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  1. #1

    Need Help Custom 320+HP Project

    Hi Guys,
    I am new to the forum but I see that there are many knowlegeable people on here and I could use some help. I currently have a 1999 Seadoo Speedster SK, I blew up one of the motors last fall so my buddy and myself came up with the idea of putting a pair of CBR 1000RR Street Bike motors in it, they are 160+ HP each. I am a mechanical engineer and I did some calculations and the stock driveshafts arent big enough to handle the torque.

    I am looking at how I can either get a 140mm impeller with a bigger splined bore and machine new driveshafts myself, or would other jet pump housings bolt up with a bigger impeller and bigger splined bore? I calculated the shaft size based on max torque and I will need around a 1 inch shaft or larger. All the help will be greatly appreciated. Thank you all for your time!


  2. #2
    Hydrotoys's Avatar
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    Considering there are watercraft running more torque and HP with smaller shafts, I'm not sure I'd agree with your calculations... but hey, engineers LOVE to be right.

    You could swap in some Seadoo RXP pumps and go with those shafts. They run 215hp straight from SD. I sure hope those thousands of watercraft running these tiny shafts don't all twist up and explode!

    Welcome to GH.

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by ride_hyperlite View Post
    ...I did some calculations and the stock driveshafts aren't big enough to handle the torque.

    ...I calculated the shaft size based on max torque and I will need around a 1 inch shaft or larger...
    How did you come up with that result?

    There are lots of watercraft running 160HP or more. A watercraft impeller is different from a road vehicle with a gearbox, and the impeller doesn't grip the water the same way a road tire grips the road and limits engine RPM rise.

  4. #4
    Yes, well when you factor in a gear box, it is putting out well over 200 ft lbs of torque at over 8000 rpm. So it is a bit of a concern with a driveshaft that is only 0.600 inch diameter, even if it is heat treated stainless shafting. I do agree with the impeller just spinning in water, but then why do the 215 hp rxp motors run with a 28mm drive shaft not a 15mm lighter weight drive shaft?

  5. #5
    Chrisrokc's Avatar
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    Quote Originally Posted by ride_hyperlite View Post
    Yes, well when you factor in a gear box, it is putting out well over 200 ft lbs of torque at over 8000 rpm. So it is a bit of a concern with a driveshaft that is only 0.600 inch diameter, even if it is heat treated stainless shafting. I do agree with the impeller just spinning in water, but then why do the 215 hp rxp motors run with a 28mm drive shaft not a 15mm lighter weight drive shaft?
    I'm not sure what some of these RXPs and GPRXPs are running torque wise at that rpm, but I know some are making 300+ 400+ horsepower. I do not believe they have a driveshaft problem.

  6. #6
    Norwegian Wiking Trond's Avatar
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    The 215hp shaft isn't 28mm

    It's a lot less where the shaft enters the engine... I think it is 22-23mm next to the splines.

  7. #7
    Hydrotoys's Avatar
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    If you are pushing a boat, the torque "APPLIED" actually increases over a ski, since it has something to actually pull against. But then again, once it overcomes the props, you are putting no more torque on it, than any ski. Most torque is down low in the rpm band "applied" with the ski/boat not being on plane. A boat, in the long run, can be easier on shafts, due to a CRAPLOAD less loading/unloading.

  8. #8
    Is anyone out there running over 160hp on those tiny .600 inch drive shafts that come stock with the 720 engines? or the 140mm pumps? I guess I will try it and see if they break even though this is not the ideal way to go and its gonna cost quite a bit of machining and customizing.

  9. #9
    Hydrotoys's Avatar
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    Call Ted Thompson. Spline and gearing is what he does. He custom builds driveshaft for many skis. He can do it for you, and/or answer all your questions. He helps me out whenever I get some crazy idea in my head.

    Don't hesitate to call back during business hours. I think he leave early on Fridays.
    http://www.greenhulk.net/forums/showthread.php?t=42454

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