Anti-clogging system for jet-propulsion

Discussion in 'Boat Design' started by alpamis34, Dec 4, 2006.

  1. alpamis34
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    alpamis34 Senior Member

    Hi all,
    I am planning to design a new model with a water jet-drive system installed in it.But as it is a commonly known issue , the clogging of the water jet inlet with debris etc. makes me sink down through a sea of ideas.Is there a way to prevent the entry of that debris and other unwanted stuff into the system without reducing the performance ? Please help me about that regard. Cheers
     
  2. ted655
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    ted655 Senior Member

    First off, I don't know a damn thing. BUT, it seems the primary intake should taper slightly at the aft end. No cross bars should be used. Just narrow spaced parrell round bars. This would self clean much of the debri by letting it pass along the intake by forward speed.
    Then a 2nd. screen or grid type placed up higher in the scoop at an angle could catch the smaller "chunks". If a low pressure area could be formed at the rear edge of this grid, the debri would flow off into this area and be ejected into a "quieter pool" that the boat would leave behind as it moved forward.
    Looks like a 20gal. aquarium tank, pump system and food coloring are in store for you.
    Once all the shapes, angles, spoilers & details are modeled, you will be halfway to BIG $$$$. Good luck.
     
  3. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    No round bars, better flat strips to avoid turbulances in the waterduct.
     
  4. DJPE
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    DJPE DJPE

    You can approach it like the PW PT6 FOD trap. Design the the ducting in as such that the debris is slung off into a lower trap when turn a corner prior to entry into the pump. This lower trap will be static powered under boat wake and would float the debis off to the surface.
     
  5. Tim B
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    Tim B Senior Member

    I wouldn't worry too much about debris getting sucked in. it will happen, but a few bits of metal over the entrance will be quite adequate. Internal marine growth is probably the most significant problem. I'd suggest contacting the builder of the waterjet about solutions to that.

    Tim B.
     
  6. marshmat
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    marshmat Senior Member

    As another option, Volvo has been known to fit F/N/R marine transmissions between the diesel and the jet.... thus you can run the engine in neutral as well as being able to back-flush the jet if too much seaweed gets in there. It's up to you whether that's worth the added weight and complexity.
     
  7. alpamis34
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    alpamis34 Senior Member

    Thanks for all your interest guys,

    If we sum up all the things you 've suggested, the best way to avoid from debris is to use bars to block the entry.But what if those bars lead to the deviation of the water flow and I might end up with an early seperation ?I reckon that would decrease the efficiency,or am I thinkin' wrong?
     
  8. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Putting flat thin strips on the waterduct inlet, do you have a protection and the loss of efficiency depends only on speed.

    It would be necessary to install such grating to avoid serious damages on impeller.
     
  9. waikikin
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    waikikin Senior Member

    I used to do some work on a powercat with twin 1100hp MAN engines & Kamewa jet units, it had longtitudinal flat bars edge on in the intakes & sometimes there would be smallish boulders(6"-8") jammed in between, so I reckon they probably saved some damage & where therefore a good scheme. All the best from Jeff.:)
     
  10. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    a couple of MAN rated 1100 bhp and Kamewajets......it would be possible to know the displacement and the speed?

    Thanks waikikin
     
  11. waikikin
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    waikikin Senior Member

    The sad truth on speed

    Rancho Otto, not sure on the displacement but she was 86' in length so probably quite heavy, originally designed for I think about 180hp a side & as a motor sailer of 56', built in a dense foam & e-glass, some contract wording apparently allowed for any equipment furnished by the owner(such as massive engines) to have to be fitted to the vessel whilst in build hence the lenght increase(and hull width etc etc) & a 1.8M project blew out to some 7M sponsered by our tax payers in a government garanteed opperation(long story made short). I think strait line performance was 20+ but cornering was described by skippers as scary & no one would push the limits on it. One of our jobs on it was to fit wedges to the aft end(from the jet intakes aft & we did 3 different sets as the back would suck down till the underwing ran on the water surface creating hideous slamming & noises to emminate from it) , never really solved much- but we got paid good!
    Sorry I cant dazzle with sparkling & impressive numbers but the main thing to be learned from the vessel besides that grates keep out rocks was to not **** with designs so much that they turn out wrong & to get build contracts worded in better favour to the vessels contructor, not really my prob as I was small fish employed to work on it & as the boats still in good service(drives around slow- maybe they derated those engines) it probably turned out ok.:) Regards from Jeff.:cool:
     

  12. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Thanks for the informations...:)
     
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