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  1. #1

    Jet Drive Efficiency

    Hey guys.. Lately I've been seeing/reading a a few articles and forum posts about the efficiency of jet drive boats vs prop boats. and all seem to say the same thing that jet drives are less efficient and need more hp to compensate for around 30% power loss delivered to the jet drives... how accurate is this? and does this mean that its the same case with PWCs? is the hp produced by the engine not the true amount of power hitting the water?


  2. #2

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    Yes, it is true, jet drive (enclosed fan) systems are less effecient as the velocity of the media (water) passes thru them. actually, at very low velocities, the jet is more effecient, but as the velocity increases, the effeciency decreases exponentially. (How's them for fancy words)
    This is why it takes more and more hp to get speed from a jet.
    Someone supposiedly put a prop on a 96XP years ago and got 70mph out of it with a stock 110hp motor. I cannot verify the truth of that, but, it's beleivable. I cannot imagine anything more dangerous than a PWC with a live prop under it though.
    Robert V

  3. #3
    NJ -> MD Jet ski trip fuel mileage champ carguy89's Avatar
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    It is absolutely true. But for safety and ability to go in low water you can't beat a jet

  4. #4
    Got Boost? psycho rxp's Avatar
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    he has run over 103 with the stock GPR engine

  5. #5
    Quote Originally Posted by s/d-daddy View Post
    Yes, it is true, jet drive (enclosed fan) systems are less effecient as the velocity of the media (water) passes thru them. actually, at very low velocities, the jet is more effecient, but as the velocity increases, the effeciency decreases exponentially. (How's them for fancy words)
    This is why it takes more and more hp to get speed from a jet.
    Someone supposiedly put a prop on a 96XP years ago and got 70mph out of it with a stock 110hp motor. I cannot verify the truth of that, but, it's beleivable. I cannot imagine anything more dangerous than a PWC with a live prop under it though.
    Robert V
    So at the low end the jet delivers its power better? meaning better off the line acceleration/holeshot? and the prop is better for top speed?

  6. #6
    Moderator beerdart's Avatar
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    One of our members put a prop on a GPR and hit 103mph..

    Here is the build thread..http://www.greenhulk.net/forums/show...-Fountain-Boys




  7. #7

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    While I had a hand in this project a few years back we felt that we had reached speeds (103+ ) that were unsafe to continue with the prop/ski so we scraped it. This speed was with a gpr 1300 efi ( 165 HP) with no engine mods. This just goes to show how ineffecent a jet pump really is. Tommy Jordan

  8. #8
    Quote Originally Posted by 2fast4u View Post
    While I had a hand in this project a few years back we felt that we had reached speeds (103+ ) that were unsafe to continue with the prop/ski so we scraped it. This speed was with a gpr 1300 efi ( 165 HP) with no engine mods. This just goes to show how ineffecent a jet pump really is. Tommy Jordan
    what was the holeshot like with the prop? was it improved?

  9. #9
    mbaseball3's Avatar
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    I last read about 107mph on that prop ski. His biggest issue was a rudder that'd actually work and would last.

  10. #10
    HORSEPOWER JUNKIE 9secZO6's Avatar
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    I have thought more than once about the surface drive for the 4-tec boat....way more speed, less holeshot, LESS FUEL

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