Wet/Dry exhaust hybrid.

Discussion in 'Boat Design' started by Red Dwarf, Feb 26, 2013.

  1. Red Dwarf
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    Red Dwarf Senior Member

    I have been looking at wet vs dry exhaust and I have a question about combining both. Would the following work?

    I like a dry stack but I do not like the chance of soot or smell. Could you build a dry stack that runs horizontally out to the transom where it takes a turn down for the last few feet before the exit. The difference is that just before the exit you have spray nozzles that give you the option of spraying water into the exhaust stream. The spray will only be on when you are concerned about soot or smell, ie going slow or in a tailwind. I think this gives all the benefits of a dry stack while keeping down soot and smell.

    Looking in Gerr's book I found a diagram called "crewboat exhaust" I have attached a picture. It looks close to what I am thinking of but the holes in the picture for the water injection look wrong. It looks more like a water jacket. Is crewboat exhaust the same as I am describing or is it different? I am confused because Gerr says crewboat exhaust doesn't stop soot. It seems if the water was injected into the exhaust stream it would stop soot.
     

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  2. viking north
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    viking north VINLAND

    Possibly I'm missing something here in your description but as I see it, You would have to run a metal exaust pipe all the way with exception of possibly the last foot or so after you sprayed(injected the water). This would #1 create a fire hazard, #2 be subject to vibration fractures, thus monoxide #3 Never pass a marine inspection for insurance purposes. Having said that it could be possible to overcome these problems (metal braded flex sections, double walled pipe and so on but I would think the cost would far outweigh installing a mixing elbow and injecting water at the engine exaust location. As I prev. stated maybe there are circumstances why this can not be done and you have no choice to go with what I would still classify is primarly a dry (hot) exaust.
     
  3. Red Dwarf
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    Red Dwarf Senior Member

    Yes it is exactly a dry exhaust construction all the way. It would be fully wrapped, insulated and cooling air ducted just like any other dry stack. Imagine a typical dry exhaust stack tipped to lie horizontal. The only difference is there will be an option of injecting water before exit. The water is in no way needed or relied upon for normal operation. The water is only used to knock down soot and odor, when desired. Why would there be any greater fire hazard or insurance risk than all the other thousands of dry stacks out there?
     
  4. michael pierzga
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    michael pierzga Senior Member

    What is the point ?

    The drawing has all the disadvantages of dry exhaust, combined with all the disadvantages of wet exhaust .
     
  5. keysdisease
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    keysdisease Senior Member

    What you describe is pretty common, except for the misguided idea of only using the water injection in certain conditions.

    Whats your problem with wet exhaust?

    Steve
     
  6. BMcF
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    BMcF Senior Member

    We designed and built a "hybrid" exhaust for a pair of Cat diesels in a prototype repowering project (11m rib) and it worked out very nicely. 75 or 80% of the run was dry and well lagged and then an injection/mixing section was inserted a foot or so before the fiberglass exhaust pipes that were integral to the hull. Connected the two with a high-temp silicon ruber flex boot.

    The water injection section was the Caterpillar product with true eductor ring injection...not like what's in the pic above, which I would only use if oriented vertically as shown.
     
  7. Red Dwarf
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    Red Dwarf Senior Member

    Thanks for the input.

    Funny, I think this has all the advantages of a dry exhaust and all the advantages of a wet exhaust.

    I don't like a typical wet exhaust for primarily two reasons. First, If the sea water pump fails you are dead in the water. Or even worse, a fire if you didn't shut the engine down immediately. Second, the sea water usage and piping lead to high maintenance. The main advantages of a wet exhaust are it is easier to route the lower temp wet exhaust and the knock down of soot and odor. Since I don't have a routing issue only the soot and odor control appeal to me. If I can accomplish the same soot and odor control with a dry exhaust I will be ahead.

    A dry exhaust done correctly will last the life of the engine with little to no maintenance, that is why it is the system of choice for commercial boats.
     
  8. keysdisease
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    keysdisease Senior Member

    That's all just fine, but why wouldn't you make the exhaust run all metal and inject water all the time, this is what I see very often on a variety of vessels.

    The either or option may actually accelerate corrosion of the mixer/spray ring via the brine/dry cycling.

    If you switch to composite after the water injection then the water pump failure/dead in the water scenario is same/same.

    Steve
     
  9. Red Dwarf
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    Red Dwarf Senior Member

    Interesting thoughts. Just to eliminate the fire risk you mention I don't plan on any switch to fiberglass after water injection.

    If I use an all metal exhaust and inject all the time then the exhaust will run cool which is always good. I guess a drawback will be putting in all the expense and weight of the drystack insulation that is really just a backup in the event of a water pump failure. I don't know if mufflers designed for dry use will be happy with water but that can probably be overcome.

    Just a thought but if water is introduced closer to the exhaust manifold then I think all the normal antisiphon and other wet exhaust protections will have to be in place which seems too complex.

    I had not considered the accelerated corrosion due to the wet/dry cycling after the water injection. It is only the last couple feet but definitely needs to be considered.
     
  10. keysdisease
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    keysdisease Senior Member

    And you haven't discussed noise levels at all. What kind of vessel are we talking about anyway?

    Steve
     
  11. JSL
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    JSL Senior Member

    I know the smell and soot factor.... yuck
    Check out the Centek water separator. Works, proven, might do the trick but I doubt anything will beat down the odour in a tailwind.
     

  12. FAST FRED
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    FAST FRED Senior Member

    One advantages of a dry stack is the ability to heat the boat , or at least drying a hanging wet locker while underway.

    With keel cooling and a dry stack , no additional water pump is required , no maint!

    Smell and soot can be handled with either better injectors , or the EZ way , just extend the exhaust stack , a telescoping pipe would do to add another dozen Ft to the height,and nor effect bridge clearance.

    For better silence than anything but a huge lift muffler , Google "Hospital Critical Silencers".
     
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