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  1. #1
    bttek1's Avatar
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    Tech Question for engineer types...

    Let's say that we are spinning 8500 RPM's with a 159mm Solas 15/20 prop through a 84mm venturi nozzle. If we decrease the size of the exit nozzle down to 81mm the volume of water decreases through the exit nozzle but the velocity increases.

    Does anyone out there have a formula to figure out the velocity difference of water exiting a 84mm compared to a 81mm opening. Also, Would this be expressed in GPM, FPS or something else?
    Last edited by bttek1; 09-06-2007 at 11:15 PM.


  2. #2
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    It would seem to me that the gpm might go down slighly w/ smaller ring, but fps goes up for sure. But to quantify the diff. it seem u need more input info, like current fps and gpm.... I know it's no help but to just say what prop and rpm...I think we need a hydrodynamics guru. Whatcha tryin' to do? See what difference the 81mm and flux-capacitor have on the pump?

  3. #3
    bttek1's Avatar
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    For so long, most know that in order to increase speed, we can reduce the exit nozzle and then adjust the pitch of the prop for the reduced exit nozzle. Even though we know that this under normal situations allows the boat to go faster, it at the same time normally loses acceleration in the process. In other words, no free lunch when it comes to pretty much anything high performnace. A gain in one area usually means loss in another.

    Back to the OT, I wanted to see if anyone had a formula to compute the actual velocity of the water exiting a given size venturi size. I am not sure how the velocity of water would be expressed... In FPM, GPM...etc..etc...


    Quote Originally Posted by 9secZO6 View Post
    It would seem to me that the gpm might go down slighly w/ smaller ring, but fps goes up for sure. But to quantify the diff. it seem u need more input info, like current fps and gpm.... I know it's no help but to just say what prop and rpm...I think we need a hydrodynamics guru. Whatcha tryin' to do? See what difference the 81mm and flux-capacitor have on the pump?

  4. #4
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    When I think of water volume and velocity, I just think THRUST. I think Land &Sea may have the formula u are seeking or something close to it.....they have a pwc dyno that measures thrust output. Just like a car dyno needs input info (tire dia. and gear ratio) so does the land and sea unit. They may even have a box for checking offf nozze dia. and prop pitch, but I don't know how they would go about measuring hp based on thrust w/o knowing the nozzle and pitch at least....

  5. #5
    Website may not be suitable for small children WOT's Avatar
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    Here's a link to a thread where some of this was discussed. http://www.greenhulk.net/forums/showthread.php?t=34479

    Water velocity is typically expressed in feet/sec or meters/sec. Mass flow rate is typically expressed as lbm/sec or kg/sec which can be converted to gal/sec if you like.

  6. #6
    Vern's Avatar
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    I agree on thrust being the focus ... In my mind, maximizing thrust is the critical focus to increase speed. But pumps are so finicky, and there is a definite relationship to intake size and exit nozzle size too. It seems like 81mm is close to optimal on our particular skis and I think that is due to intake cavity and pump size more than anything else. If you have enough HP you can sometimes gain speed by reducing nozzle size more, but jetboat race teams will also reduce the intake as well then to maximize top speed. As the boats go faster, they need less help getting water into the pump. The ultimate top speed setup seems to be a smaller nozzle AND a reduction in intake opening.

    I have often thought that if you could spend a few weeks testing, with multiple pumps, impellers, and nozzles, you could find additional speed by optimizing the critical parts and finding a great combination.

    I would tend to try to clean up the intake/ride plate/hull first, fill in holes, eliminate edges, lips, etc. Then build the engine HP, then try to optimize the pump nozzle, impeller and intake to attain the highest top speed.

  7. #7
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    Quote Originally Posted by Vern View Post
    I agree on thrust being the focus ... In my mind, maximizing thrust is the critical focus to increase speed. But pumps are so finicky, and there is a definite relationship to intake size and exit nozzle size too. It seems like 81mm is close to optimal on our particular skis and I think that is due to intake cavity and pump size more than anything else. If you have enough HP you can sometimes gain speed by reducing nozzle size more, but jetboat race teams will also reduce the intake as well then to maximize top speed. As the boats go faster, they need less help getting water into the pump. The ultimate top speed setup seems to be a smaller nozzle AND a reduction in intake opening.

    I have often thought that if you could spend a few weeks testing, with multiple pumps, impellers, and nozzles, you could find additional speed by optimizing the critical parts and finding a great combination.

    I would tend to try to clean up the intake/ride plate/hull first, fill in holes, eliminate edges, lips, etc. Then build the engine HP, then try to optimize the pump nozzle, impeller and intake to attain the highest top speed.
    I would say your mind works correctly....and speaking of intake size, I have said b4 I think this will limit the top-end of the rxpx, intake insert anyone? But orig. poster is right, when u talk bottom end and top speed, they are usually 2 diff animals......u WILL trade 1 for the other as I have never seen any set-up that could not taylored to speed or holeshot. Ya just can't do it w/o gears. When are we getting variable-pitch props? That and the wetwolf technology and we would be on to somthing

  8. #8
    Vern's Avatar
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    Agree ... and Bruce was experimenting with a variable intake grate design at one point too ... but it was difficult to design I think. The idea was a very open grate that closed up as speed went up. We all felt it would be worth 1.5 - ?? mph. I am curious if he learned anything in the process that he would be willing to share tho ...

    There is one Stg2+ guy on the forum that claimed 82.5 mph, with most of the gain coming from hull work, intake grate work/shimming. I wish I knew what he did, but there can be 3+ mph from an optimal intake grate (proven by Tommy Jordan with his modded 800 grate, which was 3 mph faster than a 1200 grate). I have some ideas on the RXP Riva intake grate, but have never touched mine ... I would be willing to throw the ideas out if anyone was interested.

  9. #9
    bttek1's Avatar
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    Some excellent points brought up. I too feel that a standardized "thrust" measurement is a lot more meaningful than to just rate an engines HP.

    For example, if one company has a more efficient pump setup, it theoratically can produce more thrust with less hp.

    Years ago, Kawasaki used to rate their watercraft in lbs/thrust but how this was measured was not devulged. If there was some kind of standard on how to measure thrust on a watercraft, this would be ideal IMO.

    Okay, so now the next question is how can a DIY type attempt to measure thrust? Anyone have ideas???


    Quote Originally Posted by WOT View Post
    Here's a link to a thread where some of this was discussed. http://www.greenhulk.net/forums/showthread.php?t=34479

    Water velocity is typically expressed in feet/sec or meters/sec. Mass flow rate is typically expressed as lbm/sec or kg/sec which can be converted to gal/sec if you like.

  10. #10
    REID2168's Avatar
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    Brian i am assuming a TAPE MEASURE WONT WORK FOR THIS????????? hahahaha just BSing bro! did u send my stuff yet? the LM1 the j-pipe bung etc etc etc... thanks let me know sorry to interrupt but i wanted to make a funny!

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