Wet exhaust setup

Discussion in 'Boat Design' started by LopiTech, May 14, 2014.

  1. LopiTech
    Joined: May 2014
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    LopiTech Junior Member

    Hey guys, I am looking for some advise on a wet exhaust setup. I have a unknown 1960's fiberglass boat that I converted to jet drive, powered by a zx-10r motorcycle engine. I have wrapped the headers in copper coils and then inject the water at the base of the headers which is below the waterline. The water runs on an electric pump wired to a switch. After the headers, I built a muffler, then it is a hose out the transom.
    I am wondering if I will ever have a problem with water backing up into my exhaust or because the top of the header is higher than the waterline it won't be an issue. Keep in mind the pump may keep running even if the engine is not, incase it gets too warm. Please see attached pic and ask any questions.
     

    Attached Files:

  2. gonzo
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    gonzo Senior Member

    If the setup is as you drew it, the engine will get full of water. It needs an antisiphon valve to start with, and it should go down from the engine to the stern. Also, the exhaust tip should be barely at the waterline.
     
  3. LopiTech
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    LopiTech Junior Member

    Would the water not just fill inside the header up to the height of the waterline, no matter where the tip exits the hull?
     
  4. viking north
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    viking north VINLAND

    Gonzo if you can read that blueprint you're a better man than me :D . Lopitech, How about a few photos or a scaled drawing with a few measurements included. A combination of both would be good as the waterline location in respect to your engine exhaust exit/ water injection plays a big role here In any siphoning or just plain backflow action.
     
  5. tom kane
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    tom kane Senior Member

    If there is any water in any exhaust system the pulsing of the exhaust and induction system can and does push the water or damp air back up to the back of the valves and into the cylinders,especially on two stroke motors. A dry exhaust cooled with a water jacket (even coils of copper) is ideal.
     
  6. LopiTech
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    LopiTech Junior Member

    transom view.jpg

    Here is a better sketch from the transom. Sorry the last one I drew in my truck in a parking lot.
    I am aware of the possibility of moist air entering the cylinder, but as I have read that is not normally an issue if the system is properly designed, as well this is a 4 stroke high performance engine with titanium exhaust as well as aluminum/titanium internals so corrosion should not be an issue for the small amount that could get in there.
    I could either exit the exhaust to the right of the muffler or straight out the back, whichever is preferred.
     
  7. Poida
    Joined: Apr 2006
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    Poida Senior Member

    Is the motor bike engine water cooled?
    I assume you are injecting water into the exhaust with water sprays and is not a problem as the exhaust pressure blows the water away from the cylinder. But in the event of what happened to me, you have a problem with an exhaust valve, it will suck the water into the cylinder. As Tom mentioned if you can use a dry exhaust it would be better.

    Fortunately my engine is a 6 cylinder and I was able to splutter back on 5.

    Poida
     
    Last edited: May 15, 2014
  8. LopiTech
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    LopiTech Junior Member

    Yes it is water cooled. I have a closed loop system for that. The water is injected at the low point of the exhaust manifold as shown in the pic.
     
  9. LopiTech
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    LopiTech Junior Member

    Here is a pic of the headers off the engine.

    image.jpg
     
  10. Poida
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    Poida Senior Member

    Looks like a nice neat job. However what your asking is how long is a piece of string, nobody can tell you if the water will evacuate the system faster than it is pumped in.

    It may be better to only have it running when the engine is going.

    What you may or may not have looked at is to make sure the engine does not get too cold. Either way it is always a tricky job with makeshift cooling.

    Good luck

    Poida
     
  11. LopiTech
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    LopiTech Junior Member

    Ok, makes sense. When the pump is off though and the boat is just sitting in water, will it flood or only fill to the natural water line? If I put the exhaust exit below the water line that is. I could always put another riser after the muffler and utilize a second pump I have to pull water out of the muffler and back in the top of that riser. Problems would occur if that unit ever failed though
     
  12. viking north
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    viking north VINLAND

    An exhaust cooling water inlet valve might not be a bad idea. You can then experiment with the volume of water injected into your exhaust. This setup is common on retrofits. Another thought that comes to mind is using a water block style of goose neck back at the transom. Never having used a 2 stroke inboard other than the old make & break ancients I am not familiar with the back suction that can occur with your power plant. (The old make and breaks used reed valves) --Good Luck and Have Fun.
     
  13. SamSam
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    SamSam Senior Member

    [​IMG]

    Here is my guess. When the engine is not running, the whole system will fill with water to the water line. Whether the engine will start with all that back pressure, I don't know.

    If the engine is running and then shut off, as the residual heat cools down, it will create a vacuum, sucking the water into the small 6 inch sections of header above the water line. Whether that small space is a big enough 'reserve' to prevent entry to the engine, or whether leaking valves will suck water into the engine while trying to start it, I don't know. As Gonzo says, "It needs an antisiphon valve to start with". Where you would put that in your arrangement, I don't know.
     
  14. LopiTech
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    LopiTech Junior Member

    Thank you all for the input. Should I raise the through hull portion to just above the water line, and hook the water pump up to the engine so it only runs while engine runs?
    Also, from what I have read the anti siphon valve is only used when the engine is mounted below the waterline. If it would actually help, it could be put in at the copper manifold near the top of the headers maybe? What would be the reason for it, and how does it help though?
     

  15. CDK
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    CDK retired engineer

    My diesel engines have the through hulls a few inches above the waterline with rubber flaps to keep waves from getting in. As an additional precaution I drilled a small hole in the lowest part of the stainless can that connects the 4" hose from the turbo charger to the 2" exhaust hose.
     
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