Dry exhaust

Hi, I'm new here and would like to hear from anyone using or interested in dry exhaust and keel cooling.

Interested in? Not sure I fit that description, but I am a dry exhaust user.

Still learning, but when I bought my boat it had one dry and one wet exhaust. Note that "dry" and "wet" are relative to the engine - at least in my case - in that water was injected into the exhaust of both engines. The water in the wet exhaust was injected prior to the max height of the riser so that there was the possibility of exhaust coolant water being next to and possibly ingested by the engine in less than ideal conditions. The water in the dry exhaust was injected after the max riser height so that as long as the injected water followed the laws of gravity, it should never come in contact with the engine.

When I had major engine work done after purchase and prior to delivery, I had the wet exhaust riser replaced with a dry on the advice of the engine surveyor. The new riser itself wasn't that expensive, but I don't recall exactly how much.

Why do you ask?

Sounds like you actually have wet exhaust on both engines as there is water injected anywhere in the system. I assume you are also using heat exchangers for engine cooling. The type of system I'm referring to is a straight dry exhaust where there is no cooling water injected into the exhaust stream. Like a car or, more aptly, a diesel truck with vertical stacks exiting above the cab. The reason I'm asking is I'm building a small trawler type boat that will be using dry exhaust and keel cooling and I'm interested in how some people have configured their systems. Thank you for your response and good luck with your boat.

I have always been intrigued with a dry exhaust. Exhaust smell would be a concern but the biggest one to me is how to keep the rain out. You don't see flappers much anymore and the bent pipes would only work if the rain cooperates and blows in the proper direction. Most working trawler have it so there must be some tricks. Great question.

bayview said:

the biggest one to me is how to keep the rain out. You don't see flappers much anymore and the bent pipes would only work if the rain cooperates and blows in the proper direction. Most working trawler have it so there must be some tricks. Great question.

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I'm a big fan of keel coolers and a dry stack exhaust. It rains up here in the Pacific Northwest a LOT, and it's pretty easy to keep to rain out by covering the end of the exhaust when not in use. Most of the fishing boats just use a bucket or a cut up fender. I made a rain cap out of 5" PVC that works very well.

If you do forget and allow a lot of rain to run down the exhaust stack, it tends to make a mess (soot) the next time you start the engine. If you keep it dry, no soot, no mess, no salt water in the boat!

All of my boats, except a sailboat I owned briefly, have had dry exhaust. But those have all been commercial boats. It's true that soot can be a problem depending on the configuration of the outlet, but there's a soot problem with wet exhaust also, just not usually topside. I have never had a problem with rain entering the exhaust system with the outlet pipe bent sixty to eighty degrees from vertical and the tip cut at forty five degrees. My primary attraction to this type of system is the simplicity. I am aware of multiple cases of engine damage caused by wet exhaust that was inexpertly installed or maintained. But I have also seem fires caused by dry exhaust suffering the same inattention. Keel cooling also offers increased simplicity by eliminating pumps, seacocks and associated plumbing. I was curious if any pleasure boats use these systems and how they got around various configuration problems. The only production boats that I've heard of using these systems is Nordhavn.

There is usually an expansion bellows soon after the exhaust manifold / turbo in these systems, and perhaps another further up the stack depending on the length of the run and if you have a muffler. The piping and components are typically resiliently mounted nowadays, especially on yachts. It is recommended that the stack itself be insulated for noise and thermally with a non combustible insulation.

For water intrusion one of the simplest ways is a 45* + angle in the pipe after it leaves the stack with drain holes in the underside of the pipe after the bend. There are other ways but that is simplest.


Dry stack uses up valuable space inside a small yacht.


There are plenty of trawler type and large yachts that use dry stack / keel cooling,

Is that kind of what you are looking for?

F53, What engine will you be using in the MWB?

I'm powering with a whooping 23hp Kubota three cylinder, I'm still working on the design of the house which is why I'm thinking about running the exhaust up through the house and overhead. I have used stainless bellows sheathed in woven stainless at the engine end and a slip joint up at the stack for expansion. What I'm interested in is inside the trunk, what is effective for sound and heat insulation. In the past I've used reflective foam with lead core and more recently stitched blankets with fiberglass inside. I intend to cruise up to Labrador and around Newfoundland with this boat so a little heat inside might be a good thing.

We've got one of those Nordhavns with dry exhaust. Like everything, there are tradeoffs.

Pro's:

- No raw water system, so no impeller, no seawater heat exchanger, and no water injection elbow. Also no seawater after-cooler if so equipped. These are big plusses in terms of maintenance and things that can break and potentially cause significant engine damage.

- No soot on and around the transom, swim platform, etc.

Con's:

- The stack takes up measurable interior space on the boat.

- You need to cover the stack when not in use, or have a stack top with integral water trap.

- Soot is a hassle, as are hot exhaust gasses. The instruments at the top of my stack are covered in soot, and when the wind is behind me, my outside temp gauge reads 120-150F from the exhaust being blown forward. Some of this could have been mitigated by better placement of instruments had I thought about this in advance. We also get some amount of soot on the boat deck and dinghy, but not real bad.

In all honesty I think it's a toss. And not all Nordhavns are dry exhaust - not even close. I believe all twin engine boats are wet exhaust, and quite a few singles too. My rough guess would be that 25-30% of the fleet is wet exhaust, perhaps more.

This was my last trawler the exhaust is barely visible behind the open array radar to port. It doesn't look it in this picture but the stack is a couple of feet higher than the radar and the monkey deck was easy access so keeping it clean was no problem but the mast, boom and rigging got soot covered very easily.

Insulation foams may be "self extinguishing," but most will burn and create a smoke hazard.
Best practice calls for incombustible insulation up the trunk with lead for noise. This can be mineral wool, fiberglass or ceramic types but given the difficulty of access in most trunks and the size you are probably looking at a few dollars more will be dollars well spent for something that will provide passive fire protection.

Good point. That's the reason I was considering an aluminum trunk but the heat and noise issue was a concern. Ceramic sounds interesting, is this also a spun fiber? Some type of panel would be nice from a space point of view but it would still need to be effective.

High quality finned heat exchangers probably are not cheap.
And of course the thru-hull connections are a source of water ingression. The one end of the heat exchanger passing hot coolant into a pipe sourrounded by cold seawater could be a problem.

Most fishing boats are whisper quiet in the harbor. But then my Willard w her lift muffler is quiet too.

I made my own keel cooler out of type K copper pipe and bronze thru hull fittings at the ends, cost about $150.00. It runs longitudinally along the keel where the boat could lay over on it's side and not crush it. The thru hull fittings only need to be sealed to raw water where they pass through the hull as internally they have engine coolant. There's not enough velocity to the coolant to cause internal issues of erosion and outside the same is true, in fact it's beneficial to direct the flow forward to increase the apparent raw water velocity to increase cooling. about the worst failure would be the incursion of salt water into the cooling system or a circulating pump failure.

Have dry exhaust with two keel tubes 6 inches running length of Hull with two Blige tanks. Never had any problems. Tubes located close to keel and we'll protected. Exhaust NOT noisy. No weed traps to worry me.

Michael, is that Dublin as in Ireland? Stopped in Galway once many years ago on a trip around Ireland and up to the Shetlands and over to Norway on an old Boston beam trawler built in 1938 that was left over there after sub patrols during the war. Converted her at Bergen Mekaniske Verkstad to a stern trawler and took her to Alaska. Very nice fishing fleet in Ireland, many more small full displacement boats than over here. I'd love to see pictures of your boat.

I have a Cummins 6CTA with keel cooling and a dry stack. 6" exhaust and the cooler is inside the keel, in fact I don't know that the entire keel isn't just filled with coolant.

A good way to insulate your stack is to use a stainless wood stove chimney, the zero-clearance kind, they are easily installed and attached and are self-supporting. With your little engine there will not be much noise; put the muffler as close to the engine as you can and wrap it. You will find the engine room heat quite manageable. Make sure that if the muffler is not mounted to the engine, there is a joint that can withstand the relative motion of a vibrating engine on soft mounts. There is a local company near here that sells exhaust pipe insulation and tape (not the stuff you wrap motorcycle exhaust with) and although it's a pita to do, it's way cheaper than custom wrap. You will also have to run coolant through your oil and Trans coolers, which means they will last longer than you will. Forget the soot issue, it's a minor inconvenience.

I have a bucket that goes on the stack just in case we get a good Southeaster, when the rain arrives sideways.

Give some thought to your crankcase vent, if you have one, you can use an airsep or run a pipe or hose parallel to the stack and out the roof. No fumes in the Holy Place!

Yes, the ceramics are spun fibers, latest and greatest. One of the trade names is Fire Master:
Blankets | Morgan Fire Protection
Also probably the most expensive but higher performance, meaning for the same thickness less heat transfer than mineral wool or fiberglass = better fire protection.

Least to most expensive:

Mineral wool: extruded mineral (rock) fibers
Fiberglass: extruded glass (sand) fibers
Ceramic: spun ceramic (clay) fibers

For noise add lead, 1 or 2 lb per sq ft.

All are considered incombustible if the proper type is used.

Fish53 said:

Ceramic sounds interesting, is this also a spun fiber?.

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My computer is acting slow so I can't post pictures. Click on my link in my sig. line and it should have some pictures of my dry stacks. They are simple, flex line coming off the manifolds into welded pipe going up and out. I'll try to edit this post to update with some pictures.

Hard to find a nice clean large yacht with dry stack. Why not- dirty upper decks. For fishing boats lots to like about dry stack with no worries about submerging exhaust with a big catch on board.

Either works, one way is really messy but many don't care about soot on the interior cabin sole.

For the last few months I have been intermingled with lots of commercial fishermen - lots of soot flying around.

"Either works, one way is really messy but many don't care about soot on the interior cabin sole."

The working fish boats frequently have high time engines , idle for hours glazing the cylinders and run near full throttle at times.

No big deal if there exhaust is foul.

On a white boat the engines are usually kept up more frequently.and seldom idled for hours.

For a refit the use of double wall stove pipe with the engine exhaust pipe installed inside works well,,

Except its hard to install a dry locker for drying clothing.

The exhaust can be silenced with a "hospital critical" muffler , usually mounted on the cabin top.

IF a wet exhaust manifold is available , the keel cooling will handle the load .

If not, there are many exhaust wraps that work well, and the ER ventilation can be upgraded.

The final thought is a by pass thermostat is required in the KK cooling circuit.

The engine thermostat may keep the engine at 180F , but the return coolant at perhaps 80F - 100F is way to cool. It should be about 160F for no thermal shock.

For below freezing area liveaboard or a boat that gets winter use , there is no finer system..

Our setup is in South FL where the thin often sandy water takes a great toll on an impeller cooling pump service life.

For our style of boating, I am delighted it was aboard when we bought it.

On a newbuild it would be a requirement.

There is no extra thermostat on my system. Engine thermostats regulate, they don't just pop wide open when cooling is required.

The muffler should be close to the engine as possible to heat it adequately. A muffler a long way from the engine will also possibly require a resonator. Besides, they are ugly outside the boat.

I don't have soot, or if I do, maybe it lands on your boat? My boat has never had a soot issue.

My exhaust is entirely dry, wrapped with Exowrap. The only salt water in the boat is the genset, installed sometime after the original build.

One boat I had two thermostats the rest just one. Up here most work boats that have keel coolers are sized for 80 degree water but our water seldom exceeds 65.

You can buy ceramic fiber cylinders that would make very good insulation for a dry exhaust. I used to use such cylinders when I built high temperature (3000F) furnaces. 2" of ceramic fiber insulation and you could touch the outside of the furnace when it was at 3000F.

When reading some responses easy to see why wet exhaust became popular nearly a century ago. The white glove trick on the boat deck of a new wet exhaust vessel is revealing.

It just comes down to one's level of soot tolerance and then operating that vessel in such a way to minimize the black stuff. With cars and pickups the regs are in place to entrap or minimize soot. New build boats are on the EPAs radar.

Once into SD hulls forget about keel coolers, hate to rip off a Fenstrum at 18 knots.

Fish53 said:

Michael, is that Dublin as in Ireland? Stopped in Galway once many years ago on a trip around Ireland and up to the Shetlands and over to Norway on an old Boston beam trawler built in 1938 that was left over there after sub patrols during the war. Converted her at Bergen Mekaniske Verkstad to a stern trawler and took her to Alaska. Very nice fishing fleet in Ireland, many more small full displacement boats than over here. I'd love to see pictures of your boat.

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Yes indeed - boat is on Shannon - steel - not really a trawler and not very good looking but lots of room and terrific in big inland seas. Getting her ready for sale - too much up and down inside for crocked knees. Going cheap !! M

FF- there is no concern about thermal shock with the 80F water coming back into the engine from the KC. No different that a truck with a radiator below freezing or a marine engine with a HX running in winter seas. The water from the cooler, of whatever type, blends with bypass flow at the suction to the circ pump impeller. It is well mixed there, and very little cold flow as tstat is nearly shut.

The little Kubota has a bypass built in between tstat housing and the circ pump housing.

Not a concern.

The usual truck (or auto) is set up so the return cooling water is about 20deg cooler than the engine output.

IF the engine mfg has an internal setup that tempers the return water thats great.

But not all do,and the chance the vessel was set up to operate in 80F water and is in 40F water exists..

Next time its cold , notice how many 18 wheelers need to cover most of the radiatorto keep from overcooling.

The bypass thermostat operates in conditioned water and its life is like most other thermostats , quite long.

A common method years ago was to install a gate valve in the coolant line to restrict flow, sort of a "manual thermostat", judicious use of the temperature gauge would have been advisable I think.