Getting it clear

Discussion in 'DIY Marinizing' started by timswait, Jul 21, 2008.

  1. timswait
    Joined: Jul 2008
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    Location: UK

    timswait Junior Member

    I just want to make sure I'm understanding terms correctly. A "dry" exhaust is one that operates in a similar way to a car exhaust, with no cooling water - is that right?
    A "wet" exhaust has water injected into the gas flow - is this right? If this is then how come you don't get clouds of steam out of the exhaust?!
    If those are right then what is the name for an exhaust which uses a water jacket to cool it but doesn't inject the water into the actual gas flow?
    Also if using a water jacketed exhaust does the cooling water pass through the exhaust water jacket before or after going around the rest of the engine?
     
  2. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    timswait,

    Yep you have it right about wet and dry.
    A jacketed dry exhaust is jsut that it is a water cooled exhaust.
    The water inlet to the exhaust is on the actual exhaust manifolf end, there is a double pipe arrangement and the water is injected into the outer pipe where it is captive for about a few inches only, usually about 9-12 inches on a 300hp for instance, this outer jacket has lots of holes that allow thye water to then come into the inner pipe, which is of course the actual exhaust pipe. The water is from the engine cooling water pipe after it has done its job of cooling everything along the way. Raw water from the sea enters the pump, through the block thriugh the head and out into the exhaust wet manifold. There may be oil, gearbox or diesel coolers inline as well, wet intercoolers/exchangers subject to the design and age of the motors.

    As for clouds of steam, yes that does occur at times, but the flow of water is usually that great the water does not actually get to boil, thence no steam.
     
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    One of the benefits of a wet exhaust is that rubber hose can be used between the manifold and the transom. That reduces noise and vibration considerably.
    With just a water jacketed manifold the gas would still be too hot for a rubber hose.
    What Landlubber describes is the old-fashioned direct cooling from the era where fuel was cheap and money was plenty. The engine could not reach a decent operating temp because the block would be eaten away quickly, so a lot of fuel was wasted.
    Nowadays the raw water passes through one or more heat-exchangers and leaves at the end of the manifold with the exhaust gas. The engine is cooled with a water-glycol mixture that prevents corrosion and allows 90 degrees C. or more operating temp.
     
  4. timswait
    Joined: Jul 2008
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    Location: UK

    timswait Junior Member

    For the purposes of a narrowboat running a 50hp engine what would typically be used? As far as I'm aware they don't use any raw water for cooling, they have a keel cooling tank and water is circulated in a closed loop. I assume either a fully dry or water jacketed system would be the norm? Can stainless steel bellows be used as the flexible section to connect the manifold and transom?
     
  5. lazeyjack

    lazeyjack Guest

    What you need consider for a canal boat is this
    If you have a wet ex going over the side, when you are locking or tied up to the bank,, you will get fumes coming over the gunnel, when you have it going aft thru transom, the fumes will flood the cockpit cos your cruise speed is usually slower than the wind, at 4 knots
    Dry stackin is great, and you can use the stack as a dryer for your wet nappies
    using keel cooling can reduce maintanence, like, no raw water pumps So for a narrow boat, keel cooled jacket water and dry stack
    Dry stacks need be lagged well, else you will burn yourself when you are attempting to attend the still that you have secreted in the engine room somewhere:))
    Nice spelling Lubs!!
     
  6. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    yeah, like usual I type and do not read it...normal for someone that is hopeless at typing mate
     

  7. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Lazyjack lives in an enormous country and cannot imagine how narrow a narrowboat is. Like he says you can dry nappies on a dry stack, but only if there is a lot of insulation and/or space to get rid of the heat, otherwise they will get scorched before you know it.
    Nothing against keel cooling, it's quite common for that sort of boat. But even then you can pump up some water and feed it to the exhaust so you can use rubber hoses or a plastic exhaust from Vetus for example.
    My twin turbo-diesels are so quiet nobody believes they are there. No fumes, no noise, the water absorbs both unburnt particles and acts as a silencer.

    I've tried stainless steel bellows on a smaller diesel engine, but it resonated and tore off near the manifold several times so I abandoned the experiment.
     
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