"Dry stack" or wet exhaust manifold ?

[carioca1232001]

I have pulled the following out of Misc.BoatRelated, hoping that someone in Building/Repair can answer it:

If noise abatement is of no concern, could marine engines operate with standard air-cooled exhaust manifolds ?

Or would the latter entail a fire hazard, especially in the smaller engine rooms of recreational boats ?

Before I DIY-marinised my boat engines with fresh-water-cooled manifolds, I had entertained the thought of completely surrounding (encasing ?) the original air-cooled manifolds with a cast piece (jacket) of copper (or bronze), through which cooling water would circulate.......some people said it would work, others the opposite.
Any offers on this one ?

[Ian McColgin]

Almost anything can be done and probably has at least once, sometimes rightly.

Purely drystacks are usually run vertically up through the deck and higher than any normal standing area. With and without mufflers, this is common on work and fishing boats. In this installation, it's quite easy to engineer sufficient insulation.

I know of two sail boats that have the simplest of dry stacks straight up from the manifold and through the bridge deck at the foreward end of the cockpit. Both have a flush plate for engin-off and an insertable 8' stack (two sections for stowage) for engin on. Unlike on fishboats (and trucks) neither of these has shielding around the exposed run and both get hot enough that a casual lean against the stack would burn.

If the exhaust runs out through the hull, insulation or cooling the exhaust can be an issue. Granuaile has such a dry exhaust for her air-cooled Deutz. That little 15KW engin produces enough heat in continuous operation that the exhaust pipe is wrapped with refractery insulation and the through-hull fitting has a huge flange to creat some dead air space. With any more power, I'd think the heat problem would be insurmountable.

I've seen water-jacketed exhaust systems but I really don't see the point in most cases. Mechanically, it's much easier to inject exiting cooling water into the exhaust stream in the normal manner.

One installation I saw where it was lovely was a verticle exhaust that went up at the back of a hanging locker. The water jacket made the locker not only useful, but really wonderful for drying out damp oilies.

A water jacket is a very fussy thing to make as it needs to be sealed against huge problems of heat expansion, corrosion and vibration. Absent no other good way to run the exhaust and no room for dry insulation, I'd not recommend it.

[carioca1232001]

Ian McColgin wrote:

....A water jacket is a very fussy thing to make as it needs to be sealed against huge problems of heat expansion, corrosion and vibration. Absent no other good way to run the exhaust and no room for dry insulation, I'd not recommend it. On a turbo-diesel, water-cooled exhaust manifolds steal available exhaust-gas-power from the turbo, which is at least one reason that would favour retaining the factory issue air-cooled manifold.

Then if you have a fresh-water cooling system, there is extra heat-load imposed on the heat exchanger.

If I were ever against exhaust-gas cooling, I am all for it now following the account of the 10 kW dry-exhaust installation on your boat, especially as mine are 100 kW a piece turbo-diesels. ;)

As for a water-cooled jacket surrounding the air-cooled manifold, it need not make contact with the latter. It would be installed half-an-inch or so away, with the surface facing the manifold painted with lamp-black, to enhance heat transfer via radiation.

Does the latter still sound ill-fated ?

[Stiletto]

I would think that a water jacket would have to make contact at least at the manifold flanges or you would have a leak. Or are you advocating wrapping a water filled tube around the manifold so the tube is in contact with it.

I have thought about this quite a bit as I have in my shed a 17hp mitsubishi that has a dry manifold that has been lagged, and then the rest of the exhaust had water injected at the elbow in the normal way.

I have thought about fabricating one completely from stainless but have enough reservations not to start until I am sure that it would be durable enough. Cast iron is a lot thicker and more durable. If you come up with a solution , make sure you post it here.

[J. A.Tones]

There are lots of dry stack exhaust systems in use here in the PNW especially on fish boats. My vessel has such a system and is dry in its entirety with the exception that the exhaust manifold is water cooled. The engine is a big old 6 cyl. Volvo diesel and the cooling water passes thru the manifold while circulating thru the engine. The actual exhaust pipe system consists of lagged heavy wall steel pipe with a short "flex" section to allow for expansion and some vibration. The "stack" passes up thru the main cabin and is enclosed in a stainless, insulated pipe which never gets too warm to be uncomfortable to touch even after several hours of running. It then extends up above the cabin top by about 8 ft and is clamped to the aluminum mast for support. Never a worry about cooling water in the system or of something comming apart and sinking the boat - would not have it any other way!
Just one boaters view <grin>
John Tones MV Penta
Sidney, BC

[Gary E]

carioca1232001

It is POSSIBLE to do any fool thing you want to, but WHY?

As I understand you have a twin screw diesel powered boat, a pleasure boat, not a fish boat, not a work boat.

If you were to run a dry stack, it must run verticle from the manifold, which still must be water jacketed and cooled or you will create so much heat in that small space, you wont have any problem baking a turkey and 10 pounds of potatoes as well as a pie or two. Now if you still want a verticle pipe running up from your engines they can have a dry muffler in there but it's gona get as hot as hell and burn anything that come near it. For an examople lift the hood on your car and get near DO NOT TOUCH the manifold that after you started the car and let it idle for 10 mins, Now imagine actually letting that engine actually do some work the heat is tremendus. One more thing, spose you like all this, how much cabin or wheelhouse space will you loose to these stacks of hot pipes and mufflers?

Get the water cooled manifolds, freshwater cool the intire engine, and run the seawater from the heat exchanger out the exhaust.

[carioca1232001]

Stiletto wrote:

I would think that a water jacket would have to make contact at least at the manifold flanges or you would have a leak. Or are you advocating wrapping a water filled tube around the manifold so the tube is in contact with it.
I was thinking of my turbo-diesels.....the cooling-jacket device manufactured from carbon-steel could be welded to a rectangular flange ditto as per turbo template and thereby share the same studs and bolts - get me ?

For a naturally aspirated diesel like your 18 HP MITSUBISHI, a master welder could fabricate a cooling-jacket (raw or fresh water) device from carbon-steel that will "sprawl" over your exhaust manifold, top and bottom. A stud (or two preferably) tapped into a convenient location in your manifold could provide anchoring points for this device - I´ve seen some engine manifolds that have provision for just this.

I´ve seen similar "heat traps" (waterless, though) as factory issue in our alcohol-powered vehicles in Brazil, wherein the exhaust heat passes on to the incoming alcohol-air charge (intake manifold) for a more complete vaporization of the fuel. Admittedly, this device is lighter, in the form of a heavy gauge steel plate, not the larger mass asociated with a cooling-jacket.

You have correctly pointed out that the raw water could still be injected into the hot gases at manifold exit (elbow), and thereby exit the hull in a dignified way ;

I wouldn´t use SS - it does not take lightly to extreme temp. variations and tends to crack.
Use standard carbon-steel, much cheaper, and the welder will not automatically class you in the loaded-wallet boating bracket !

As for cast-iron, I´ve seen factory-issue water-cooled, cast-iron jacket castings contigous with the hot exhaust section of turbos - literally bed mates, this lot are !

Gary, I do appreciate the fact that you would not want to see us making dangerous blunders on our boats, but we are just exchanging views, experiences and observations.

Stiletto, Í shall get back to you to-morrow as I have to rush off to an appointment now... let´s examine this subject exhaustively.

[Stiletto]

With the sprawl type manifold casing you are suggesting, does the cooling water actually touch the original manifold?

I have found air cooled engines and dry stack exhaust systems to be noisy, which for me, is best avoided.

[ 10-09-2004, 04:56 AM: Message edited by: Stiletto ]

[brian.cunningham]

Why not put a separate water cooled muffler downstream?

[carioca1232001]

Stiletto,
I am back again......after succesfully sorting out some health-related matters in my immediate family.

The energy balance of an IC (internal combustion) diesel water-cooled engine reads roughly as follows:

- Useful (work)power output : 30 % or thereabouts

- Heat carried away by cooling water: 30 %

- Heat carried away in the exhaust gas : 30 %

- Heat lost from engine surface: 10%

I assume that you have an adequately sized heat exchanger, and associated raw-water pump, to handle approx. 4,0 kW ( 0.3 x18HP x.75 kW/HP) of heat rejected to the cooling water at rated power (18 HP).

From what I gather, the engine you have in hand has an air-cooled manifold lagged with some sort of heat-insulating material (asbestos tape ?) and a wet elbow into which the raw water could discharge after making it´s rounds through the main heat exchanger, gear-box oil-cooler etc, before finally exiting the hull in a :cool: and dignified way :D .

Time to look at some numbers - the heat power that would be lost from the surface of your 18 HP engine at FULL LOAD, with an air cooled manifold, would be of the order of (18HP x.75 kW/HP x.1) = 1,35 kW, akin to a measly electric fire.

A good 70 % (950 Watts)of the overall heat power of 1350 Watts would be radiated from the red-hot air-cooled manifold and the remaining 30% from the rest of the engine block, which you will agree is a great deal cooler.

However, with a lagged air-cooled manifold, perhaps only 20 % of the 950 Watts leaks out through the lagging and so the temp.of the hot gases exiting from this particular exhaust manifold is likely to be somewhat higher than normal. But then again, the raw-water-injection in the wet-elbow will promptly quench out any excesses smile.gif

So, if you leave it as it is, viz., a lagged air-cooled manifold, wet elbow exhaust cum main heat exchanger, in my mind you need just:

1. To make provision for a blower to direct a gust of fresh air on this engine set up;

2. Another blower in the transom to evacuate the hot air.

If you are at all worried about the lagging breaking down or slowly deteriorating , size the upstream and downstream blowers to take care of the overall 950 Watts - no big deal !

Now if you are concerned about the lagging reaching ignition temp., this is another issue.... I have had people tell me that asbestos tape can ignite under special (?) circumstances.

In this case, have a welder fabricate a cooling-water jacket to sprawl over the top and sides of the air-cooled manifold, effectively boxing the latter within. No need to cover the undersides - hot air will rise and contact the jacket. Be sure to paint the underside of this water-cooled jacket with lamp-black as black bodies are supreme radiators and(receivers) of heat energy.

Use your imagination as to how to best anchor this water-jacket over the air-cooled manifold. I´ve told you that in a turbo diesel one can use the manifold flange (for the turbo), the threaded rods and nuts.

I´ve been on a boat with a 6BTA 170 HP turbo-Cummins "DIY-marinisation" with exactly this set up - NO WATER COOLED EXHAUST MANIFOLD, just a raw-water-cooled jacket fabricated out of 3/16" SS plate, with top and sides, bolted on the turbo flange, sprawling over the exhaust manifold with a wet-elbow at turbo exit. :cool:

In a normally aspirated (NA) diesel, you could perhaps use the flange where you bolt up the wet-elbow ? Are there any threaded studs available on the existing exhaust manifold ? Can the manifold be tapped and a threaded rod inserted somewhere for installation of a water-jacket ?

The cooling-water-jacket need not make contact with the exhaust manifold, as the heat is transferred partly by radiation (Stefan-Boltzmann law) and partly by convection. In fact it would be better for these two bodies not to make contact, if not for any reason, just to simplify the vibration problems.

Sorry if I have gone over the top, but I just want to be helpful !

[cbob]

There are dry stacks and there are dry exhaust manifolds, and there are jacketed exhaust systems (pipes) and there are wet exhausts, water into the gas. These are all different things. Boats with dry stacks usually have water cooled manifolds, either raw water or coolant. Boats with wet exhausts usually have water cooled manifolds, either raw or coolant. Marine installations that have uncooled exhaust manifolds, are such because they have no external coolant supply, an air cooled engine or a water cooled engine with a contained system, like a radiator cooled by air blown across the radiator by a fan, deck mounted gensets, emergency generators above the main deck and such, if this clears anything up? cbob

[carioca1232001]

cbob wrote:

Marine installations that have uncooled exhaust manifolds, are such because they have no external coolant supply, an air cooled engine or a water cooled engine with a contained system, like a radiator cooled by air blown across the radiator by a fan, deck mounted gensets, emergency generators above the main deck and such, if this clears anything up? cbob, nice to see that you are around, concise and to the point !

The downside of water-cooled exhaust manifolds are briefly:

1. Reduce power output in turbo-diesels as they cool the hot-exhaust gases that spin the turbo.
2. Increase the heat-load on the main heat exchanger somewhere between 25-35%
3. Should they "hole", create COSTLY HAVOC $$$) in your engine.

I´ve tried to show that a 18HP engine with air-cooled manifold (stationary or vehicular type) could perform well in a boat if fitted with a main heat exchanger cum raw water pump (or keel-cooler system), water-jacketed air-cooled exhaust manifold and wet-exhaust.

[Stiletto]

Thanks Carioca, now you have got me thinking smile.gif

[carioca1232001]

Stiletto wrote:

Thanks Carioca, now you have got me thinking Much obliged !

BTW, a modern day alternative to lamp-black are the ceramic(black)paints packaged in aerosol cans for motor-cycle exhaust systems.