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  1. #1
    Vern's Avatar
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    RXP speeds vs rpm and impeller pitch

    I am trying to better understand why some modded RXP owners are reporting better speeds than others. In theory, boats with similar pitched propellers should run about the same speed IF they are turning the same rpm. That does not seem to hold always though ... I know most of the Solas 14/19 props out there have been tweaked by PPG, but then my prior statement should still be somewhat true.

    I tried to capture some examples, and would love to hear input from other members (esp with modded RXPs) on what their impeller pitch, rpms, and speeds are. There is more to achieving speed on these boats than raw power ... and in particular, the pumps seem especially sensitive to the exact right pitch and nozzle size and wear ring condition.

    Examples:
    Sundance PPG stage 1 + RR IC, wedge, PPG nozzle, stock ECU, PPG tweaked Solas 14/19, 75 mph @ 8150 rpm.

    First customer Riva Stage 2, WestPalmGPR Riva IC, wedge, etc, tweaked Solas 14/19, 75 mph @ 8150 rpm. 3 bars fuel, trim 2 clicks down from top. He is going to try the Solas 15/20 next week he said.

    PPG customer PPG stage 1 only, tweaked Solas 14/19, 71.6 @ 7950 rpm, added PPG nozzle, ran 73.8 @ 8000 rpm, tank gas.

    Jerry, RR stage 1, Impros repitched stock prop, no wedge or nozzle, 73.8 full tank, 76+ low fuel, rpms??
    Jerry, RR stage 1, Solas 14/19 tweaked, added wedge and nozzle, 73.8 full tank, 76+ low fuel, rpms??

    Jerry, your speeds are lower now than in the past, but are you running different props or rpms than before the head/cam mod?

    I think Mudbug made it very clear that some GPR owners have made extensive mods to their intake grate, trim tabs, ride plates, sponsons, pumps, impellers, pump shoes, etc, all adding to their top speed. I think there are a lot of things we can explore with the RXPs, but I thought a good place to start would be to compare speed, impeller pitch and rpm, to give us some baselines.


  2. #2
    Site Admin Green Hulk's Avatar
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    My speeds are lower now because i recorded my best speeds when it was only 60 degrees outside and water temp was in the 50's.

    Unfortunately i have no idea what pitch i am running as PPG doesn't advertise their pitch. I ran 74'ish this past weekend at 8400 RPM's in 90 degree heat. The day I pulled my head off i ran 74.5 in 80 degree heat, but i'm running more pitch in my prop now. Once things get cold again my current prop will hit the rev limiter and i will need to put more pitch in it to bring the RPM's down and my speeds should increase.

  3. #3
    Jarrett's Avatar
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    How much do those riders weigh? I have detailed numbers for myself, but I am typically 1-1.5 MPH slower than other riders because of my weight. I may do 72 MPH on my ski while Franko does 73.5 MPH on it.

  4. #4
    Jarrett's Avatar
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    Vern, something else to take into account: Agenda

    Some posted speeds on various boards are probably from the manufacturer under a pseudonym. Others are people wanting everyone to think they are faster. Gotta consider the sources.

  5. #5
    Vern's Avatar
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    Good point. I think the 'good' numbers we hear are with 150 - 200 lb riders typically, so I would go with numbers when Franko rides to keep things comparable. Last I saw, Franko ran 73.5 on yours with a repitched Solas 14/19 at 8300 rpms - is that correct for your latest runs?

  6. #6
    R.I.P. 11's Avatar
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    It has never been possible to relate speed to RPM on skis because there are too many factors involved.

    Speed depends mainly on thrust and drag, and thrust is affected by trim angle, nozzle diameter and rpm.
    Then the hydrodynamics of the impeller and the shape of the vanes will also
    affect the pump's efficiency and therefore the thrust.

    More rpm can actually degrade the efficiency of the pump and therefore the thrust and the speed.

  7. #7
    Vern's Avatar
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    Jarrett, that is true ... and kinda what I want to get to the bottom of. I do wonder why more of us are not running 75+ mph with tweaked 14/19 props and similar mod setups ... makes you wonder ... I know heat and humidity is a factor, but most of these posters are in the South too.

  8. #8
    Site Admin Green Hulk's Avatar
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    i often wonder about the speeds people post as well

    it's very misleading.

  9. #9
    Vern's Avatar
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    11, yes, there are a ton of variables ... the ones you mentioned, but even with 'identical' boats you have slight clearance differences in the pump, hulls with more or less hook or rocker, 'untrue' ride plates, etc, etc. And all those affect speed. Even so, with identical boats and setups, there is usually some correlation ... enough to help to start to zero in on potential problems with the slower boats. I used to play with GPRs a bit and have even heard people report that by switching different custom props between boats, BOTH GPRs gained speed. So, yes, each boat is unique and each pump can have a different 'sweet spot' as far as speed, thrust, rpm. But I wanted to try to collect some data so we can try to improve our knowledge of these boats in general ...

  10. #10
    Moderator Franko's Avatar
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    Most people want to believe the best number their GPS flashes at them. Some people just lie.

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