Why marinize the exhaust manifold?

[Dockhead]

Originally Posted by Terra Nova

While this may be possible, the real cost of such a system is often the smell and sound of diesel exhaust in your cockpit, plus living with a potentially serious fire hazard.

Indeed. There are good reasons why you almost never see this.

Besides engine room heat (you can never lag every bit of it), there is a significant design challenge to create a connection to the hull opening which is at the same time watertight, well-insulated (to avoid burning the hull) AND flexible.

It's not an accident that the wet exhaust is almost universal on small inboard powered boats.

[DeepFrz]

It is possible to build a system which has significant reliability advantages over the conventional wet exhaust. And it can be done at a significant cost savings.

Given the fact that this is a raw water cooled engine you still need to pump sea water. Why not use it to cool the exhaust. Besides, installing a keel cooler into this particular boat would be very expensive. I'm not saying that dry exhaust systems don't exist but they are irrelevant to this conversation.

[brownoarsman]

Remember though, we're talking about the manifold only, not the exhaust, though assuredly a water-cooled manifold cools the exhaust somewhat. With the dry manifold, the exhaust would still be cooled through the mixing elbow, which enters right above the lift pump. The question is whether an insulated dry manifold would destroy the surrounding components and/or the riser and/or start a fire and whether the water from the mixing elbow would be enough to cool the exhaust so that the exhaust doesn't destroy the lift pump, the $23/ft exhaust hose, or start a fire at the hull connection.

John, would still love to hear what you would use JB Weld for, if anything.

Got the engine reassembled. The lack of threads on one of the studs cut the experiment short, however. I was trying to use a rubber gasket for the thermostat, and couldn't compress it enough because of this deteriorated stud. Ugh. Letting it dry out for a bit and I'll try again with rtv. Then, mr. happy drill and mr. mean tap come out. Already had to tap the other stud hole. I'm learning bunches of new skills!

The patch seemed to hold but I've identified a small exhaust leak in another part of the line. Boats, huh?

The good news: the new water pump works great! Water all over the engine bay courtesy of the bad stud/leaky gasket. Next old boat with an old diesel will also at least have an old outboard ...


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[GILow]

I am not sure of the validity of the following comment but I will share it anyway as it came up a number of times when reading up on insulating marine exhausts, a job I completed just recently on our Nissan Ed33, a 96 HP conventionally aspirated truck engine.

Again and again I found remarks cautioning that good heat insulation on our cast iron exhaust would have a negative impact of shortening the lifespan of the pipe. For this reason I guess I am glad to have a water cooled manifold as the exhaust gasses hitting the insulated section of cast iron before the mixing elbow are surprisingly cool. The point being, if the warnings are correct, a well insulated manifold might well cause problems downstream in the exhaust system, particularly when combined with very unnerving setup where the main exhaust runs dry until the engine has heated enough to allow water through. Overall I feel that the combination could be enough to cause serious risk of fire or at least damage to the remainder of the exhaust pipe.

On balance I personally would keep the cooled manifold but maybe look for after market sources to mange the cost. I got a terrific quote from a local engineer for ours when discussing a what-if scenario since I know ours is not commercially available any more.

Matt


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[Pauls]

In reply to the various concerns voiced, I'd like to say that dry exhaust, properly done, is a significantly more reliable and trouble free system than wet. There is no reason why there should be any increase in exhaust odor. Why should there be? If exiting at the same location?

Likewise, it is very possible, and I would consider it mandatory, to insulate all the exhaust system. You start at one end and work to the other.

I'm an engineer familiar with the design of thermal systems. And I've designed and built some boats. It is my opinion that the almost universal use of salt water cooled exhausts is popular for two reasons: it is a very well developed system which is well understood, and it is the simplest system for a builder to install in a hull. It is not the cheapest system for the owner, either initial cost or down the road expenses.

To do one of these systems well requires stepping off the beaten path a bit. It is very doable, but it does require one to engineer his own system.

The boat I am currently building has a dry exhaust exiting just above the waterline at the stern. The engine is keel cooled, ie there is no saltwater in contact with any part of the system. No raw water pump, thru hull, strainer, etc. This has enabled me to use a industrial diesel at about half the cost of a marine version. This approach eliminates many of the more common failure modes seen on marine diesels.

[Pauls]

Hi Matt,

I can't comment on the sources that said that insulating cast iron manifolds causes premature failure. What I can point out is that insulating manifolds, and turbo housings on blown engines, is very common. It is seen rather commonly in the industrial world, and it is well recognized in the performance world as being a method to gain a small horsepower increase out of an engine, particularly turbo'd engines, where the hotter exhaust gas improves turbo efficiency.

At a basic level, diesel engine exhaust is much cooler than gas engine exhaust. Our exhaust systems don't see anywhere near as much heat as gas engines do. They're made out of similar materials. There's a fairly large built in safety factor there.

[Stu Jackson]

Originally Posted by Pauls

This approach eliminates many of the more common failure modes seen on marine diesels.

Which are?

[Dockhead]

Originally Posted by Stu Jackson

Which are?

He means:

1. Clogged intake strainer; overheated engine, burned-down exhaust, risk of sinking

2. Failed impeller; ditto

3. Overcranked engine; hydrolock; severe engine damage

etc.

His point is well taken, and who would not love to have the simplicity of a dry exhaust. But the bottom line is heat -- small sailboats can't cope with that much heat in the sometimes very small and sometimes poorly ventilated engine compartments, and it's really hard to engineer a safe, flexible, hull outlet for uncooled exhaust gasses for a plastic boat (metal boat somewhat different story).

As to "engineer it yourself" for such a safety-critical, difficult system -- there seems not to be any smiley for "shudder".

Yes, wet exhaust systems are expensive and complicated, with a number of failure modes. And extra holes in the boat. But like so many things, they are expensive because -- they are worth it.

[GILow]

Originally Posted by Dockhead

...And extra holes in the boat....

That one got me scratching my head... (I do not have fleas). As far as I can tell with our system there are no more holes than there would be with a dry exhaust, since the exhaust outlet doubles as the raw cooling water outlet. And the raw cooling water inlet would be needed regardless since keel cooling is not an option on our dear old tupperware boat. So actually, a dry exhaust on our boat (insert shudder smiley here) would need MORE holes.

But you are right, we really do need a shudder-smiley.

Matt

(normal smiley!)

[GILow]

Originally Posted by Pauls

...and it is well recognized in the performance world as being a method to gain a small horsepower increase out of an engine, particularly turbo'd engines, where the hotter exhaust gas improves turbo efficiency.

Yeah, I realised that when I went to buy the exhaust wrap and found it in the "performance" section of the local auto store. It made me laugh because they had just seen me pull up in my ancient Volvo brick wagon. They just HAD to be jumping to the wrong conclusion.

Matt

[Terra Nova]

Only a few of the yachts I've ever worked on had dry stack exhausts. But at least one of those had a serious exhaust related fire. This was on a virtually brand new boat that had been built by a longtime builder, not some DIY lash-up.

The enclosed space taken up by these dry stacks can be considerable. It is within these enclosures that most of their fires start.

[RaymondR]

I made this one out of SHS and RHS stainless tube.

[GILow]

Originally Posted by RaymondR

I made this one out of SHS and RHS stainless tube.

Very tidy and innovative. To clarify please, what make/hp of engine?

Matt

[RaymondR]

The engine is a Kubuto V2203 50 HP. It is raw water cooled and the four devices which can be seen under the manifold are the brass sleeves which were installed by interference fit in the block where the welsh plugs were removed to allow fitting of anodes. The bell housing was fabricated from alloy plate using only hand tools and I fitted two starter motors, one each side. All in all it is surprising what you can achieve when you set your mind to it.

[DefinitelyMe]

I've been on two boats with dry exhausts (well, one was a ship really). As others have said, heat is the only real problem, but it can be a big problem. The carbon deposits that build up from normal running of the engine have to be cleaned out or at least burned off regularly. We had a fire once because we weren't running the engine hard enough, the carbon built up and then ignited. We had 30-foot flames shooting out the top of the stack. It was fun.