Why marinize the exhaust manifold?

[brownoarsman]

The water passage on my exhaust manifold has somehow leaked past the gasket and caused an uneven surface on the manifold (the bottom corner of the water passage has essentially expanded). It's patched for now with a compressible, high heat gasket material (fel-Ramic 2499) and high temp RTV. I soaked the gasket material in water for 48 hours to make sure the fiber layer could withstand water immersion first.
As the thoughts of water leaking into my cylinders and exhaust poisoning don't appeal to me, I also priced out a new manifold. My universal 5411 is actually a kubota z500, and I was struck by the $80 for a kubota manifold and the $500+ for a universal one.
I understand the manifold is one of the parts marinized to allow for raw water cooling of the manifold/exhaust, but why? Raw water cooling it directly seems to be a potential source of considerable engine damage when, not if, the manifold fails. Raw water passages in the manifold are even a SOURCE of failure.
At least on my engine, everything from the engine back to the lift pump is metal, except for a short section of exhaust hose dropping into the lift pump. Above this drop, the cooling system injects raw water into the piping. When the thermostat is letting the engine warm up, which can take up to 10 minutes according to some owners, no water reaches the exhaust system as it's all being recirculated to the engine, except for the small passage in the manifold leading to the thermostat which cools the manifold and whatever is inside of it. So how crucial is a marinized manifold? It would be fairly easy on my engine to mount the thermostat on the wall and have it feed the mixing elbow, bypassing the manifold. Has anyone tried it?


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[Cheechako]

You can run it dry, all must be metal and must be insulated... the insulation is usually done like plaster of paris ...wet and then it hardens. But not always. Some soft wraps exist. Your boat will be hot though. Run a stack straight up like a tug boat. haha

[brownoarsman]

Like every young boy I always dreamed of driving a tug, now is my chance!

Insulating the manifold would be hard. My mixing elbow is insulated with soft wraps, but the manifold is tight in, close to the block, so I couldn't really get anything around it. Doesn't help that the alternator wires currently run above it. Drat.

If it won't kill me or my engine though, it may be worth the $420 experiment. I'll see what it would take to move the manifold off ...


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[boden36]

In a motor vehicle there is a huge amount of air flowing past to take the heat of the manifold away. In a boat your engine is enclosed. You will be able to cook a chook in there unless you organize substantial ducting and fans.

Regards,
Richard.

[svadagio]

If you live near a fishing port you might go and look at what they do. Many fishing boats use dry exhaust and dry stack. You will need insulation. Maybe the cheaper manifold will allow for this. In my limited experience if the manifold is insulated it is not that hot so good ventalation will work.
Bob

[brownoarsman]

Boden, that's great actually! I need another cooking source besides my butane stove and crockpot maybe I'll fit a cast iron grill plate on top of the manifold

When I get the engine back together, I'll zap the manifold and see what temperature it is running at. Whenever the engine runs dry (which has been quite often recently due to a recurring water passage issue) I assume the manifold is scorching hot and nothing seems to have melted. I can also always move the alternator which is relative simple and it is the only plastic part. As someone said, the kubota manifold should be smaller which will allow for a degree of insulation. It seems sort of silly to make a big fuss about insulating the riser and elbow due to fire risk, but leave the manifold uninsulated. The exhaust in the riser can't be hotter than the exhaust coming out of the manifold by definition. I guess the water in the manifold keeps that block of metal cooler, but still, it must heat up awfully when dry.


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[DougR]

For years and years commercial vessels have run dry exhaust systems, and even dry turbo chargers on the exhaust housing side, but it's not often that you see an engine with a dry exhaust manifold. There are two major considerations that have to be addressed with dry exhaust; heat flow into the engine room and fire safety.

Dry turbos and exhaust pipes are always wrapped or lagged in an attempt to keep engine room temps down to a reasonable level, as well as provide protection against burns from inadvertent touching. In the old days this wrapping was done with asbestos "tape". Today you can buy Fiberglas pipe wrap by the roll and achieve the same end. Wrapping a manifold is somewhat more difficult as the physical space just isn't there, and that's why most marine manifolds are wet.

You have to remember that diesel exhaust gas temps are in the 950 +- degree F range, and that means that the manifold temp will also be in that temp range. As about 30% of your fuel BTU value goes out the exhaust pipe there is plenty of
energy available to keep the pipes hot.

Your engine h.p. was rated at a maximum air temp of probably 120degrees F, and when engine room temperatures start to climb much higher than that, power output starts to suffer. So you will loose power, the exhaust might start to get smokey, or more smokey, and the transom sooted up. Not only that, if engine room temperature gets too high other things mounted in there, like electronic devices may suffer.

Lastly, those old commercial vessels with dry exhaust systems also suffered from the occasional engine room fire. Diesel fuel "auto-ignites" at about 500 degrees F, and fuel leaking onto a 900 degree manifold or turbo isn't my idea of a fun weenie roast. A hot manifold is just that much closer to a fuel source than an exhaust pipe.

DougR

[Lancerbye]

Having owned a commercial fishing boat with dry exhaust I know that certain requirements of heat control in the engine room is a must. The dry exhaust system was not a dry exhaust manifold. The cooling water for the manifold was part of the closed fresh water system. There was no raw water pump. The fresh water was cooled by a keel cooler which in my case was along bronze pipe running the length of the boat next to the keel. The connection from the exhaust manifold to the stack which included a muffler, an expansion joint and the exhaust pipe had to be lagged with insulation to reduce the extreme heat being generated beyond the exhaust manifold. The exhaust pipe is run through the center of the vent chimney to provide a heat discharge from the engine room. Most smaller pleasure crafts do not have the room going up through the center of the boat to provide such a system. As DougR stated you have to deal with some seriously destructive heat. There are reasons for the exhaust systems that presently exist on most boats.

[darylat8750]

Beta Marine engines also use that Kubota block. You might contact them and see if their exhaust manifold would work on your engine and if so what the cost would be.

I've read where one guy wrapped his tractor like exhaust with copper tube (and pumped water through it) then insulated well on top of that. It kept the temps in the engine room down nicely.

[Bill Seal]

Brown; unless you have a very strange cooling system, any time the engine is running, the raw water is running through the through the heat exchanger and out the exhaust. The thermostat just controls the amount of coolant going through the exchanger.

[darylat8750]

Here is a link to Rob Whites page on machinery in his Rescue Minor. He is a little unconventional but gets the job done. Check out his cooling system. He uses this launch for access to his island home and it gets a lot of hours on it every year.

Robb White, Boatbuilder, Thomasville, Georgia

[Cheechako]

Originally Posted by brownoarsman

Like every young boy I always dreamed of driving a tug, now is my chance!

Insulating the manifold would be hard. My mixing elbow is insulated with soft wraps, but the manifold is tight in, close to the block, so I couldn't really get anything around it. Doesn't help that the alternator wires currently run above it. Drat.

If it won't kill me or my engine though, it may be worth the $420 experiment. I'll see what it would take to move the manifold off ...


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Use a cooled manifold and insulate everything aft of that. Instead of running your cooling water out the exhaust just run it overboard.
But realistically, just leave well enough alone and get 'er fixed!

[Dockhead]

Very bad idea! You will cook your engine compartment, which will cause you no end of problems, and kill a lot of gear. Don't do it!

[Terra Nova]

Originally Posted by brownoarsman

The water passage on my exhaust manifold has somehow leaked past the gasket and caused an uneven surface on the manifold (the bottom corner of the water passage has essentially expanded). It's patched for now...

No it isn't.

"essentially expanded", is that what they're calling "rusted through" these days?

Originally Posted by brownoarsman

...When the thermostat is letting the engine warm up, which can take up to 10 minutes according to some owners, no water reaches the exhaust system as it's all being recirculated to the engine, except for the small passage in the manifold leading to the thermostat which cools the manifold and whatever is inside of it. So how crucial is a marinized manifold? It would be fairly easy on my engine to mount the thermostat on the wall and have it feed the mixing elbow, bypassing the manifold...

Stick to what you know so no one will be injured.

There's something wrong with your cooling system if water isn't injected into the exhaust from the moment the engine is started.

[SkiprJohn]

I had a friend with a dry exhaust on his 36 foot homebuilt cutter. I think it was a Lister. Great engine. I think too that it was air cooled. I can't remember that part. Here was the major problem. The exhaust stack come up through the cabin top and sooted the sails as well as made them very hot. Not a good situation. He finally bought an old Volvo engine and installed it. He was much happier with it but it didn't have a heat exchanger either just flow through saltwater cooling.