Wet or Dry Exhaust

[Owtmoist]

Hi all,

Newbie question.

Can anyone comment on the advantages / disadvantages of a dry exhaust with keeling colling vs wet exhaust system?

During our search for a boat the majority have the wet exhaust setup, but a few have a dry exhaust/keel cooling. Managed to find a bit of information about the pros and cons, but certainly doesn't seem clear cut.*
As for the keel cooling I've only heard disadvantages.

For info I'm talking about 38ft approx Motor Cruiser / Cutter / Trawler boats with single screw 80 - 140 hp engine.

Thanks

 

[Duke 239]

Dry stack with keel cooler advangaes are decresed number of through hull fittings and sea water hoses to break and flood your engine room. Also, there are no heat exchangers with saltwater on the inside that require service. Disadvantage is bridge clearance. Most dry sack boats are 25+ feet tall and cannot be lowered. Other disadvantage is fouling of the keel cooler on the outside. This is mainly for people who ignore fouling in general. For folks taht ignore fouling and just sit at the dock hosting oyster colonies, I would suggest wet exhaust since the sea cocks are always closed anyway.

 

[tiltrider1]

Keel cooling dry exhaust eliminates a lot of failure points and expenses. You eliminate raw water pumps and raw water heat exchangers.

Unfortunately what you gain is more noise and a dirtier boat due to the exhaust settling on the boat.

No free lunch in this world.

 

[RT Firefly]

Greetings,
Welcome aboard.

 

[mvweebles]

Dry stack with keel cooler advangaes are decresed number of through hull fittings and sea water hoses to break and flood your engine room. Also, there are no heat exchangers with saltwater on the inside that require service. Disadvantage is bridge clearance. Most dry sack boats are 25+ feet tall and cannot be lowered. Other disadvantage is fouling of the keel cooler on the outside. This is mainly for people who ignore fouling in general. For folks taht ignore fouling and just sit at the dock hosting oyster colonies, I would suggest wet exhaust since the sea cocks are always closed anyway.

I hope TwistedTree finds this - when he built is Nordhavn 68 a few years ago, he went against Nordhavn orthodoxy and opted for wet exhaust. He can go into details, but in short, servicing wet exhaust can be done within the boat. Keel coolers and such need to be serviced outside the boat. They also take quite a bit of interior space.

I'll add that while there may not be a raw water pump, they frequently require massive blowers that will cause about as much trouble if they fail as a raw water pump.

Peter

 

[Lepke]

I've had many dry and wet exhaust boats. Dry exhaust doesn't meam you have to use a keel cooler. You can still have heat exchanger cooled engines.
I like keel coolers the best. Close to zero maintenance and no salt water pump to maintain, no impellers to change. But you do have to keep the cooler free of growth. In warm water you need a bigger cooler and sitting idling at a dock with no current, the cooler might not keep the temps down.
Dry stack usually means an exhaust trunk going thru your cabin. It's also noisier. Wet exhaust usually goes out the stern and less noticeable. You can exhaust under water for even less noise.

 

[Mako]

I’ll throw in that if you have a metal boat then it’s a no-brainer to go dry because of the simplicity of welded pipe cooling integral with the hull. Plus cost. Those Fernstrums cost thousands. Split pipe costs a few dollars.

 

[twistedtree]

I hope TwistedTree finds this - when he built is Nordhavn 68 a few years ago, he went against Nordhavn orthodoxy and opted for wet exhaust. He can go into details, but in short, servicing wet exhaust can be done within the boat. Keel coolers and such need to be serviced outside the boat. They also take quite a bit of interior space.

I'll add that while there may not be a raw water pump, they frequently require massive blowers that will cause about as much trouble if they fail as a raw water pump.

Peter

Peter summed it up pretty well, but I'll admit that it's largely a toss up between the two which is why the debate never ends. It ultimately is picking which poison you like better.

For us, dry exhaust was one of the things we really didn't like about our Nordhavn 60. Here's why.

- It's dirty, and in all the wrong places. We didn't get greasy soot particles raining down on the boat like some people, but we did accumulate a haze over all the upper mast instruments. And washing it off was a royal PITA. You had to climb the stack, hold on with one hand, and scrub everything with the other hand. And you are scrubbing oddly shaped and often delicate devices. All the black goo would end up all over you, and wash down all over the boat. So cleaning the stack actually meant a top to bottom wash of the whole boat. In contrast, any haze left by wet exhaust accumulates on the side or transom of the boat where it's easy to wash off.

- The dry exhaust, even with a "hospital grade" muffler, was loud. Not straight pipe, DD loud, but significantly louder than a well muffled wet exhaust. I found it very uncomfortable to even be on the fly bridge when underway. At the pilot house helm, the N60 was 63db which by most measures is pretty good. But my 68 with wet exhaust is 52db. The flybridge was 80db, and the boat deck (loudest place) was 85db. The loudest place on the 68 is the cockpit, and that's 75db/ Every other place on the boat is in the 60s or lower.

- I hate changing impellers. But my keel cooler required just a frequent maintenance for cleaning and/or zinc replacement. And as Peter mentioned, the impellers you can change from inside the boat where the keep cooler work is done from in the water. If you are always in warm water and/or regularly have your bottom cleaned that might be just fine, but for us in cold water where you don't get the bottom cleaned, it was a pain.

- A dry exhaust will last a long time, but when it's time to replace the muffler, it's an ugly project. I have followed a couple of them and it's a big job. Other than impellers, a wet exhaust is largely maintenance free. You just need to keep an eye on the injection elbow or ring.

- Heat exchanger and/or aftercooler maintenance. Yes, both suck with a wet exhaust, and most dry exahaust engines. Yup, it sucks.

- I think the whole "fewer thruhulls" thing is blown way out of proportion. You will have one less thruhull. It will vary by boat, but I still have thruhulls for the wing engine, generator 1, generator 2, watermaker 1, watermaker 2, HVAC, anchor wash, and hydraulic cooling. And if I didn't have a thruhull for the main engine, which in turn feeds my dripless shaft seal, I'd have a thruhull dedicated to the dripless shaft seal. You're gonna have raw water running through your boat. Just accept it and do a good job on it.

- Cleaning strainers sucks. With dry exhaust you won't have to do that for your main engine, but you will still have to do it for everything else.

- Another gotcha if you are looking at a new build is that some engines now require a raw water loop for the aftercooler regardless of how you cool the rest of the engine. I think it has to do with maintaining different temperatures for emissions, but I'm not sure.

So you can see there is good, bad, and indifferent for both. I prefer the wet poison over the dry poison, but I question it every time I have to change impellers, all of which are due shortly....

 

[Pascall]

What I haven't seen in this thread yet, the disadvantage of a wet exhaust is often too high exhaust back pressure.
This can have adverse consequences, especially for modern engines.
Next disadvantage, if a waterlock is placed, there is always a container of water in a closed environment near the pistons, which can cause corrosion.
A well-constructed dry exhaust can be just as quiet as a well-constructed wet exhaust, downside, it takes up a lot of space.
With a dry exhaust, you don't have to winterize the engines.
We have our own system with a wet exhaust, to reduce the back pressure, 80% of the cooling water goes overboard before it is pumped into the exhaust.
The remaining 20% is for the coolness of the exhaust.
To properly cool the exhaust and reduce noise, an exhaust mixer was installed in the hose and a two-chamber waterlock was made.

Mvg,

Pascal.

 

[sunchaser]

Two of the newer Nordhavn models are the 41 and 51. Both have twin engines and wet exhausts. Both are well into backordering and quite popular. It would appear times are changing a bit for Nordhavn.

 

[Serene]

I haven't seen it mentioned in this thread, but what about heat? With the dry exhaust you have hot gases passing up through the salon, which in turn would heat the boat.

I do understand a lot of these installations have blowers to move the hot air, but it is still a consideration. Well maybe not if you are in a cooler climate!

 

[klee wyck]

I run one of each. The John Deere are wet. The Mercedes is dry.
I have a significant preference for dry. On the dry stack/keel cooled boat, zero raw water enters the vessel. No impellers, no zinc.
On a steel boat, it almost seems like a no brainer.

 

[Comodave]

I haven't seen it mentioned in this thread, but what about heat? With the dry exhaust you have hot gases passing up through the salon, which in turn would heat the boat.

I do understand a lot of these installations have blowers to move the hot air, but it is still a consideration. Well maybe not if you are in a cooler climate!

We live in Michigan and still use A/C a lot in the summer. Maybe Alaska???

 

[sunchaser]

Hummm, the boats I’ve lusted over the most are:
- Fleming 58
- KK 52
- OA 65
- DF 57
- Aleuthian by GB 58
- N57
- Northern Marine 64

On the above list only the N57 comes with a dry stack but with a few made with wet exhaust. I’d take an 57 equipped either way in a minute; although I did pass on an N57 with a dry exhaust manifold on its Cummins NT 855 with help from Tony Athens on spotting on engine exhaust leaks.

It is hard to imagine a Fleming with dry exhaust. Likewise it is hard to imagine a commercial fishing boat with wet exhaust - why , because of a variable waterline due to loads moving the waterline up and down.

Then comes the yacht owner who feels the need for no cooling exhaust water in the boat. OK, but how then does one account for through hulls associated with shafts, rudders, active stabilizers, AC water, gensets, get homes, transmission cooling, big reefers, fish boxes and holding tank discharge?

Would I pass on the right boat with dry stack? No. But I’d do the white glove test as I’m a neat freak.

 

[Fish53]

I've had several of both, though dry has been on commercial boats mostly. My current boat is dry with keel pipe and the reasons are all mostly stated previously, my biggest reason however is using my boat in winter. I live in Maine and until recently our harbors force over but now they freeze just a little. With dry exhaust and keel cooling I have no raw water in the boat hence no freezing issues and I can enjoy boating at 10 degrees.

 

[long-cours.62]

If I have the choice I will prefer the dry exhaust, particularly on an alloy boat hull, you can don't use a "keel cooling but a "hull tank" .
I know few boat (and lot of barge) who doing that.
Also one disadvantage of the wet exhaust it is : when you stop your engine, he became cooler it "suck" salt humidity air from your waterlook...Probably I am not clear .
I don't see any reason, if well fitted and sized, for have bigger fans with dry exhaust than wet one ?


Also with dry exhaust and keel or tank cooling you still able to start your engine even even when your boat is "molded" in ice. with alloy boat it could happen (with our former Long Cours 62 the blade of the rudder was "molded" down to the bottom 1.30 below wl.)

 

[mvweebles]

I always chuckle at the argument against an engine thru-hull, as if these are a common failure point. I mean seriously - look at how relatively fragile air con pumps are with plastic pump-heads and how they are rarely connected with robust hose and double clamps. And somehow a bronze Groco seacock well backed into the hull with heavy wire-reinforced hose double-clamped to bronze nipples is going to sink the boat?

TT's explanation was informative to me. Of all the selection criteria for lusting over a boat, dry/wet exhaust is way down the list. Bridge clearance (ICW) and sooting are issues you'd have to deal with everytime you use the boat so I'd definitely lean towards wet exhaust, but I could make a dry stack work.

Peter

 

[Bmarler]

I haven't seen it mentioned in this thread, but what about heat? With the dry exhaust you have hot gases passing up through the salon, which in turn would heat the boat.

I do understand a lot of these installations have blowers to move the hot air, but it is still a consideration. Well maybe not if you are in a cooler climate!

Yes, there is heat, but that’s why people say they take up too much space. You need an insulated box on the interior of the boat to run the stack through.
I like a dry stack, we always had them on the fishing boats. We scrubbed the deck regularly so sooting didn’t seem like an issue. I guess with pristine white gel coat I could think differently. But wet exhaust isn’t without sooting, I certainly have to clean the transom and cockpit of my current boat regularly.
And height? I have a 28 foot air draft with the mast up. Dropping it is a pain, so…

 

[Fish53]

I don't see the blower issue, most larger vessels have them regardless of exhaust type and smaller vessels get by without. There's about ten thousand lobster boats in Maine that get by fine without blowers. Obviously with proper ventilation.

 

[sunchaser]

Possibly being over propped, excessive blow by or bad injectors are the cause.

 

[Bmarler]

Possibly being over propped, excessive blow by or bad injectors are the cause.

Of course those are things to consider. My problem isn’t excessive, but does require maintenance. My engine has a fair amount of hours on it, so it’s not surprising to have a little smoke at this point in its life. I know I’m not the only one with wet exhaust that gets a little soot on the transom though.

 

[rslifkin]

Soot with wet exhaust depends on both the engine and the boat. Depending on how much soot a given engine puts out, how well the exhaust knocks that soot into the water, and where the exhaust is placed on the boat (and how air flows around that) determines whether you get soot on the hull. Many boats with older, dirtier burning diesels do get some soot.

 

[twistedtree]

My new boat gets soot on the sides and transom, and I agree that's normal. All boats will soot, though the amount will vary.

My point about soot is that it's easier to wash it off the side/transom than to climb the mast and wash it off all the nav instruments, and then wash the rest of the boat because everything you washed off the mast is now all over the lower parts of the boat.

 

[long-cours.62]

My new boat gets soot on the sides and transom, and I agree that's normal. All boats will soot, though the amount will vary.

My point about soot is that it's easier to wash it off the side/transom than to climb the mast and wash it off all the nav instruments, and then wash the rest of the boat because everything you washed off the mast is now all over the lower parts of the boat.

Not necessary to climb the mast for clean à dry exhaust...some dry exhaust are at the same place than wett one

 

[sunchaser]

Our wet exhaust vessel had no discernible soot. The placement of the wet exhaust discharging under the transom was one reason. A well thought out mixing elbow and water lift muffler helps too. Then comes the engine health and propping.

If your vessel cannot meet full rated RPM plus 50 RPM or so the chances of a sooty wet exhaust increase. The worst wet exhaust soot I’ve seen is from side discharge exhausts for what ever reason, possibly less mixing time in the exhaust run.

RickB who used to be on TF designed a soot and noise abatement system for large yacht gen sets. It is a science IMHO.

 

[rslifkin]

Our wet exhaust vessel had no discernible soot. The placement of the wet exhaust discharging under the transom was one reason. A well thought out mixing elbow and water lift muffler helps too. Then comes the engine health and propping.

If your vessel cannot meet full rated RPM plus 50 RPM or so the chances of a sooty wet exhaust increase. The worst wet exhaust soot I’ve seen is from side discharge exhausts for what ever reason, possibly less mixing time in the exhaust run.

RickB who used to be on TF designed a soot and noise abatement system for large yacht gen sets. It is a science IMHO.

I'd definitely expect more soot from a shorter run with less mixing. Side exhaust will also have some of the exhaust gas travel along the hull for at least a short distance, providing an opportunity to deposit soot. With a transom exhaust, the gases may or may not make much contact with the boat depending on the exact placement of the exhaust, airflow around the boat, etc.

 

[Capt Dan]

I don't see the blower issue, most larger vessels have them regardless of exhaust type and smaller vessels get by without. There's about ten thousand lobster boats in Maine that get by fine without blowers. Obviously with proper ventilation.

We used to kid around when a dry stack lobster boat would come by and say, "What, can't afford a muffler?"

Nowadays I think most Maine Lobster Boats have wet exhaust.

 

[Daveo]

I’ve had both. Currently a dry stack. Either is good but the devil is in the details. Most of the negative issues raised with either type can be ameliorated with good design. My dry stack adds no noticeable heat to the boat. It is well insulated. All the heat comes from that massive hot lump of moving metal below it. In fact, because the exhaust enclosure is effectively a chimney venting the engine room, it probably helps. My exhaust runs all the way to the top of the mast, nothing up there to collect soot. Never a hint of exhaust odor on the boat and I’ve spent days underway with zero apparent wind. Now that I am in the frozen north I really appreciate not having to winterize, one less thing to worry about. So I like the dry but don’t think wet is worse, just different.