduce it? I mean if you look at diesel cars soot was an issue 40 years ago when you were able to see blackened car trunk but nowadays it is really not the case anymore.

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Old 05-07-2021, 06:24 PM   #8
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If we are taking a vote, I vote for wet exhaust in any boat under about 100 feet.

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Old 05-07-2021, 06:31 PM   #9
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Paid four figures to clean diesel soot off my trawler after crossing the Pacific aboard a ship.
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Old 05-07-2021, 06:41 PM   #10
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My commercial boats were all dry exhaust and private boats were wet.

No matter how you insulate dry exhaust, it's usually nosier. And it takes up space in the center of the cabin structure. If the dry exhaust piping isn't made out of stainless, you replace it every few years. There are always soot issues. Even with a proper combustion balance, moisture gathers in the piping after shutdown and next startup, is expelled as sooty water. I have been in commercial fishing boats that were dry exhaust and quiet, but had a foot or more expanding foam between the engine room and living areas. Plus about a foot of foam around the exhaust structure and 6' of foam on the inside hull. And other sound deadening material.

With some effort, wet exhaust can be quieted down close to zero. I ran covert patrol boats in the USN. They exhausted under water, the engine room was well insulated, and twin Detroits couldn't be heard at idle. That was sound deadening tech of 50 years ago.
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Old 05-07-2021, 07:27 PM   #11
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Originally Posted by skyhawk View Post
I'm curious...especially from those of you that have had comparable boats with both.

daydreaming and gawking at some of these boats, it seems they could be ordered with either type of exhaust system. Some Nordhavn's for example...

Assuming both are properly designed for the boat, quality components, etc...which would you choose? why?

Soot is largely a function of the engine. Some are soot creators, some not so much. My CAT 3306 doesn't produce much soot, and in 15 years of running, I suppose I've cleaned soot a dozen times, if that. I;ve cleaned soot from the genset that is wet exhaust many, many more times over the years. The space required is a raceway that butts up against the galley counter. On one side I mounted a Newport diesel heater, which we use all winter. Because of where the raceway is located, its practical impact on usable space is essentially zero.


I do run the engine up to 80% WOT for 10 minutes before returning to the slip and that seems to blow out any accumulated soot. I also built a a stainless steel faux stack mounted over the actual exhaust stack that keeps rainwater out. Sound wise, I measure 62 db in the pilot house, and the exhaust is located immediately behind the PH in a "stackhouse" that also holds the hot water tank and Kabola boiler. So noise is hardly a problem.


Bottom line, like most stuff on boats, if properly thought through and well designed and the engine is not one that produces a ton of soot (thinking about DDs, CAT 3208s, etc.), a dry stack/keel cooler is a simple, quiet system. You mileage may vary.
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Old 05-07-2021, 07:34 PM   #12
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I'm fine with either, slight preference to wet . Twisted Tree is on his second Nordhavn build and went with wet which definitely snapped my neck towards wet. His logic is solid. Maintenance item in a wet exhaust is mostly the raw water impeller which is inside the boat and easily accessible. Dry exhaust has a keel cooler which is outside the boat.

Not a huge difference, but looking purely at wet cs dry stack doesn't tell the whole story. There are other components.

I suspect trawler folks bow go the alter of Beebe when it comes to dry exhaust. Back in the day it made more sense. Now I'm not so sure.

Peter
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Old 05-07-2021, 07:40 PM   #13
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Commercial boats tend to have working deck, where the exhaust is more than just a nuisance, but can be a serious health hazard, so it has to be directed elsewhere. The logical alternative is to send it skyward.
Pleasure craft don't need the engine running when working on the aft deck, so there is no problem letting the engine exhaust right below the stern.
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Old 05-07-2021, 08:03 PM   #14
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My engine's wet exhausts exit to port slightly above waterline, often requiring closure of port pilothouse door to minimize smell, but no problem with soot.
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Old 05-07-2021, 08:18 PM   #15
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Commercial boats can have lots of variation in load. Get the tailpipe buried under water and that makes engine safety more complicated. And most commercial boats don't have a flybridge so engine exhaust note is not that big a deal. And most are keel cooled, so no need for a sea water pump. Soot on deck, who cares!! All adds up to make sense on commercial boats.

Recreational boats, wet exhaust makes the most sense for the opposite reasons.

I do like the coolness factor of dry stack. Had two boats myself with such. Kinda like riding a Harley. Fun for five minutes.
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Old 05-07-2021, 08:55 PM   #16
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We have a dry exhaust. All of my other boats, and those I worked on, had wet exhaust.

Some of the wet exhaust boats had relief ports on the side of the hull (Broward). I preferred scrubbing the soot from the transom and hull sides vs. random places on the upper decks, now.

I think the heat in the ER would me much less with a wet exhaust.

As Twisted Tree explained very well in another thread, with a wet exhaust, you do all of the maintenance INSIDE the boat. No diving or haul out required.

My preference would be wet exhaust if I were ordering new. Otherwise, we got what we got with the boat we wanted...dry.
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Old 05-07-2021, 09:06 PM   #17
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As others have noted, I built a Nordhavn 60 with dry exhaust. Then I built a Nordhavn 68 and switched to wet exhaust, and it was frankly in the top 5 list of reasons we switched boats.


That said, there are plenty of pros and cons for each. I still have friends with dry exhaust, I still speak to them, and they still speak to me. So if you have a personal preference, go for whatever that is.
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Old 05-08-2021, 07:07 AM   #18
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one of the youtube channels I watch is MV Freedom. I recall a video fairly recently where Sean was talking about a rain cap he puts over their dry stack while not running to eliminate (or more likely minimize) soot.
Does that sort of thing offset that issue enough?
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Old 05-08-2021, 07:49 AM   #19
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one of the youtube channels I watch is MV Freedom. I recall a video fairly recently where Sean was talking about a rain cap he puts over their dry stack while not running to eliminate (or more likely minimize) soot.
Does that sort of thing offset that issue enough?
Some use a flapper, others a soup can.
On the Nordhavns, I have seen pics of a rather large external muffler and a pic of where the owner had run the exhaust all the way up the aft mast.

Does anything work? I dont know.

The AT engine exhaust is on the stbd side below the water line plus a small exhaust all the way aft stbd side.

I remember we were all taught, we have about 30 seconds to check for a good flow of water out the exhaust. These days, that bit of advice holds little meaning, if the exhaust is below the water line.
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Old 05-08-2021, 08:00 AM   #20
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Flappers can keep rain out of the plumbing, but they have the annoying tendency to rattle under way.
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