dual jet drive bottom layout

Discussion in 'Jet Drives' started by yipster, Dec 22, 2009.

  1. yipster
    Joined: Oct 2002
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    yipster designer

    i'm placing jet drives in drawings, one drive usually sits in flat triangle rite?
    now two drives i'm not sure, ok they better come down much as possible to the center but do they cant as in pic?
    to top it off, how should it look in a transverse stepped hull when they hit the step as shown in pdf
     

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  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The intake body shown (is that Alamarin?) has a V-shape forward of the inlet grating. For double installation that must be trimmed down to suit eventual bottom curvature. Note that the cl of the intake opening must follow the buttock line, which means that the engine cl is NOT parallell to keel (engine fronts stand closer than their aft ends), due to the shaft inclination!

    There may not be any longitudinal spray strakes or steps inside the buttock line just outside the intake body, nor any transverse steps. Also no through hull fittings like water intakes or outlets, logs or equivalent, in the bottom area in front of the inlet.

    If I understand your dwg, you intend to install the jets outside the longitudinal step?? If so, forget it! The aeration from the spray crossing the step will cause a complete loss of pumping power. Those waterjet pumps can only handle about 5 percent of air volume in the inlet; so take all steps possible to avoid air following the bottom into the intake. Even deadrise angles less than ~10 degrees should be avoided for this reason.
     
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  3. Submarine Tom

    Submarine Tom Previous Member

    yipster,

    Why two jet drives?

    -Tom
     
  4. CDK
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    CDK retired engineer

    Twin jets in a V-hull is asking for trouble. Believe me, I've been there.

    Unless you can obtain or make adapters to mount the jets horizontally above the inclined intake grate, you'll have insufficient clearance for the lower engine parts. Obtaining a vertical steering axis by rotating the nozzle heads and drilling new holes may be a solution but only if the flanges are cylindrical.

    And as Baeckmo pointed out, the presence of air will degrade the performance beyond the point of usefulness.
     
  5. yipster
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    yipster designer

    thanks for the pro help and advice gentleman !
     
  6. Submarine Tom

    Submarine Tom Previous Member

    Still wondering yipster...
     
  7. yipster
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    yipster designer

    altho my first thought too, to answer your question Tom: i am asked to do so
    and these reply's do shine light so thanks again, marry xmas and a happy new year
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Ahhh, a customer request, I get it,

    "No correlation between brains and money."

    Thanks for the reply.

    All the best to you,

    Tom
     
  9. jim lee
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    jim lee Senior Member

    Is there any spec saying how far apart the two have to be? About 8 years ago there was a legal battle between, I think Aggressor and another group over a dual jet design. I saw the "other" one, forgot who, run their machine. It worked ok I guess, but not all that hot. I recall that both jets bolted to the same intake casting. One engine and gearbox. Something like that may help get around your mid hull problem.

    I really wonder why they think dual jets would be a good idea.

    -jim lee
     
  10. yipster
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    yipster designer

    oops Tom, was gonna mention something similar and not just becouse i linked "No correlation between brains and money." to this thread
    guess its meant as "top of the line" rib drive setup. interesting as it may be i figger this hull suits dual surface drives better
    on the other hand i still have, reading anthony's post above the boat may well be for people in the water, oops
     
  11. mark775

    mark775 Guest

    Lots said that two jets are bad but nothing about why two jets are bad. Is it just that they are set up for singles or is there something you believe to be inherantly bad?
     
  12. anthony goodson
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    anthony goodson Senior Member

    I think the logic goes something like this ,if you have had a bad experience with a jet drive ,through no fault of your own ,which many people have ,then jets are bad ,and if one jet is bad then it follows that two jets must be twice as bad.
     
  13. CDK
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    CDK retired engineer

    Not necessarily true Anthony.
    I've had very bad and costly experiences with twin jets, absolutely my own fault because in a moment of weakness I believed the text of a Californian website.

    Yet for a rescue vessel that must be able to cope with impossible conditions, twin jets seems a good idea even to me. I would use diesel engines though.
     
  14. anthony goodson
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    anthony goodson Senior Member

    CDK you have taken my post too literally,it was written tongue firmly in cheek. I am of course aware of your problems with jets from your previous posts and have every sympathy with your feelings about them .As I have said here many times the people who have been to blame for the bad reputation that jets enjoy,are ,with a few exceptions the manufacturers themselves but I feel that things are improving in this regard ,shared information on forums like this one helps a lot. A correctly specified jet will work very well in certain roles ,but these roles are limited and specific, in the best case scenario they are unrivalled in the worst they are next to useless.
    Now to attempt to answer the man's question with my tongue out of my cheek, there is nothing inherently bad about twin jets in the right application ,in the smaller vessels we are talking about numerous lifeboats and safety boats use them and there are two outstanding military examples that come to mind The British CSB and the American PBR which created its own legend in the Vietnam war Merry Christmas
     

  15. apex1

    apex1 Guest

    Would be interesting to have baeckmos comment again.

    Richard
     
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