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(RC) Water jet impeller design?

Discussion in 'Off Topic [BG]' started by T-Bird, Apr 30, 2009.


  1. Hi.

    A friend of mine called (multiple times) last evening. He was quite drunk, but what I could understand from his explanation was that they would like me to partly design and manufacture a few impellers for their RC water jet boat.

    I have a bachelors degree in mechanical engineering and have quite an extensive array of machinery and the skills to use them so the designing and machining won't be much of a problem.

    What is a problem is that I have ZERO experience in propeller/impeller design.

    The specs I know ATM are as follows:

    Boat: 1500mm long, 350mm wide.

    Engine: 39cc, 14Hp, 14000 rpm. :scowl: http://www.motobikespares.com/39-cc-engine-watercooled-p-243.html :scowl:

    Jet: Twin 50mm dia. Three blade. 20000 rpm

    What bothers me is the pitch and lenght of the impellers they need/want. Lenght=1.8*D. Pitch 50mm or 75mm or 100mm.

    That's nowhere near the dimensions I looked at in the net. All the impellers were shorter and had a pitch much less than the diameter.

    Personally I think that their ideas won't work, at least not well, the proven ones will ;).

    If anyone has any info/sites to point me to, that'd be awesome. And if someone has a pic of the Graupner (or any other) 49mm impeller, that'd be cool too. It seems to be missing from all the sites I visited???

    Math/physics, even quite advanced, are no problem, but I do need some laymans explanation to give to my friend also.

    Regards
    Sam
     
  2. MakiSupaStar

    MakiSupaStar The Lowdown Diggler

    Apr 12, 2006
    Huntington Beach, CA
    So uh. I've got nothing.
     
  3. wazzel

    wazzel

    Dec 27, 2007
    Houma, LA
    I would look at the impellers that are jet ski's and down size them. You are going to take a performance hit because of the small size. Make sure you have lots of intake cross section to keep losses down.

    You could try the units that are used the in "jet airplanes" https://shop.graupner.de/webuerp/servlet/AA
     
  4. Phalex

    Phalex Semper Gumby Supporting Member

    Oct 3, 2006
    G.R. MI
    Balance is important. That's all I got.
     
  5. L-A

    L-A

    Jul 17, 2008
    Eh?
    Isn't pitch in degrees and not mm?
     
  6. Hi.

    Thanks, guys.

    Just got this link e-mailed to me:

    http://www.modeljetboat.com/jetboat/jetboat.htm

    They want me to duplicate that.

    Maki, thanks anyway ;).

    Wazzel, yep, that's what I was thinking about, and that was the approach I took when started to gather data. As You can see, the design on the above link is totally different. Archimedes type screw may work somehow, but as an engineer, a impeller with a compression stage woud work better. Naturally the compression will cost a bit powerwise, but all the impellers I found were like that.

    Phalex, especially with the design they have in mind.

    L-A, I thought so too, but it seems that it's used (incorrectly?) to tell the advance in the helix, in mm or inches.

    I got also some sketches with the link, and while they will pump water, that's about it. IMHO.

    If someone has any math or physics on this subject, that would be great. I'd like to take a scientific approach to this rather than: "let's try that and see what happens". This stuff has been around for 50 years IIRC, no point in re-inventing the wheel, or a propeller in this case :).

    Regards
    Sam
     
  7. ehque

    ehque

    Jan 8, 2006
    Singapore
    It seems like that's the exact thing

    did. Sorry, i have nothing.
     
  8. wazzel

    wazzel

    Dec 27, 2007
    Houma, LA
    I would not use an Archimedes screw. I think you would have sever rotational speed limitations and no pressure increase. Jet boats need a stream of presurized water to move. IIRC archimedes screws need to move slow and are only used for lifting fluids without adding pressure.

    For what its worth I am an engineer and use to work for an industrial pump manufacture. Never did any impeller design tho.
     
  9. Hi.

    I wouldn't either, but that's what they want, are willing to pay for and that's the easiest I can manufacture.

    It's hardly optimal approach to the jet (or any for that matter) design, but I'll go along this time. It doesn't take me much to make a few mock-ups and patterns for them to work on.

    Let's look at the bright side: At least there's a great possibility for improvement later on ;).

    Regards
    Sam
     
  10. ehque

    ehque

    Jan 8, 2006
    Singapore
    You might be right. From wikipedia:

    Propellers remained extremely inefficient and little-utilized until 1835, when Francis Pettit Smith discovered a new way of building propellers. Up to that time, propellers were literally screws, of considerable length. But during the testing of a boat propelled by one, the screw snapped off, leaving a fragment shaped much like a modern boat propeller. The boat moved faster with the broken propeller.

    In my opinion, the radial component of the velocity of the water increases with the length of the screw (and conversely the efficiency decreases). Therefore using a short high speed impeller would perform much better than a low speed Archimedes screw.
     
  11. txbasschik

    txbasschik

    Nov 11, 2005
    Leander, Texas
    Too bad my hubby's not a bassist, and a member here. He's a boat mechanic. He repairs many, many imprellers. He could definitely tell you what *doesn't* work. I asked him if he could provide some input here, but he says, "Baby, its my day off. I don't want to think about impellers today, 'k?"

    Can't blame the man.

    Cherie
     
  12. txbasschik

    txbasschik

    Nov 11, 2005
    Leander, Texas
    Hubby sez a water-jet impeller is, basically, a condensed Archimedes screw. FWIW.

    Cherie
     
  13. Hi.

    Thanks Cherie, just the kind of information I was looking for.

    It's true that the design is basically an Archimedes screw, but the compression makes it effective in propulsion AFAIK.

    It would be great if Your hubby could provide information about the approximate compression ratio, ie effective cross section area ratio difference, between recreational waterjet impeller and race water jet impeller. From pics it seems to me that the recreational impellers are about 2:1 area ratio. I Couldn't easily find any pics of the race-only impellers (obviously ;)), just pics of recreational use impellers.

    Regards
    Sam
     
  14. I think the most important thing to remember is that it's not the size of the boat, but the motion in the ocean.
     
  15. wazzel

    wazzel

    Dec 27, 2007
    Houma, LA
    There are different beast that are similar.
     
  16. anything that moves is usually raced as soon as the second one is built and id bet that the racing community can tell you all you need to know about the trade offs of impeller design
     
  17. Chrispurchase

    Chrispurchase

    Oct 24, 2007

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