Wet versus Dry exhaust

[ksceviour]

Love the Nordhavn 40 however my only experience with dry exhaust stacks was a converted fish boat I charted many years ago. What I recall from that was the occasional bit of soot on the deck. Not much and only until the engine warmed up but soot. Our current boat is a wet system.

I'd like to get some input on the experiences, good and bad, associated with dry stacks.

 

[Sabre602]

We have a dry stack and my late stepfather had one, too (both diesels).

I wouldn't want a wet exhaust after running these boats. The peace of mind from not having hot salt water circulating through the engine room is incalculable. Besides, the average Dickinson diesel stove puts out way more soot. And, after all, a bit of light smoke on start-up (maybe, depending upon make) but other than that, a properly-tuned diesel isn't putting out black smoke.

Yes, you must keep a bucket on your stack to keep out the rain...no sweat.

We hose down the boat after a trip to rinse off the salt anyway. The local fish boats and gulls probably contribute more to polluting our topsides than does the diesel exhaust.

 

[dave907]

Dry exhausts are great with one big but very rare exception: Fire hazard.

I grew up in the commercial fishing industry in Cordova, Alaska. All the diesel boats had dry stacks, including all of the boats owned by my family. 99% of the time they are simply bullet proof in my experience, but if you allow dirty fuel to cause soot it can build up and cause a problem.

It's my opinion that wet exhausts and the impellers associated with them are far more likely to cause a problem than a dry stack. I once had a wet exhaust hose split on me on the CHB I owned and it could have been a big problem (like boat sinking problem) if the weather had not been perfectly calm and we were miles from a leeward shore.

I've seen maybe 2 fires from dry exhausts in my life, and I've been around hundreds of boats putting 12-18 hours a day on the engine for most of the summer. It just wasn't an issue.

YMMV. Luck.

Dave

 

[Lepke]

I've run dozens of boats, wet and dry. I don't have a preference. For me the issue would be sparing room for the dry stack and you'll get some exhaust rumble in the cabin. Wet usually puts all the exhaust noise behind you.

 

[sunchaser]

Boats with variable water lines are not candidates for wet exhaust. This includes commercial fishing boats and ships. Once away from these obvious cases and into recreational vessels, either works. It then comes down to one's acceptance of soot, ( both inside and out) ER temperature control and concerns about neighbors' decks.

The N40 is a fine vessel and they each have a different level of previous soot issues dependent upon engine age and previous care. Plenty of larger Ns out there with wet exhausts took, meaning there is not necessarily unanimity in the Nordhavn camp.

Hoses and pump parts need care and attention on both wet and dry exhaust. For grins, find out the date of the last coolant change on a used N you're considering. Then ascertain coolant volume.

 

[FF]

I love a dry exhaust , but some folks claim the dry is noisier than a wet exhaust.

It can be if done poorly. A big truck muffler is a great help, and cheap to do.

On a new build I would look up "Hospital Critical" muffler systems.

Yes 100+ pounds and not cheap but silence is GOLDEN!

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Hapco Super Critical Grade Exhaust Silencers (VCS,SCSC) ... 32 dBA* Recommended application for the VCS grade silencer is Hospitals, Ships, Tugs, ... and Construction equipment, Military equipment, Air Intake silencer for Diesel Engines.

 

[Ski in NC]

Dry exhaust stack has a certain "coolness factor" that I really like.

I like to hear (a little) "stack talk". Better than the woosh woosh or gargle from a wet exhaust.

But dang that Harkers boat with 4-53 Detroit with dry stack was just plain LOUD.

It is a challenge to do dry stack right. You have to build in flex pipe, it has to be properly insulated, muffler has to be well chosen and mechanically supported, box tends to intrude into interior, gots to keep water out of stack and muffler chase, and yes, some soot.

Stack fires are real too. Had a stack fire on one boat doing sea trials. Boat had logged lots of hours at putt-putt speed, loaded the muffler with goo and soot. Powered up and flames and sparks out the stack, muffler and pipe red hot. Fortunately nothing was in a wooden chase or we could have had quite the fire. We monitored it and changed power levels, successfully burning it clean. Sketchy, though.

Wet exhaust and seawater cooling has its own issues, and certainly can be engineered wrong. But my opinion is wet is easier than dry, depending on the application. And most trawler type engines are designed from the start for wet.

I still like dry. But my personal boat has wet.

 

[Delfin]

Any system on a boat can be done well, or poorly. Done well, dry stacks have advantages. The alleged disadvantages, IMO, are the result of poor design, and/or operation. I might see a fleck of soot once every 20 hours of operation, probably because I run the engine up to 80% power or so for 15 minutes before idling down and shutting off. The noise level in the pilot house is around 60-62 decibels, which is conversation level. The stack is 3' behind the pilot house but well insulated with a Cowl muffler. I don't worry about fire because I don't let combustibles build up in the stack, and it is encased in insulation anyway.

I've heard some wet exhaust Bayliners approaching from three miles off, they are so loud, and have literally have had to hold my breath to avoid exhaust fumes billowing up from water level exhaust on boats near mine in the marina.

There's no ER temperature issues because the stack is well insulated and puts out no more heat than a wet exchanger in any case. On this boat, as compared with wet systems I've maintained on others, I've never changed an impellor, or cleaned an exchanger after an impellor disintegrates, or dealt with sea water corrosion in the exhaust system or changed an exchanger zinc. Never cleaned the keel coolers because their design obviates that requirement of some other designs. Maybe I'm missing out on other positive attributes of a wet system, but I'm having a hard time coming to with what they might be, other than a trade off of slightly more space in the ER for slightly less in the saloon, which seems close to a wash to me.

 

[sunchaser]

Delfin
Your points are well taken. Your vessel is well designed and powered and propped correctly, but not all dry stack yachts are so fortunate IMHO.

I well remember the N55 I had an offer on that went south because the soot was so bad, both in the chase and exterior. Another N55 I was looking at had the same engine and no soot, but was propped differently - no "cruising" prop. Or the new N6? at our dock that had the Series 60 engine removed due to excessive soot issues. The top side fabrics and bimini had to be replaced. A new JD1225 was put in and no soot.

BTW, my RW pump impellers come out looking very good after about 500 hours (18 - 24 months) , throw them away anyway. No broken bits and pieces plus the engine plumbing design screen would catch them right after pump if they did go. And yes, I replaced my exhaust elbows. Doing the genset elbow for the second time this spring.

I won't get into the keel cooler plumbing leaks on my friends N55 this past summer. Boats and maintenance, no matter what exhaust design, it seems there is always something.

 

[caltexflanc]

The peace of mind from not having hot salt water circulating through the engine room is incalculable

I don't understand what this has to do with dry stack vs wet exhaust. Whether you have salt water circulating in the ER or not is dependent on how the engine is cooled. What am I missing here?

 

[Delfin]

Delfin
Your points are well taken. Your vessel is well designed and powered and propped correctly, but not all dry stack yachts are so fortunate IMHO.

I well remember the N55 I had an offer on that went south because the soot was so bad, both in the chase and exterior. Another N55 I was looking at had the same engine and no soot, but was propped differently - no "cruising" prop. Or the new N6? at our dock that had the Series 60 engine removed due to excessive soot issues. The top side fabrics and bimini had to be replaced. A new JD1225 was put in and no soot.

BTW, my RW pump impellers come out looking very good after about 500 hours (18 - 24 months) , throw them away anyway. No broken bits and pieces plus the engine plumbing design screen would catch them right after pump if they did go. And yes, I replaced my exhaust elbows. Doing the genset elbow for the second time this spring.

I won't get into the keel cooler plumbing leaks on my friends N55 this past summer. Boats and maintenance, no matter what exhaust design, it seems there is always something.

Tom, you're quite right that some engines are better than others in terms of soot. For whatever reason, the CAT 3208 seems to smoke a lot more than the 3306, and Detroits have a reputation for being a bit dirty in that department. And, I had neglected to mention the importance of propping, although I think that getting the whole system hot enough to burn out any carbon goes a long way towards addressing any potential issue. In my case, that is running the EGT up from around 475 to 750 - 775 degrees.

I believe someone mentioned the need for a "bucket" over a dry stack to prevent water infiltration. When I see that, I really wonder what the designers were thinking since there are easy ways to prevent that. Delfin came with a gravity cap that clattered like crazy at idle, so I had the below built in polished stainless and to my knowledge, have never had a drop of rain water get inside the stack.

 

[Sabre602]

I don't understand what this has to do with dry stack vs wet exhaust. Whether you have salt water circulating in the ER or not is dependent on how the engine is cooled. What am I missing here?

No, we have no raw water impeller that could disintegrate, no exhaust elbow that corrodes away from the inside, no wet exhaust hoses that could sink the boat if they split. The keel cooling system is a closed system that contains engine coolant; no salt water enters the boat at all.

Of course these are all things that must be maintained, and I've lived with them on other boats, but I really like NOT having them.

 

[klee wyck]

Thru hulls....anyone in love with these things over a 40 year span of a boats life? For me it is part of this debate. Dry exhaust and keel cooling eliminate a lot of them.

People tend to talk their own book. I have two volumes, so think about both most days. Let's assume that both of these Dutch built steel boats were 'done right' for their different cooling and exhaust systems in the power train.

Having owned both for a bit, all other things being equal, I would pay a ten percent premium at least for the dry exhaust, keel cooled boat. Although I understand not always, they tend to go together(keel cooling and dry exhaust) as do the treatment of the main(s) and gennies. The simplicity of the engine room on Libra with all spinny things keel cooled with dry exhaust is...simply stunning and refreshing. The two Mercedes (main and genny) do not seem to create a soot problem and certainly less than the diesel furnace on either boat and I do love my diesel heat. If soot, out the side or top is not a big deal to me. (no flying bridge)
The chase that holds the exhaust behind the pilot station upstairs is neither obtrusive nor loud. When standing on the finger outside the boat I need to look up at the flapper on the exhaust to see if the boat is even running. Not true on the wet exhaust on the other boat that I can hear from 10 slips away. And so what, the low rumble of power is sort of pleasing to me in some odd way.
This is a lot like the question of steel versus plastic..... I say drive what makes you happy. There is not likely one right answer or all boats would be built the same. My boat is always the best....wanna buy it?

 

[caltexflanc]

The keel cooling system is a closed system that contains engine coolant; no salt water enters the boat at all.

Again, my point was that this is independent of having a dry stack exhaust, which is the subject of the thread so that's where I got confused.

 

[twistedtree]

I think anyone considering this should start with the assumption that it just doesn’t matter. Both, when well implemented, work well. So prime focus should be on that.

Then, if there is some particular aspect of one vs the other than you fall irrationally in love with, let it sway your decision. Otherwise move on to more important selection criterial like anchors, chain, and filter micron ratings.

 

[RT Firefly]

Greetings,
Mr. Saint tt. "... more important selection criterial like anchors, chain, and filter micron ratings."

 

[Sabre602]

Again, my point was that this is independent of having a dry stack exhaust, which is the subject of the thread so that's where I got confused.

Again, whether or not a dry stack boat has a keel cooler, absolutely no sea water is injected into the exhaust. The entire point of my post was that the dry system is both far less maintenance AND less worry in the event of a failure. That comparison is, indeed, the subject of this thread, right?

 

[sunchaser]

I think anyone considering this should start with the assumption that it just doesn’t matter. Both, when well implemented, work well. So prime focus should be on that.

Then, if there is some particular aspect of one vs the other than you fall irrationally in love with, let it sway your decision. Otherwise move on to more important selection criterial like anchors, chain, and filter micron ratings.

 

[FF]

For the steel boat folks the ease of having a "keel cooler" that is built in behind the outer skin means no extra drag , or worries on haul out about lifting strap positions.

 

[ksceviour]

I think anyone considering this should start with the assumption that it just doesn’t matter. Both, when well implemented, work well. So prime focus should be on that.

Then, if there is some particular aspect of one vs the other than you fall irrationally in love with, let it sway your decision. Otherwise move on to more important selection criterial like anchors, chain, and filter micron ratings.

I agree with you there. My intent with this post was just to get input from owners/operators that have used the dry stack extensively. That said, it wouldn't be the overriding concern on a purchase. Like you say, there are many more important issues surrounding the decision to buy a particular vessel.

I'm partial to a dry stack myself for many reasons however it never hurts to get input on issues that just wouldn't occur to someone with little or no experience with the system and this forum is great for that. For instance, I hadn't heard of potential fires from buildups in the stack. Important thing to be aware of but I just had never heard of that. Same goes for the bucket. Whenever I saw that on a stack I always assumed it was because there was an issue with whatever normally kept the rain out not that it never came with a way to do that in the first place.

 

[twistedtree]

You are smart to ask. With any system like we are talking about, there are always the “well known” attributes. But there is always a deeper story that comes from day to day use and hands on experience. Boating magazines never get pst the veneer on the surface, so I greatly prefer blogs and forums.

Here’s an example argument against wet exhaust. We all know you need to change impellers. But less is said about what happens when the impeller is used too long and buts if it break off. Then you need to take apart the raw water system until you find and remove all the pieces. Now that’s a pain. It’s avoidable through proper maintenance, but I expect many people learn the hard way. And there is also the issue of running the pump dry and melting down the impeller, something the fancy Globe impellers are prone to do. They initially survive much better than a conventional impeller, but when they go you get a blue solid glob stuck to the inside of the pump.

 

[FF]

"Then you need to take apart the raw water system until you find and remove all the pieces. Now that’s a pain. It’s avoidable through proper maintenance,"

Better engineered , tho costly , systems can save lots of hassles.

If the sea water is fed to a filter AFTER the rubber impeller pump , loosing the impeller is still a PIA , but no reverse flushing or chunk searches are required.

If low enough in the boat a centrifugal pump would work .

An Alfa Laval or similar) centrifuge can save a lot of filter maint although a proper fuel tank with a sump can keep the tank clean too.

A bypass lube oil filter can extend engine oil changes , perhaps so a mid passage change can be extended till dockside.

 

[Delfin]

You are smart to ask. With any system like we are talking about, there are always the “well known” attributes. But there is always a deeper story that comes from day to day use and hands on experience. Boating magazines never get pst the veneer on the surface, so I greatly prefer blogs and forums.

Here’s an example argument against wet exhaust. We all know you need to change impellers. But less is said about what happens when the impeller is used too long and buts if it break off. Then you need to take apart the raw water system until you find and remove all the pieces. Now that’s a pain. It’s avoidable through proper maintenance, but I expect many people learn the hard way. And there is also the issue of running the pump dry and melting down the impeller, something the fancy Globe impellers are prone to do. They initially survive much better than a conventional impeller, but when they go you get a blue solid glob stuck to the inside of the pump.

The impellor on the hydraulic cooling line blew up a couple of years ago, which is what prompted me to replace it with a Grunfos electric that is completely quiet and has no maintenance parts. What surprised me was that this loop was coolant, so you'd think the impellor being constantly lubricated would have lasted, but nope.

I did learn something, however. I put a small mesh filter on the return line to catch the bits of flotsam and jetsam (never sure which is which) so the pump wouldn't jam. I didn't bother cleaning out the exchanger because the system cools fine, but I've caught a handful of rubber bits and more keep showing up. The interesting part is that the filter is just one of those little 50 mesh ones, so bits smaller than 50 mesh have been passing the Grunfos just fine. This leads me to think that if I want to replace the impellor pump on the genset with a Grunfos but filter the raw water for the exhaust through a properly sized algae 50 mesh filter, it should work just fine. Since the genset is constant speed, the same amount of water is needed all the time for the gas/air separator and water muffler, so it should work. Not sure what the downside would be....

The cartridge (i.e. the pump separate from the motor) for the Grunfos is just 4 allan head screws, so swapping out a replacement, should it ever been needed, is really easy.

 

[DDW]

To answer one question posed, closed circuit cooling and wet exhaust are not completely orthogonal. To have a wet exhaust you need a source for the water to cool it which involves most of the bits required for a raw water circuit. Hence they tend to go together. Dry exhaust offers the opportunity, though not the necessity, of closed circuit cooling.

 

[Delfin]

To answer one question posed, closed circuit cooling and wet exhaust are not completely orthogonal. To have a wet exhaust you need a source for the water to cool it which involves most of the bits required for a raw water circuit. Hence they tend to go together. Dry exhaust offers the opportunity, though not the necessity, of closed circuit cooling.

And vice versa. Wet exhaust can be closed circuit keel cooled. My genset is wet exhaust and keel cooled because I couldn't figure out a graceful way of providing dry stack exhaust for it.

 

[Portage_Bay]

.....

flotsam and jetsam (never sure which is which) so the pump wouldn't jam.


.....The cartridge (i.e. the pump separate from the motor) for the Grunfos is just 4 allan head screws, so swapping out a replacement, should it ever been needed, is really easy.

Flotsam floats after a sinking. Jetsam is what you jetison to keep from sinking.

Grunfos pumps do wear out, but it takes a looooong time. I've seem them go more than a decade runnin 24 X 7. I'd guess on a recreational boat they will last the lifetime of the systems. But by the time they do wear out they are obsolete and you wind up replacing the entire pump and motor.

 

[Delfin]

Flotsam floats after a sinking. Jetsam is what you jetison to keep from sinking.

Grunfos pumps do wear out, but it takes a looooong time. I've seem them go more than a decade runnin 24 X 7. I'd guess on a recreational boat they will last the lifetime of the systems. But by the time they do wear out they are obsolete and you wind up replacing the entire pump and motor.

Ok, then it was flotsam I cleaned out....

And I see I mis-wrote. The cartridge includes both pump and motor. The base remains in place. And you're right, I'm on our second Grunfos pump for a circ loop in our house after 26 years. Changed it about 10 years ago.

Does it surprise you that they don't jam with bits of rubbish <50 mesh size? Kind of did me, since they seem to have fairly tight tolerances, but clearly not.

 

[Portage_Bay]

I don't know about chunks of rubbish. I did kill one by not flushing the lines on a large hydronic system well enough after changing the boiler out. Black crud, most likely from the boiler's death, killed the pump in just a few days.

Repeated flushing and back flushing until it ran clear and the new pump lasted many years.

 

[Steve DAntonio]

Love the Nordhavn 40 however my only experience with dry exhaust stacks was a converted fish boat I charted many years ago. What I recall from that was the occasional bit of soot on the deck. Not much and only until the engine warmed up but soot. Our current boat is a wet system.

I'd like to get some input on the experiences, good and bad, associated with dry stacks.

There's no wrong answer here, each have their advantages; but it is a bit like asking about anchors, some strong opinions.

It's a question I hear often enough that I wrote a column on the subject for ProBoat last year. https://www.proboat.com/2017/11/clearing-air-case-dry-exhaust/

 

[Steve DAntonio]

You are smart to ask. With any system like we are talking about, there are always the “well known” attributes. But there is always a deeper story that comes from day to day use and hands on experience. Boating magazines never get pst the veneer on the surface, so I greatly prefer blogs and forums.

Here’s an example argument against wet exhaust. We all know you need to change impellers. But less is said about what happens when the impeller is used too long and buts if it break off. Then you need to take apart the raw water system until you find and remove all the pieces. Now that’s a pain. It’s avoidable through proper maintenance, but I expect many people learn the hard way. And there is also the issue of running the pump dry and melting down the impeller, something the fancy Globe impellers are prone to do. They initially survive much better than a conventional impeller, but when they go you get a blue solid glob stuck to the inside of the pump.

Agreed on the Globe impellers. This article shows a photo of the remnants of just such a melted impeller...

https://www.proboat.com/2010/07/offshore-passage-prep-hydraulics/