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Thread: Got an idea

  1. #1
    MDGPR's Avatar
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    Got an idea

    I dont know if this has already been tried or if it would benefit but here it is...

    My idea is to take out the stock driveshaft that is im guessing 3/4" diameter and replacing with different shaft that reduces down to 1/2" as soon as it comes out of the hull and then goes back to 3/4" at the impellar. I think it would be a good idea to use a stronger metal to prevent it from torque to much ti bends. with it staying 3/4 at each end it will not require any additional mods to make it fit. I am sure it would be an easy project for a machine shop to just copy the stock one except reduce the middle down to 1/2"

    Now if it will work, with it being a smaller diameter it will increase the area of the pump tunnel possibly reducing cavitation and increasing top speed maybe? I dont know enough about my ski to do the mods but not really why most of them work so i might be totally wrong on the benefit.

    What do you all think?


  2. #2
    Moderator RX951's Avatar
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    Quote Originally Posted by MDGPR View Post
    I dont know if this has already been tried or if it would benefit but here it is...

    My idea is to take out the stock driveshaft that is im guessing 3/4" diameter and replacing with different shaft that reduces down to 1/2" as soon as it comes out of the hull and then goes back to 3/4" at the impellar. I think it would be a good idea to use a stronger metal to prevent it from torque to much ti bends. with it staying 3/4 at each end it will not require any additional mods to make it fit. I am sure it would be an easy project for a machine shop to just copy the stock one except reduce the middle down to 1/2"

    Now if it will work, with it being a smaller diameter it will increase the area of the pump tunnel possibly reducing cavitation and increasing top speed maybe? I dont know enough about my ski to do the mods but not really why most of them work so i might be totally wrong on the benefit.

    What do you all think?

    Good thought, but I think it will promote premature tunnel stuffing at higher speeds.
    I think the tunnel area needs to be reduced more to help us out.

    Just my thought......

  3. #3
    MDGPR's Avatar
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    what do you meen by premature tunnel stuffing?

    I dont know metals real well but im sure their is somethign that is both stronger and lighter so we could maybe shed a couple pounds as well.
    Last edited by MDGPR; 02-17-2007 at 12:43 AM.

  4. #4
    Moderator RX951's Avatar
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    Quote Originally Posted by MDGPR View Post
    what do you meen by premature tunnel stuffing?

    At the higher speeds some are running, 78+, they encounter a problem where the impeller is no longer pulling water....water is getting forced into the tunnel. the term" tunnel stuffing" is what I am referring to.

    the end result is the "Buck" some experience as their boats pump area gets overstuffed with water volume form the speed they are running.

    If you look back at the Yamaha Superjet, they sell a tunnel stuffer that takes up some pump tunnel area to help the ski increase in speed and lower the amount of water volume that the pump is taking in.

    The 800 grate help reduce the amount of water that comes into the pump area via the ramp and scoop angle. the pump area is recieving less water, thus the pump does not get as loaded vs. the 1200 grate.

    A larger pump area is probably more beneficial for CC racing where they need all the hook-up they can get.

    Yamaha increased the pump volume on the GPRs by adding the 2" pump spacer as seen on the 2003-current GPR's.

    .02
    Last edited by RX951; 02-17-2007 at 12:51 AM.

  5. #5
    Chrisrokc's Avatar
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    Is anyone working on something like that for the GPR? I saw the picture for the Superjet.
    Last edited by Chrisrokc; 02-17-2007 at 12:52 AM.

  6. #6
    Moderator RX951's Avatar
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    Quote Originally Posted by Chrisrokc View Post
    Is anyone working on something like that for the GPR? I saw the picture for the Superjet.
    reducing the pump tunnel area? yes. Spud has some testing he needs to do when the weather gets warmer.

    http://www.greenhulk.net/forums/show...tunnel+stuffer

    http://www.greenhulk.net/forums/showthread.php?t=16509


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    Last edited by RX951; 02-17-2007 at 12:55 AM.

  7. #7
    Me Hahn and Joe H204YOU's Avatar
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    Quote Originally Posted by Chrisrokc View Post
    I think he means that we are not able to turn the impeller fast enough to get the water out fast enough. So more water in the pump tunnel could be less speed. Less water would be optimal at the speeds we are reaching now.
    Dang,you still at it Chris?Its not so much impellor speed as it is too much water for the size hole.It would cause the infamous buck to happen sooner.

  8. #8
    Me Hahn and Joe H204YOU's Avatar
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    Billy is 100% on it.The closed coarse pro shoe allows much more water through and is more streamlined for better flow,but after 75mph not much good.

    Now how bout a super jet stuffer modified to fit a GPR ?Inquiring minds wanna know!
    Last edited by H204YOU; 02-17-2007 at 01:02 AM.

  9. #9
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    Lol, yea I am still at it. I have been slammed with homework and nothing sounded exciting tonight so I decided to stay in.

    Hopefully this weather will get nice and next weekend I can get to work on the ski!

  10. #10
    Me Hahn and Joe H204YOU's Avatar
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    Quote Originally Posted by Chrisrokc View Post
    Lol, yea I am still at it. I have been slammed with homework and nothing sounded exciting tonight so I decided to stay in.

    Hopefully this weather will get nice and next weekend I can get to work on the ski!
    If your bored tomorrow gimmi a call,I need to go to the prop shop tomorrow and pick up my pro pump shoe and Judge modded intake grate.My scat 14 vien set back just came in from down under.To see these compared to stock would shed some light on pump flow 101.I wouldnt mind seein that ff plate too!

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