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  1. #1
    Vern's Avatar
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    More RXP speed thoughts

    My RXP is sitting in the garage, so I am bored and thinking ... so I thought I would throw out some ideas for discussion ...

    Thoughts on speeds on modded GPRs – Good ride plate = 4mph gain, 800 grate = 1 mph gain, tabs = 1 mph gain, = 6 mph total gain that we can’t get on the RXP. Then add the better thrust angle from the nozzle that is flush to the ride plate, which makes the whole ski run higher up out of the water …

    Things to try on the RXP – clean up the hull/intake/ride plate bottom, add some type of droop snoot to get the thrust centerline lower, intake grate rear bar thickening (bottom of the pump gets more water at high speeds than the top does usually), determine optimal distance from impeller to impeller vanes.

    I measured the GPR thrust line vs the RXP – measure the centerline of the thrust nozzle then measure down to the hull bottom running surface. The RXP pump definitely sits higher above the hull bottom than the GPR, approximately 2 inches, depending on how much angle your GPR plate has. Read this, off the web, about the Berkeley Droop Snoot (for full size jet boats): “Lowers the thrust point of the jet, allowing the jet pump to have better leverage to lift the bow up, which in most cases will increase your top end speed. A Droop Snoot is designed to create lift on the aft end of the boat, by lowering the thrust line of the jet nozzle prior to exit. (Works well on 20’ and under boats)” Just a thought, lowering our thrust line would make the ski more efficient, pull the bow up, and reduce porpoising. It would require a different venturi (thrust nozzle), with a flat, perpendicular bottom, a top that comes down more, and then a wedge would still give you flexibility to get the right angle, but at a much more advantageous thrust line. From the American Turbine site: “A droop snoot is designed to create lift on the aft end of the boat. By doing this you will gain approximately 3 to 5 miles per hour in speed. Droop snoots generally work best on 20’ boats and under.” We won’t know how much we would gain on an RXP until we try it.

    A possible way to drop the thrust centerline:
    Cut the thrust nozzle just behind the bolt flange, and add in material at the top or cut out some from the bottom, then reweld it, angling the nozzle down. Cut the end (where the nozzle rings are) and angle it back up to neutral or more (2 or 3 degrees up) and reweld it. This will drop the thrust line. Not sure if it is better to add in material at the top or cut out material at the bottom – is it better to add volume to the thrust nozzle or remove it? Also the vanes in the thrust nozzle would likely be cut out with this mod – would this have an impact? Possible to leave them in, but would require some fabrication work maybe to extend or trim them.

    From another performance jet boat site:
    The above mentioned jet pump parts are further enhanced by making subtle changes within the bowl. This is called "Flowing" the bowl. By changing the internal surfaces that the water strikes when entering the bowl from the impeller, water flow is increased through the bowl. The more water that flows through the bowl faster, yields greater thrust at the exit nozzle.

    The remaining parts that boost efficiency, yielding further improvements, are the Place Diverter, or similar trim device, and the top loader intake grate. The top loader will charge the intake with equally pressurized water on the top of the intake and the bottom of the intake. At speeds in excess of 60 mph, the intake has a differential pressure between the top and bottom, the top being the low pressure. In essence, the bottom of the impeller is doing most of the work. By equalizing the pressure, the whole impeller is working and packing the bowl with more water. This is why a good toploader grate can sometimes add speed.


  2. #2
    Site Admin Green Hulk's Avatar
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    Damn good info Vern!

    Not only would it increase top speed it would dramatically increase accleration.

  3. #3
    Im having a Yami friend help me true my rideplate and tinker with the pump some to see if we can find some extra speed. We're sure there's some there. Won't be able to test until next May though.

  4. #4
    While rideplates are on my mind, whatever happened to the aftermarket one for the RXP by that company from Idaho or wherever? Any more info?

  5. #5
    Matthew K ReDevilRXP's Avatar
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    That is some good stuff right there and from looking at the riva grate it doesnt look like rocket science to make one. someone shoudl really look into making and testing some diff style grates

  6. #6
    mxl16's Avatar
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    heres the site and a pic of a drop snoot.

    http://www.americanturbine.com/produ.../BERKsnoot.htm

    does anybody have a pic of the backs of a gpr and rxp side by side?
    Attached Images Attached Images  

  7. #7
    EZ Dock of Long Island Shibby1485's Avatar
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    we need an aftermarket rideplate in such a bad way!

  8. #8
    Good reading, alright all you metal fabricators, get to work with your cnc`s...PR...

  9. #9
    Moderator The Bandit's Avatar
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    I could make one out of JB weld....lol

  10. #10
    Vern's Avatar
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    No kidding ... a better ride plate would definitely help our speed ...

    I was thinking about the best way to lower the thrust centerline without changing the trim and steering cables, etc. The best I could come up with is a modded nozzle. I wish someone made a nozzle that had a flat bottom and a steeper sloping top, but I don't know of one like that that will fit our 158 mm RXP pumps. So, cutting an RXP nozzle in two places and changing the angles seemed like the next best idea. I have two spare nozzles, but will have to try to find a shop that can cut, weld in a small patch piece of aluminum, then reweld. I wish I had the tools to do it all myself, but I don't. Hmmmm ... just had a thought ... since the wedge will really not work with this mod, maybe I sacrifice the wedge and use a piece of that to 'patch' into the top of the nozzle ... flip the wedge upside down, cut it and use a piece of it. Or, (never tried this) install the wedge upside down, and cut the nozzle area and angle it back up to achieve a 2 - 3 degree up angle at the exit of the nozzle. I am at work, so I can't check if this even would work fit-wise on the ski or if an upside down wedge would get the exit nozzle close enough to the ride plate to be effective ... but I think any way we can get the nozzle down low, then angled back up at the exit, should help. The next trick is to keep the nozzle smooth and efficient inside with those changes.

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