Water jacket exhaust manifold - epoxy?
Tom wrote 05/29/2019 at 13:18 • 0 pointsI'm looking into what engine to put in my boat. Marinised engines generally have several parts water-cooled that are usually air-cooled on an auto engine. The most extreme of these is the exhaust manifold - in a car, there is constant air flowing over this to cool it, but if you just put the engine in a boat with no cooling air, it can easily glow red hot. So it's usual to use a manifold with a water jacket around it. This is added to the engine's existing water cooling loop.
Some engines have water jacket manifolds available for them but most car engines don't. So it's not that unusual for people to make their own. This is usually made by building a box out of sheet steel or aluminium, cutting holes in it where the manifold pipes will enter and exit it and then welding it around the manifold. I'd like people's thoughts on something a bit different.
A water jacket could be made by packing a layer of something water-soluble around the manifold and then laminating glass roving over this with a high-temperature epoxy. An epoxy such as this might be suitable: https://www.permabond.co.uk/1-part-epoxies-ch9o. It could be cured just by running the engine under enough load to bring it up to temperature. The mold material could then be gradually flushed out be squirting water into it, and eventually running water through it.
How durable might this be? My particular worry would be the join between the steel manifold and the epoxy. It would be hard to predict how hot this would get; not as hot as an air-cooled exhaust manifold, but potentially still well over 100 degrees in places a little distance from the water. It's worth remembering that the water is not kept as cool as possible; the thermostat only switches the heat exchanger in at a certain temperature to improve engine efficiency. The epoxy is marketed for applications like bonding heatsinks, so presumably it can maintain a bond through repeated temperature cycling, but how long exactly is it likely to last?
This could perhaps be improved somewhat by replacing the studs that bolt the manifold to the engine block with longer ones and cutting another piece of sheet to cover the edges of the outlet ports; the ends of the glass roving could then be sandwiched between this plate and the manifold ports. This would reduce the dependence on the strength of the epoxy somewhat by backing it up with a mechanical clamp on the roving.
I'd like some informed feedback on this. As far as I can tell, no-one has tried this before. Is that because it's a stupid idea?
Bear in mind that a boat's engine is a fairly safety-critical piece of kit; you don't want to be stuck in the middle of the ocean without a working one. And a sudden leak in the cooling system is a serious failure for an engine.
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isuzu has some marinised variants of their engines. Yanmar and Yamaha are also available marinised. Not just cooling is different. Oil, water and filter points, serviceable areas may be slightly different areas in marine version. Probably better idea that you buy a marinised engine with purpose built heat exchangers, overheating is the biggest root cause of failure by far. Unless you have someone who knows what they are doing to do the customisation-conversion.
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anyways, I'd keep the epoxy for emergency repairs and use marine grade ss for the elbow. If you design your own heat ex, remember that salt water and any two different metals makes a battery and corrodes quickly, so care must be taken at as, copper, and aluminum interfaces.
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ahh ... methinks you perhaps need a heat exchanger for the COOLANT ... If there's a water jacket, coolant runs through it, not sea water. The heat exchanger like kryzystof says, uses copper tubing, and, I guess is separate, so they can be removed, cleaned, repaired, or replaced as needed. What kinda boat and engine?
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its called an exhaust elbow. Forget the epoxy. Buy one premade, or weld your own out of 316. Its a crucial part, and if it fails, you could end up pumping sea water into the boat, fairly rapidly, making for an, uhm, bad day.
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I think cooling exhaust manifold directly with water will make it corrode. When exhaust manifold is cool, water from exhaust will condense. Normally that water will evaporate soon when manifold is heated. I suggest wrapping it with copper tubing (6-8mm external diameter) and not even using temperature conducting paste. Just run water through tubing and it should be enough to cool manifold to required temps and not too much aggresive cooling. That way you don't have problems with leaks.
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the sea water does not go through or around the manifold. The engine only touches coolant. The heat exchanger cools the coolant with salt water, that is then injected into the exhaust elbow. That cools the exhaust gases enough to go into heavy duty tubing. The water collects tn a second chamber called the water lift (below the exhaust elbow, usually below the waterline), which can be safely made out of fiberglass, where every so often some of it is pumped up and out of the boat stainless steel exhaust thru hull by the exhaust gases.
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A quick Internet search came up with Duralco 4700 which is good up to 315 degrees C!!! They quote: "Other uses include repairing cracks in pipes carrying hot oil at 250°C and at 1000 psi." But it is very expensive, that may change your mind.
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