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Originally Posted by NIC SURRY
Listening on here to the various corrosion problems Valves rusting etc why do the majority of inboard engines use raw water cooling and also risk potential impeller failure’s, I know a sealed system can be used but seems to be quite a heavy solution still relying on a pump/impeller. I was wondering why a sealed Air cooled radiator system is not used I understand this could be difficult with a conventional boat but possible on a race boat with sufficient room for ducting in fact most modern cars have a full under tray with surprisingly little vents to let hot air out of the engine bay. During my Autograss days we had major issues with the radiators getting blocked with mud so used to mount them at all different angles usually adding additional radiators to increase the water volume and I was always surprised at the relative lack of air flow required to keep everything cool, a lot of grassers now use bike engines with revs in excess of 15,000 revs and use tiny radiators to keep them cool. I only ask this as using a sealed system would allow Alloy engines to be used with massive weight savings and long life expectancy(Corrosion wise) and also opening the door to other Automotive engines readily available on the second had market. My second question is why are the majority of exhaust manifolds water cooled is this just to save heat build up or quieten things down or another technical reason. I only ask these questions because I’m nosey and for no other reason.Nic
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Valves rusting etc is not too much of a problem if you use the right stuff i.e. stainless steel valves and seats. And the problem arises from reversion of the cooling water as it exits the exhaust so it would be raw water in any case.
Air cooled radiators are out of the question on a boat mainly because we struggle anyway with getting enough air into an engine bay simply for combustion let alone use it for cooling.
Impellers are not as big a problem as some think, in fact they should be doing little to no work at all at speed which of course is where the heat is created if the inlets are designed correctly. Most overheating problems tend to be a blockage of some kind rather than the pump's capacity to stick water through.
Exhaust manifolds are cooled because they get too hot in a confined space like a boat and will at any rate take the water at some point for it to exit the boat, unless of course you fancy a red hot tell tale of water spouting out the back.
Closed cooling systems are fine but you are adding weight with a heat exchanger.
Aluminium heads can be used in a closed cooling system but the downside is that you are hanging a fair bit of weight in exhausts on a stud into ally and they tend to tear out as has just happened to my brothers PCM engine in an Air Nautique.
Finally, automotive engines. This has been done to death but the reasons automotive are no good is that the torque curves created are useless for marine purposes. We need high torque low down in the rev range to swing a big enough propellor and the torque to stay fairly flat so that the propellor does not over rev the engine. Automotive don't need this as they have gears. This is also why turbos don't achieve what we need.
That combined with the fact that your average bmw engine is not designed to stay at sustained high revs punching out top horsepower for 2 hours solid. How long do you think your M5 would last if it only ever saw a little idling and then everywhere between 4 and 6 thousand rpm in top gear?
Come to that how long do think your licence would last?
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