Dry exhaust

I have been doing a lot of reading as I am looking for my first boat. It seems that one of the weak points of a marine engine is the wet exhaust. So the question is .... has anyone converted to a dry exhaust, (I know some boats, customs and some tugs) are built that way.

I have never converted a boat to dry exhaust but in 40 years of commercial fishing every boat I have run had dry exhaust. Both wet and dry have their good and bad points you just have to decide what is most important to you.

It is unlikely that in a boat designed with wet exhaust that there will be room to convert it to a dry exhaust. They require quite a bit of room and most people don’t want to give up that much room in a retrofit. The large majority of boats have wet exhaust and with a little bit of maintenance they are very reliable. Also that may be part of the reason you see quite a few problems is that there are just so many more of them out there.

Commercial boats usually have dry and yachts have wet unless they're big and have a stack. Dry stack is usually noisier and you need some interior cabin space the dry exhaust runs thru. Unless the exhaust trunk is well insulated it adds a lot of heat and more noise in the cabin. If the dry stack isn't stainless, it rusts out in a few years.

In addition to conversion to dry stack exhaust, which won't be easy, you have to deal with engine cooling with a keel cooler or a raw water pump system. Also if raw water after cooled you have to deal with that as well.

Basically it is almost never done.

David

The changeover is almost never done as others have stated.
Even for pleasure craft that have dry exhaust, there are issues, just different issues compared to wet exhaust.
A good friend had dry exhaust on his 50 foot steel hulled trawler. He had to cover the stack or birds would nest in there if not run often enough, sometimes they would persist adding stuff for days in a row even after it being "blown out". Sometimes his top deck area would get covered with an oily black soot that was very hard to remove, coming from his stack.
Others have mentioned the heat, noise, and space issues. Then there is the regular cleaning of sea life from the keel cooler (heat exchanger) mounted outside the boat for engine cooling. A blocked HX is not very effective at cooling.

Wet exhaust, requires a system of regular maintenance to avoid or reduce the associated problems. Lack of doing this, is the main reason you hear about problems with wet exhausts. Ensure that the thru hull is clean and clear and that the valve is regularly exercised (opened and closed) and greased occasionally. Change the impeller in the pump regularly (every year or two). Check and change engine anodes as needed. Major service all other raw water components such as fuel coolers, aftercoolers, gear oil coolers and heat exchangers by taking them off the engine and properly cleaning them (rodding out the passages and maybe acid bath) and then pressure test (with new seals or o rings), and then reinstalling. The exhaust elbow should be checked and cleaned as needed. This major service interval should be every 3-5 years depending on several factors like local salinity and growth, shallow water operation, and whether or not the engine is freshwater flushed or not regularly. If operated in freshwater or regularly freshwater flushed, the time interval can be extended and maybe even doubled.
No two ways about it, if you neglect your wet exhaust, that is one of the major causes of engine problems on boats. They are made up of items that require regular maintenance to operate reliably over many years. JMO

MNdiver said:

I have been doing a lot of reading as I am looking for my first boat. It seems that one of the weak points of a marine engine is the wet exhaust. So the question is .... has anyone converted to a dry exhaust, (I know some boats, customs and some tugs) are built that way.

Click to expand...

Downside of dry exhaust is the noise and if vented up, the sooty mess which develops over time on topside surfaces.

I hope Twisted Tree chimes in. He is having his second Nordhavn built and decided to go with wet exhaust. If you read this blog post from his first Nordhavn, you get a sense of what it takes to cool a dry exhaust.

In short TT went with wet exhaust because both systems need maintenance. Yes, heat exchangers clog up, but so do keel-coolers. Yes, heat exchanges need zincs replaced. But so do keel coolers. Yes, wet exhaust has a water pump, but many dry exhaust installations require an ungodly amount of blowers that are less reliable than engine-mounted gear-driven pumps. He decided he preferred to do maintenance on the inside of the boat instead of outside the boat. Hard to fault his decision.

TT converted me from my prior worship of All Things Beebe and his exaltation of dry-stack exhaust. It deserves a fresh look.

Peter

EDIT - there is a hybrid approach whereby the engine is keel-cooled vs a heat exchanger, but exhaust is raw-water cooled. Depending on your outlook, it's either the best of both worlds or the worst of both worlds. It's a relatively inexpensive conversion if so desired.

Thanks for all the great replies.Really good info. here. Good to get facts and perspective
from those in the know.

Fishboats, and other commercial boats that require crew to be on deck, working, while the engine is running, need to protect those crew from the smell and possible harmful effects of breathing in an exhaust rich environment.

A dry stack solves that problem, though as others have mentioned, there is a trade-off to a noisy, dirty, and sometimes smelly stack spewing that exhaust above the boat.

I had a dry stack+keel cooler. You'll have a beautiful efficient engine, but the noise is significant and takes away from the aesthetics of cruising. I also ended up replacing the keel cooler and it wasn't cheap. I wouldn't own one again.

One of my customers was very sensitive to diesel exhaust. He converted two Bertram’s to dry stack exhaust by running it all the way up the tuna tower.
The first one, Shooting Star, is still in service as a charter boat in Fort Lauderdale. The second larger Bertram caught fire in Biscayne Bay as it was returning from the Bahamas.
I don’t know if the fire was related to the exhaust or not. https://shootingstarcharter.com/fort-lauderdale-charter-boat.html

I had a dry stack on my Nordhavn. I liked it. ER was HOT HOT HOT but, I still liked it. Every 2 years I had the keel cooler removed, boiled out and reinstalled. Would I have another, you bet'chum Red Rider. Gotta learn not to open the cabinet where the exhaust pipe is located.

OD have a lots of questions aand hope you can address them.
Hear of people opening the engine room door to heat the accommodations. Have you used this ploy to lower ER temps? In other words open that door let’s say 15 minutes before an ER check. If so does ambient air temp matter? Does that work in the tropics?
Have been lead to believe shaft output is more efficient with dry stack than wet exhaust. That you can run a lower HP engine and get same performance and greater range. Talking with local commercial lobster men and small stern trawlers they tell me that’s a reason they choose dry stack. Is there truth to that?
Over the years have had concerns with impellers, mixing elbows, heat exchangers and pumps. Service/replace at fixed intervals not waiting for failure. Can you offer any insight as to which set up is more expensive, time consuming and difficult to maintain?
Peter’s blog link was very appreciated. Did you have difficulties with engine room heat? How high did ambient temp go in your boat? What model nordhavn did you have? Did you do any modifications? Where did you cruise? Temperate?sub tropics? Tropics?
Thanks in advance and please anyone else with dry stack experience do chime in.

mvweebles said:

there is a hybrid approach whereby the engine is keel-cooled vs a heat exchanger, but exhaust is raw-water cooled.

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That’s interesting.

The commercial shipyard I’ve been negotiating with for a new build sets all their boats up with raw water cooled engine via a sea chest, with a dry stack exhaust.

MVWeebles summed up my take on dry vs wet pretty well, but I would make one important correction....


Yes, I had an ER heating problem, but I don't attribute ANY of it to the dry exhaust. That's because the exhaust is well insulated, and barely warm to the touch. So it radiates very little heat into the ER. The ER temperature issue and my feelings about dry vs wet are really two completely independent things. The heat in the ER all comes from the surface area of the engine and all it's accessory parts. If the exhaust were uninsulated the heat contribution would be huge, but that's not the case.


As for dry/wet, both require maintenance - just different maintenance. I found the noise unacceptable with the dry exhaust, though dedicating more space for more mufflers would surely help. And cleaning up after the dry exhaust was a real pain. I had to climb up on the hard top, scale the mast, hold on with one hand while scrubbing with a bucket of soap, a brush, and a hose using my other three hands. And all the rinsed off black sooty water would end up on me, and showering down over the rest of the boat, so it necessitated an complete boat wash. That's a LOT of maintenance that nobody talks about very much. It's much easier to wash soot off the side of your hull, below the gunnels.


Now I HATE changing impellers, especially bigger ones in main engines. You get salt water all over everything, they never come out easily, and it's always a wrestling match to get a new one squeezed back in . It's kind of like trying to stuff a cat into a bag. But at least that can be done from inside the boat, warm and dry. Keel cooler maintenance has to be done from the outside, in the water. It's kind of like being on a sail boat rather than a trawler. So gear up with the dive tank, jump into the cold water, change the zincs, and clean the crap off the cooler tubes. The zincs are a twice a year thing, and the cooler probably once a year. Now in lots of locations people have a regular diver clean the boat bottom and changing zincs is just a matter of saying "change the zincs", but in the PNW that's not the case, and the water is COLD. So in you go.

Another article, this one by Steve D comparing dry vs wet.

https://www.proboat.com/2017/11/clearing-air-case-dry-exhaust/

Years ago, I was aboard one of the first DDs built by Seahorse. The owner, a novice, had delivered her on her own bottom from China! Boat had originally been designed with dry exhaust but builder was unable to satisfactorily resolve ER heat issues, so was converted to wet exhaust with the exhaust through the side of the hull vs stern.

I cannot overstate the size and capacity of the blowers used in most recreational dry stack installations. These are not the little 12vdc jabsco blowers many boats have but rather multiple 120vac Delta industrial blowers. They are impressive and are only slightly less optional than the gear driven raw water pumps they replace.

The final consideration depends on your cruising aspirations: air draft. Dry stacks tend to be pretty tall in an effort to not throw soot on decks. Impressively, many Nordhavns have circumnavigate. Few, perhaps none, have completed The Great Loop.

I have no bones to pick with dry stack. But it is invariably part of the original design and build of a recreational boat. I only point out the negatives as they are often under-considered when comparing to wet exhaust.

When Twisted Tree (TF member) posted he was going with wet exhaust on the build of his N68, it snapped my head around. First, he's as wonky and technical as they come for trawlers - an incredibly articulate writer. Second, he's building a very expensive boat that customarily comes with dry stack.

I'll note that another wonky expert - Steve Dashew - went wet on his FPB globe girdling boats. Here's his discussion

https://setsail.com/wet-or-dry-exhaust-which-is-best/

Back to the OP, as stated in the linked Steve D article, conversion from wet to dry is unlikely to be practical. Can I ask why you're asking? Perception that dry is more robust and reliable?

Peter

There is almost no risk involved in a wet exhaust. Most boats are designed so that while underway the final exhaust port is above the waterline. In the event of an exhaust hose failure you would be adding a lot of cooling water to your bilge, but also a lot of exhaust fumes and gasses. You should be able to smell or hear it before serious amounts of either are in your bilge.

It would be pretty strange to have an exhaust hose fail while at the dock with the boat unattended but I suppose it could happen. That would be bad, very bad.

A hose failure at idle or trolling speed would also be bad but only an oblivious captain would not notice his boat wallowing.

Keep a big exhaust sized plug on board and enjoy the contented gurgling of your engine with a wet exhaust.

pete

Peter keep them coming great links. Living half in New England and half in the eastern Caribbean would note. Fouling is much less up north so keel cooler cleaning should have a longer interval than 2 weeks. Down in the tropics in water maintenance wasn’t a biggie even without a hookah or tanks. So think this is a trade off as down south we spent a lot of time snorkeling. Before taking off in the dinghy or when returning usually jumped in the water to look (and maintain if necessary) the bottom. Up north a short haul out of the spring/ fall rush is short money. Wonder what you use to clean the keel cooler? We use plastic scrapers for hard stuff and rough towels for slime and soft. Don’t like hard paint so always want the least abrasive. Is stiff nylon sufficient or do you need brass brushes for the cooler? Nylon was enough on the Frigoboat keel coolers for frig/freezer.
Really like the Dickerson DP cap but see a lot of simple buckets around here on dry stacks.

A dry exhaust is frequently just a truck muffler welded in place.

Folks preferring a quieter exhaust should look up "hospital critical" mufflers .

There not light , not cheap, but do a great job of quieting , especially since few cruisers are at full throttle for very long.


https://www.industrialexhaust.com/silencers.php is one of dozens of choices.

My goal is not to convince anyone that Dry Exhaust is bad - it's most definitely not. However, over the years, between Beebe and Nordhavn, dry exhaust is considered sacrosanct - a badge of offshore preparedness for a trawler. Given the nature of the OP's question - can a boat be converted - made me wonder if dry exhaust hype underpinned the OPs question. My 1970 Willard 36 (more or less coincided with Beebe's generation, which is why the W36 is pictured in his original book) originally came with galvanized pipe for exhaust, and was probably straight exhaust at the time - not very reliable. Dry stack made a lot of sense. Since then, wet exhaust has come a long, long way. To their credit, PAE has done an amazing job of designing and fitting dry stack too - not all designers and builders are similarly skilled/committed.

20-years ago, I was influenced by the dry-stack hype and therefore worshipped at the dry-stack alter, but not anymore. I personally would not select a boat based on its exhaust system provided it was properly designed and installed. Wet exhaust is remarkably trouble-free. I'm okay with either. I certainly would not go through any heroic measures to convert from one to the other.

Peter

Oh well, I'll comment too.
I do have a dry stack but with a heat exchanger. Have had the boat now for 35 yrs. Had to have part of the piping replaced 25 yrs ago at approx $5,000. The actual mufflers are original, 43 yrs. It has been a good system.

However if the boat had a wet system we would have bought it also.
I certainly would NOT change from one system to the other if a boat under consideration already was equipped one way or the other.

If you find a dry stack boat that suits you OR you find a wet exhaust boat that suits you don't let that alone deter you.

As pointed out both will work. What is important is how either design is done AND maintained. If done poorly then then you are going to have trouble. If done well the system should give good service.

But I will say don't let hype, either way, affect you too much.,

And if you really want an exhaust education then go to Seaboard Marine and peruse Tony Athens' articles about exhausts and the MOST important points about weak points .
He has offered huge amounts of info for free

www.sbmar.com

C lectric said:

Oh well, I'll comment too.

.....However if the boat had a wet system we would have bought it also.
I certainly would NOT change from one system to the other if a boat under consideration already was equipped one way or the other.

If you find a dry stack boat that suits you OR you find a wet exhaust boat that suits you don't let that alone deter you.

As pointed out both will work. What is important is how either design is done AND maintained. If done poorly then then you are going to have trouble. If done well the system should give good service.

But I will say don't let hype, either way, affect you too much.....


www.sbmar.com

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Nicely reasoned.

mvweebles said:

I hope Twisted Tree chimes in. He is having his second Nordhavn built and decided to go with wet exhaust. If you read this blog post from his first Nordhavn, you get a sense of what it takes to cool a dry exhaust.

In short TT went with wet exhaust because both systems need maintenance. Yes, heat exchangers clog up, but so do keel-coolers. Yes, heat exchanges need zincs replaced. But so do keel coolers. Yes, wet exhaust has a water pump, but many dry exhaust installations require an ungodly amount of blowers that are less reliable than engine-mounted gear-driven pumps. He decided he preferred to do maintenance on the inside of the boat instead of outside the boat. Hard to fault his decision.

TT converted me from my prior worship of All Things Beebe and his exaltation of dry-stack exhaust. It deserves a fresh look.

Peter

EDIT - there is a hybrid approach whereby the engine is keel-cooled vs a heat exchanger, but exhaust is raw-water cooled. Depending on your outlook, it's either the best of both worlds or the worst of both worlds. It's a relatively inexpensive conversion if so desired.

Click to expand...

I ran a 45 foot crew boat, aluminum hull with keel cooler ( the running strakes were angle welded on the bottom and rigged as a massive keel cooler) and water cooled exhausts. The original engines were Detroits and later changed to John Deers.

I own and run one of each.
I agree with the concept that neither are deal makers or deal breakers within a boat you might like and cannot conceive the thought of retrofitting one to the other for the benefits or risk of one over the other.
Having said that, if I were looking at two boats that were otherwise identical, I would choose the keel cooled, dry stack boat. While I don't really lose sleep over it, no saltwater inside of a boat and no thru hulls tips the balance of plus and minus of each for me personally. That could be related to the build materials I prefer.

Hippocampus said:

OD have a lots of questions aand hope you can address them.
Hear of people opening the engine room door to heat the accommodations. Have you used this ploy to lower ER temps? In other words open that door let’s say 15 minutes before an ER check. If so does ambient air temp matter? Does that work in the tropics?
Have been lead to believe shaft output is more efficient with dry stack than wet exhaust. That you can run a lower HP engine and get same performance and greater range. Talking with local commercial lobster men and small stern trawlers they tell me that’s a reason they choose dry stack. Is there truth to that?
Over the years have had concerns with impellers, mixing elbows, heat exchangers and pumps. Service/replace at fixed intervals not waiting for failure. Can you offer any insight as to which set up is more expensive, time consuming and difficult to maintain?
Peter’s blog link was very appreciated. Did you have difficulties with engine room heat? How high did ambient temp go in your boat? What model nordhavn did you have? Did you do any modifications? Where did you cruise? Temperate?sub tropics? Tropics?
Thanks in advance and please anyone else with dry stack experience do chime in.

Click to expand...

My Nordhavn was a 46, (dry exhaust) great boat for a couple and friends. It was destroyed in a shipyard accident.
My current boat is a AT34 with wet exhaust. It has 2 installed resistance heaters and I have one ceramic heater. Starting the engine to get warm is always an option.
I am in south FL so if it gets cold, I put on long pants, sweater and socks.
I know, if I get really cold, I can always run the engine with the access deck plates open and risk falling into the engine room. SMILE

The rest of this is just my opinion.
Efficiency is based first on boat designed, engine selected, speed, sea state, current etc.
I would not consider changing or modifying the exhaust without factory personnel.
Wet or dry exhaust, each have their own set of maintenance situations.
Which is better? You pretty much have to accept what was on the boat originally, by design.
I think I answered all your concerns, if not ask again.

twistedtree said:

Now I HATE changing impellers, especially bigger ones in main engines. You get salt water all over everything, .....

Click to expand...

This is why I always do a fresh water flush before changing the impeller (or any other work on the raw system). Much better to spill fresh water inside than salt. On a large boat with a good watermaker, you don't even need to be at the dock to do this. It doesn't take that much fresh to flush well beyond the raw pump.

DDW said:

This is why I always do a fresh water flush before changing the impeller (or any other work on the raw system). Much better to spill fresh water inside than salt. On a large boat with a good watermaker, you don't even need to be at the dock to do this. It doesn't take that much fresh to flush well beyond the raw pump.

Click to expand...

I'd like to do something like that. How do you have things plumbed up to do the fresh water flush? I think there are lots of ways it could be done, and I'm always interested in examples and how well they work.

twistedtree said:

I'd like to do something like that. How do you have things plumbed up to do the fresh water flush? I think there are lots of ways it could be done, and I'm always interested in examples and how well they work.

Click to expand...

Each vessel is different. Access is good on our DeFever making the impeller change out and clean up task not too difficult.

What I do on ours is close the through hull, drain the engine raw water by opening the little petcock on the bottom of the strainer and have a bit of fresh clean up water available for mopping up any released raw water when impeller cover is removed. Ditto the genset.

Don't forget that after you clean the soot off your decks your dock neighbors may be asking you to clean your soot off their decks, and some not so nicely.