Jet DIY

Discussion in 'Jet Drives' started by Duco84, May 19, 2022.

  1. Duco84
    Joined: Feb 2014
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    Location: Montenegro

    Duco84 Junior Member

    Hi,
    I need a help with Jet design. I’m planning to make it and install on boat
    Lwl-6m
    B-2.7m
    T-0.4m
    Displacement-2.5T
    Two DIESEL engines, 80hp each, 4200rpm.

    Please advise at least impeller diameter, inlet angle and nozzle.


    I already tried with impeller 190mm, and nozzle 110mm, but engines achieved rpm, speed only 7kn.
     

    Attached Files:

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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I do not know anything about water jet design, so I will be very interested in seeing what others on here who are very knowledgeable on the subject will say.

    I do know though that the design of the jet and the impeller is absolutely critical, and that if you are just slightly 'off' from what the optimum should be, your performance will suffer drastically.

    And your hull seems to be rather heavy (relatively) at 2.5 tonnes, and 160 hp is not an awful lot of power - especially when you consider that a pair of 80 hp outboard engines would probably have better efficiency with their standard propellers than your impellers.
     
    Last edited: May 20, 2022
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  3. Duco84
    Joined: Feb 2014
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    Duco84 Junior Member

    Yes, I know, but this is let’s say some kind of experiment.
     
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . . .

    Wow, that's a lot of work.
    Can you show the hull in pictures?
    Have you had outboards on this hull before?
    Heavy at 2.5T.
    What diesels are they? Hours?
     
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  5. Duco84
    Joined: Feb 2014
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    Duco84 Junior Member

     

    Attached Files:

  6. Duco84
    Joined: Feb 2014
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    Duco84 Junior Member

    Engines are from AlfaRomeo, 1.9 tdi, in good condition. No I didn’t try with outboard.
     
  7. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    That hull corrugation right in front of the intake isn't helping. Weight plus small diameter jets isn't helping either. Upping impeller size and jet size isn't really all that easy.
     
  8. Duco84
    Joined: Feb 2014
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    Location: Montenegro

    Duco84 Junior Member

    Yes I know, but I’m ready to try. It is a lot of work, so thats the reason ghat I’m looking for dimensions in order to achieve best efficiency
     
  9. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . . .

    Thanks for those answers Duco84.
    Any other hull pictures?
    Is it a catamaran?

    This is like trying to find a needle in a haystack!
    If you're looking for efficiency ("in order to achieve best efficiency") props would be a better way to go and much easier to size.
     
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  10. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    Yeah. Jets aren't used for efficiency, they are used for extremely shallow draft applications.
     
  11. Duco84
    Joined: Feb 2014
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    Duco84 Junior Member

    Yes I need it for shallow water
     
    Last edited: May 20, 2022
  12. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Waterjets are extremely sensitive to inlet conditions. The grating shown, together with the "bump" in the hull bottom is a complete disaster in terms of inlet shaping. You will have cavitation setting in just in the speed range before hump speed, where maximum thrust is needed, i.e. the engines are working on the rev limit with reduced fuel injection volumes, while the jet pumps are blocked by cavities.

    From the pictures it looks like you have built new stators and nozzles to old jet bodies. What is the origin of the units? Unfortunately, with the existing inlets, there is no realistic chance to increase flow by using a bigger impeller, because the inlet tip speed will increase correspondingly. As cavitation in this speed region is mainly a function of tip peripheral speed, that's not a possible way.

    So, first of all, let's see better pics of the bottom upstream of the inlet, and get rid of the gratings so we can see what the "real" inlet looks like. And take off a stator to show what the impeller and stator vanes look like.
     
  13. Duco84
    Joined: Feb 2014
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    Duco84 Junior Member

    All parts of jet are made. I’m ready to start from beginning. I’m sure that I have problems with cavitation, due the bad inlet design. Please advise dimensions and angle of inlet. Also, I worry abt diameter of impeller, engines achieve rpm without problem.
     
  14. Duco84
    Joined: Feb 2014
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    Location: Montenegro

    Duco84 Junior Member

    On the last photo you can see that impeller is simple immersed in water, almost no inlet pipe. Also inside, he installed pipes for cooling of engines (I’m sure, shouldn’t be there.
     

  15. anthony goodson
    Joined: Mar 2007
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    anthony goodson Senior Member

    Hi Duco you haven't made it clear how much of this project you have inherited ,and how much you have made . Is this a failed experiment by someone else that you are trying to put right or have you produced these parts yourself ,and what is your motivation for doing so .?
     
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