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  1. #1
    Vern's Avatar
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    Hi Pressure pump speed

    I am curious if anyone can answer this ... the GPR seems to gain at least 2 mph usually from converting from the old style pump to the newer pump with a bigger hub. What I am curious about is WHY this happens?? Usually the faster you can move more water thru a pump, the more thrust you achieve ... this must be happening, but it seems counter-intuitive ... with a bigger hub, there is less pump volume, which would seem to be more of a bottle neck, allowing less water thru and less water speed. Yet the opposite must be happening since the speed gain indicates MORE thrust with less internal pump volume. Anybody know why this works?

    Only thing I could come up with is that the water flowing along the outside edge of the pump would likely be flowing the fastest, and the water flowing along the hub would be the slowest moving. Does a bigger hub balance this out, creating more flow speed along the hub (by eliminating the area of slowest water flow), balancing the speed of the flow significantly enough to improve pump throughput overall and generate more thrust??


  2. #2
    Geez Vern, that sounded so sophisticated I can't answer, but it sounds good.

  3. #3
    I like Boobs. wetwolf's Avatar
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    the larger hub taking up more space helps keep water velocity up. You are moving the same amount of water, but thru a smaller hole, so it goes faster.

  4. #4
    Moderator RX951's Avatar
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    "less pump volume" from the larger hub with the power of the motor will develop more "pressure"

    More pressure = more ft. lbs of thrust = more speed

  5. #5
    Vern's Avatar
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    Thanks Frank! Image is everything ... not! Hey I hope your ski owns the competition next summer!

    OK, next pump question ... for top speed are less vanes generally better, or no real impact?

    And, one more ... for best flow are super smooth pump surfaces better, or is a slightly rough surface better? Super smooth seems good but head porters have found that slightly rough surfaces promote better laminar flow (the air on the port wall goes somewhat slower, almost 'rolls', somehow allowing better overall airflow numbers), but I have no idea if/how this applies to water flow thru our jet pumps.

  6. #6
    Moderator RX951's Avatar
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    Quote Originally Posted by Vern View Post
    Thanks Frank! Image is everything ... not! Hey I hope your ski owns the competition next summer!

    OK, next pump question ... for top speed are less vanes generally better, or no real impact?

    And, one more ... for best flow are super smooth pump surfaces better, or is a slightly rough surface better? Super smooth seems good but head porters have found that slightly rough surfaces promote better laminar flow (the air on the port wall goes somewhat slower, almost 'rolls', somehow allowing better overall airflow numbers), but I have no idea if/how this applies to water flow thru our jet pumps.


    Salty ! Where are you at !

  7. #7

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    smooth pump

    generally it is known that if the pump has more vanes it will be slower than a pump with less vanes although the pump will be much quicker out of the hole. for the second question ,it is hard to compare a transfer port to a jet pump bacause we are dealing with two entirely different things one being a vapour and the other a liquid.i think to have a rough finish to a jet pump would create turbulence and promote cavitation although i dont know if a polished finish is the answer. in the field of flow it is generally accepted that a smooth walled cylinder will flow water faster than one with a rough wall. hope this helps

  8. #8
    I like Boobs. wetwolf's Avatar
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    a rough surface is better for flow. It helps maintain the "boundary layer". Which is the layer of air or water between the pump vane ans the flowing water. the rough edges create a little grip to which a layer oif water clings to, the boundary layer..... Everything else flows over this layer. A smooth surface doesn't give any "grip", so the b/l sloughs and runs, disturbing laminar flow. On transfer ports, a rough finish helps create a good b/l. on exhaust ports, you want them highly polished so that carbon has nothing to cling to. once carbon starts building up on the exhaust port, it never buiulds up a nice even surface and thus the b/l eventually becomes very bumpy and flow is reduced. In a pwc jet pump, you're just as well off getting rid of casting flash and doing a rough one over the surfaces.

    Da Wolf

  9. #9

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    Quote Originally Posted by Vern View Post
    I am curious if anyone can answer this ... the GPR seems to gain at least 2 mph usually from converting from the old style pump to the newer pump with a bigger hub. What I am curious about is WHY this happens?? Usually the faster you can move more water thru a pump, the more thrust you achieve ... this must be happening, but it seems counter-intuitive ... with a bigger hub, there is less pump volume, which would seem to be more of a bottle neck, allowing less water thru and less water speed. Yet the opposite must be happening since the speed gain indicates MORE thrust with less internal pump volume. Anybody know why this works?

    I've done a lot of reading on this and am just learning myself, so I will take a crack at the subject. In a nut shell though, Da Wolf is dead on right. I'm just going to take the long route and add some BS to make it sound cool! I also reserve the right to pre confess.... I could be wrong.

    To maintain a consistent speed, a boat's pump must take the surrounding water and accellerate it to the speed that the boat is traveling through the water (velocity). Do not forget though that you also need a certain VOLUME.

    . Every pump operates best at a certain range of "head" pressure. We will just define "head" pressure as that force that acts to push water into the intake gullet.

    Lets say that our pump operates best at a range of 40 to 68 lbs of head pressure (and coencidently, our boat accellerates best at 40 to 68mph).
    Our boat at idle experiences only atmospheric pressure (about 14 lbs) plus the pressure that about 8 inches of water can exert (the boat is in about 8 inches of water) . During hole shot, we do not have enough pressure forcing water into our intake. Our pumps can process more water than it can receive... and so it creates a negative pressure and the pump cavitates. A larger intake can offset this problem because there is more VOLUME in a bigger intake to process. A more agressive scoop helps here as well (1200 grate vs 800 grate).

    Here's the tricky part, and the part, Vern that I think that you are missing. Once the boat moves forward we have dynamic pressure... the intake is acting like a funnel or scoop, and the water that we move through is being forced into the pump at boat speed. Once we reach 40 lbs of head pressure (40mph), the boat begins to accellerate well until we reach 68 lbs of head pressure (68mph). This is our midrange.

    At this point, you pump is very efficient, processing all of the water that is coming into the intake. After 68mph, your engine is having to work harder and harder because it can no longer process all of the water coming into the intake. The boat increases in speed, but not near as fast as when you are in mid range. You are outside of efficiency, but sheer horsepower is increasing your speed. Your boats top speed is in part due to increasing hull resistance, but also because the intake is taking in more water than it can process. Some people call it "overstuffed". Because of this "overstuffing" the velocity of water entering the pump begins to slow down before it is processed.

    Picture yourself with a water hose about 2 inches away from a 3 inch funnel with a 1 inch drain. The drain is pointing down You can increase your water pressure (more horsepower), and water will pressue out of the drain spout with more pressure. Proportionally though look at all of the water that you have overflowed out of the funnel and onto the ground. The represents the intake and the water that overflows represents water that went into the intake but was not processed. Not only that, because there is such excess water, the velocity of that water slowed down. Your engine is using more and more power to increase this velocity. When your engine can no longer fight the hull resistance and process no more water, you have reached top speed.

    What if you could reduce the total size or volume of the intake? with a smaller intake, you could get to a higher velocity (boat speed) without getting to the point that your pump can no longer process more water. That is what a bigger hub on a pump does for you. It reduces the amount of water that is allowed into the intake at any given time.

    If you were to carry this futher and fill in certain parts of your intake gullet with fiberglass, you would find that your boat would go even faster. The problem with this thinking is that the intake must not only be filled in, it also has to have a specific shape to prevent incoming water from getting turbulent. You can really mess up you boat changeing the intake shape.

    You are right Vern, you do decrease the volume with a bigger hub, but you must also take into consideration that at high speed, the pressure and overall volume of the water going into the pump is much greater than at idle.

    Bottom end: high volume of water (big intake) with low head pressure to get velocity.... and therefore speed.

    Top end: low volume (reduced intake size due to a larger hub) intake with high pressure water to get water velocity.... and therefore speed

    Once again, I could be wrong.

  10. #10
    cheatin' piston popper addicted's Avatar
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    Reading this and I happened to stumble upon this at the X.http://www.x-h2o.com/showthread.php?t=10807 http://www.tbmracing.com/images/prod...ma_cone_lg.jpg

    I was thinking of trying something like it to make my raider pump more like the HO version on the gpr's.. . If i could make it myseld or find one that's CHEAP! Just sucks c'ause I would have to hack it up to get my AAT to fit.

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