Dry exhaust

Beautiful boat. Always preferred the look of the wheelhouse being toward aft.

guy with a boat said:

My main engine has a dry stack and we have never had a sooting problem. Not any noticeable soot on our boat and no complaints from neighbors. We have a recent (2015) Deere 6090 and I do run it up to 70% power for 15 minutes or so sometime in the last hour of a long run, and those factors may help.

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As noted by you and TT, newer Nordhavns with Tier XXX engines seem a good fit for dry stack. My less than positive experiences with dry stack soot are on pre 2010 vessels. A 2017 N52 I'm familiar with has not appreciable soot issues.

It's a debate that almost rivals that of anchor types. I covered it in this article https://www.proboat.com/2017/11/clearing-air-case-dry-exhaust/

I inspect (and have been involved in the construction of) a lot of dry exhaust vessels, and while there are undeniable advantages to dry exhaust, and keel coolers, there are some drawbacks as well. Bottom line is, there's no free lunch, and as others have said, conversion is almost never practical, the vessel needs to be designed around it.

My boat was built with dry exhaust and keel coolers welded right into the hull, using the outside aluminum hull plating as the exchange surface. I love the setup. I don't use any salt water for engine cooling at all, and there is really zero maintenance, beyond changing coolant out every few years, which you have to do on wet and dry systems. It was set up originally with really powerful engine room ventilation, so heat is not a problem. You do loose a little floor space in the back of the pilot house for the chase that the exhaust pipes, and hot air ventilation shaft run through. The noise level seems very reasonable to me, but I don't have a ton of experience on wet exhaust boats to compare to. Certainly it is not much louder outside the boat, than a typical boat with wet exhaust.

guy with a boat said:

Lastly, I have never put a bucket or any other cover over the exhaust. I’m not sure how to explain the difference between my experience and those of some others, but we have zero complaints or issues with the dry stack.

I think either system can be equally desirable, and the quality of design and construction of the boat is far more important than the type of exhaust system.

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YOur location may play a big part in the lack of need to cover the tip. Also the orientation of the boat during rain. NOt suggesting you don't get rain but it may not be enough to cause you trouble. Maybe Nordhaven had a few tricks up their sleeves that we are not aware of.

I cover mine because we get lots of rain and if the tip is aimed into the rain then, and almost only then, do I get sooting. May not be in the rainstorm but days later when I start the engine again.
So I cover it any time the engine is not running, summer or not.

THere was a thread years ago in Boatdiesel about a dry stack equipped boat in, I think France, who had his engine damaged by rain entry from an uncovered stack. They figured out the water was FRESH, not coolant, not seawater. He had a tough time understanding how the rain made it to the engine since he had never needed to cover before and he had owned the boat for a number of years. What finally convinced him was the amount of rain that year and the direction the boat was oriented in relation to the rain.

Under the right conditions I could see that happening to me thus the cover.

We have clean burning JD 6068’s with dry stacks/keel coolers and can’t complain. They are muffled, insulated and have expansion joints. Reasonably quiet, engine room is hot but not excessive and have had minor soot on the aft deck only twice in 100’s of starts. These were during the winter after several weeks without starting. The discharges have weighted flaps and fairings which prevent rain intrusion. I think that sooting tends to occur in cold humid weather causing some degree of condensation inside the stacks.

I do clean the keel coolers twice a year during Florida-Bahamas cruising but have not encountered engine overheating.

FWIW, Nordhavn has done several vessels with wet exhaust over the years and their latest, the N41, is as far as I know, wet exhaust only at this time. Both wet and dry have pros and cons.

There is extensive turbulence behind a deckhouse and vortex shedding off a rounded mast. The conversations I've had with owners burdened with sooting have been on single engine CENTERLINE exhaust installations.

Having the exhaust stack near the beam ends (like DCDC with twins, or just asymmetric with a single) would help greatly. Even better would be extending the exhaust laterally to the full width of the boat.

Get the exhaust into clean laminar air flow as much as possible.

It's been quite a few years since I modeled cars/spoilers and airplane wings sections in a wind tunnel. Thinking back on those days, it would be interesting to experiment with a small high lift wing mounted immediately below the outlet on a single, centerline exhaust. Upside down of course! That would direct air upwards. This could be something that you could retrofit on an existing installation.

I wonder if I can get NASA to advance a little cash on this idea $$$

Mako
Very good points. Stack placement though is not applicable when a soot prone vessel is idling at the dock with neighbor's vessels moored nearby.

Some years ago we had an acceptable offer on a Nordhavn 55. The vessel was moved from Lake Union to Elliot Bay. During that slow transit the boat show cleaned upper decks received soot. Upon further inspection a commissioning yard found the chase quite sooty, the exhaust line failing and the vessel over propped by owner's choice. We walked.

The worst sooting I have had is from a twin.


Wind is generally unpredictable.


If in a certain radius of some dry stack boats...I wouldn't bet on single or twn to make the difference.


Either the setup soots a little or a lot is the big question...and that can be for several reasons.

There is one Nordhavn, I think a 46, that ran the exhaust to the top of the mast.
That should have taken care of any soot on the upper deck.

OldDan1943 said:

There is one Nordhavn, I think a 46, that ran the exhaust to the top of the mast.
That should have taken care of any soot on the upper deck.

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Nope...look at older ships with the stacks very high and soot all around th stack on deck.


See my last post.

Look at the winged arrangements of the newer cruise ships for inspiration.

You guys talking aerodynamics remember we are discussing 8kt boats, right?

Maybe I’m overlooking the opportunity to add some wings and increase my cornering capability.

Has anyone sprayed water into the stack to capture all particulate?

guy with a boat said:

You guys talking aerodynamics remember we are discussing 8kt boats, right?

Maybe I’m overlooking the opportunity to add some wings and increase my cornering capability.

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No wind there?

psneeld said:

No wind there?

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We do have wind. My problem is that it keeps changing direction and then I have to start over on my wing calculations.

I never had a soot problem with my N46.
Maybe a fouled injector?

That’s what hearing as well. Seems to depend on health of the engine and wether exhaust is well engineered, executed and if necessary maintained. But with appropriate materials and components maintenance should be minimal.

Hi MrX

Xsbank said:

I have bought new injectors too, looks like its a lot more complicated on a Cummins than a Lehman! I also have a plastic drain pipe with a handle for the stack.

I have no salt water in my boat except for the genset. If I get soot on startup, I have a battery-powered leaf blower that takes care of it. No saltwater, no coolers with salt water, no saltwater pumps. Oops. I do have a salt-water fire pump. And it has one of those cheap-ass Jabsco rubber impellers that I hate...

My keel cooler is inside the keel so no worries there. There are better ways.

Besides, nobody would reject a boat simply because of its exhaust, you live with it.

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Hi Mr. X

Could you please let me know the " better ways " to cool vs your current keel system

Soot is incomplete combustion , fix the engine.

Exhaust Noise is from a cheap muffler , easily fixed.

From a guy with patents on how to clean up genset exhaust.....


"Most large yachts use a wet muffler system on their generators. Just like the main engines in small yachts, water is sprayed into the exhaust upstream of the muffler but in this case the muffler is constructed to function as a gas/water separator. The muffler holds water at a certain level and the excess drains out the bottom and to an underwater through hull. The exhaust gas along with water from the spray ring/mixer enters the muffler below the internal water level and "dry" gas is then discharged overboard.



The exhaust is thoroughly scrubbed by water spray and moving through a water column. Even with this type of system, particulates stain the hull sides. Nothing short of particulate filtration will remove all the soot.



Some of those boats described above incorporate an option to direct the exhaust overboard at the side or transom or up a dry stack before the water injection point. They have a choice of both worlds.



What many people don't understand about exhaust is that for every pound of fuel burned, close to a pound of water is created along with a few grams of particulates. Those particulates mix with water vapor and in the case of a "dry stack" that vapor will condense on the interior walls of the stack piping and "plate" the walls with a layer of soot. Eventually, with heating, cooling, and vibration, chunks of that soot layer will break off and land on the deck or in the water.



Spraying water in the stack is probably the worst way to reduce particulates. Heating the stack can produce good visual results in some circumstances but the amount of power required to keep the exhaust hot just means more fuel burned and more particulates generated.



If there was an easy or cheap solution this would not be a topic of conversation."

Am I wrong to believe that the more the exhaust run is insulated throughout its course the less soot you will see with a dry stack?
Of course insulation keeps the engine room a bit cooler and given it’s nature may decrease noise a bit but one would think keeping the exhaust gas as hot as can be would mean more vapor and less liquid water. Also if the exhaust gasses are hotter they will rise faster and further after leaving the terminal exhaust pipe

Hippocampus said:

Am I wrong to believe that the more the exhaust run is insulated throughout its course the less soot you will see with a dry stack?
Of course insulation keeps the engine room a bit cooler and given it’s nature may decrease noise a bit but one would think keeping the exhaust gas as hot as can be would mean more vapor and less liquid water. Also if the exhaust gasses are hotter they will rise faster and further after leaving the terminal exhaust pipe

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I will let my expert chime in when he sees this...but my view is the same...insulation keeps the heat in the stack till exhausted...thus less soot buildup...but I am NOT an engine guy...


It works on wood stoves in my experience.

Here is another reply ....


"Am I wrong to believe that the more the exhaust run is insulated throughout its course the less soot you will see with a dry stack?"


Not wrong at all but practice is removed from theory.



Even with really good (read thick bulky expensive) insulation the exhaust cools with expansion and a huge temperature drop occurs right at the stack outlet. If wind doesn't move the gases away instantly the resulting mist of soot and water droplets will fall to the deck or stick to the stack top, antennae, masts, or whatever else is in the way. Look at photos of dry stacks, note the extreme deposits around the outlet.


We have seen great dry stack improvements with additional exhaust heating. If you can keep the gas hot enough that condensation occurs some distance removed from the outlet (ala the condensation trail behind a jet at altitude) the issue may be mooted to some degree but this is not a practical solution for many reasons.


A visibly clean exhaust plume doesn't mean there is no soot either. Consider that an EPA Tier 4 engine can still put just over 2 ounces of particulates per day out the stack.

Capt. Nemo, I already have the better way, the box keel contains the coolant.

The only disadvantage was my fault, my anodes are welded on and when replacing them I welded the new ones in the same place. One year, after grinding off an anode, I tried to pry off the second side (it looked like a poor weld) and I made a very small crack in the keel and coolant began to weep. As you can guess, impossible to get a good weld with coolant present so I had to drain, filter and reload 200 litres of coolant, which added a few hours to my yard bill. 200 litres comes out of a garboard plug with a vengeance; I had a coolant shower, sticky, smelly and gross.

I now choose different and random locations for my anodes.

My experience with soot has been it only rears it's ugly head at startup and I use a nice battery powered leaf blower to remove it if it gets on other's boats.