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  1. #1
    Vern's Avatar
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    Impeller to pump clearance - RXP

    I was working on two impellers yesterday, my Solas 15/20 4 blade and a Skat/Riva 15/19 3 blade. I have run the 15/20 all last year ... what I noticed though is that when I install the impellers, and spin them in the housing when the impeller is facing up, it rubs on the pump hub ... when I tip the pump upside down and spin them, they don't rub. I have a Skat modded 3 bearing pump. The stock impeller does not rub at all and there is noticeably more clearance between the impeller hub and the pump hub. Sooo, I found some thin steel washers and will likely put a washer on the threaded shaft on the pump so that the impeller is slightly less close to eliminate possible rubbing on the hub.

    Anyone else notice this? It also makes me wonder what the optimal (for speed) clearance is between the pump vanes and the impeller ...


  2. #2
    GTXX's Avatar
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    it's a speed trick to machine down inside the hub of the impeller so it will sit closer to the pump, then use a thin shim to space it out just enough that it doesn't contact the pump.

  3. #3
    Moderator The Bandit's Avatar
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    Vern,
    I just installed my Skat prop in my pump. It does not rub at all. Mine is not a modded 3 bearing pump though. I turned it upside down and pulled on the prop and there is no play..

  4. #4
    Moderator Snoman's Avatar
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    Quote Originally Posted by RXP Steve View Post
    Vern,
    I just installed my Skat prop in my pump. It does not rub at all. Mine is not a modded 3 bearing pump though. I turned it upside down and pulled on the prop and there is no play..
    Get your numbers Steve you are more than likely gonna have to tweek her.
    When you do she probley wont go back in.your gonna have to file down the edges to make her a tight fit again

  5. #5
    Site Admin Green Hulk's Avatar
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    Getting the prop as close to the veins as possible is desirable. It also improves hook up. Ever hear of "set back" props. This is what they are talking about.

  6. #6
    Vern's Avatar
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    Yup, I used to run a set-back Nujet and Solas on my GPR. Getting them close to the vanes supposedly helps with hookup, but too close and some people say it wears out the pump vanes much quicker. There were also rumors that set-backs could trade off a little top speed for better hookup, and the closer the prop is to the vanes, the 'steeper' it feels, so getting it closer behaves like adding pitch. Like everything else in the pump, subtle changes can have a measurable impact on speed, so I have always wondered if having the impeller too close unnecessarily increases the difficulty of turning the impeller and makes things slightly less efficient. I might experiment with getting my impeller as close as possible, getting numbers, then adding shims and getting numbers again.

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