Jet boat gear box?
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Jet boat gear box?

  1. #1
    Senior Member fred hammett's Avatar
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    Default Jet boat gear box?

    My buddy is not a jet boat guy, but was telling me about a 2 speed gear box that goes inline with the driveline to the pump!!! Anyone hear of these and are these worth a shit???

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    Fred look up the Worlds fastest water car, look for the Python. Thats what you can use your gear box for.

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    21 Daytona Outlaw's Avatar
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    A gear vendor?



    Sent from my iPhone
    #55

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    Are you talking about like what the boat " HEAD OVER HEELS " Has in it ?.

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    Senior Member fred hammett's Avatar
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    Default 2 speeds hmmm???

    When it was talked about I thought a 2 speed trans of some sort for a jet boat sounded pretty cool, but have never seen such a thing and thought somebody could shed some light!!!

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    Senior Member H20MOFO's Avatar
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    I had the same idea back when hot boat was around. What they r gonna tell ya is that by shifting and dropping your r's....you are gonna be slower because your motor makes less hp at 4k as opposed to 5k...or what ever. Your mileage may vary.
    Another Hot Boat refugee

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    Just Me snoc653's Avatar
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    Quote Originally Posted by H20MOFO View Post
    I had the same idea back when hot boat was around. What they r gonna tell ya is that by shifting and dropping your r's....you are gonna be slower because your motor makes less hp at 4k as opposed to 5k...or what ever. Your mileage may vary.
    This should play back into the does it take HP or TQ to make a jet boat fast. Obviously you have to have one to get the other. But if it takes TQ to get the pump turning, the reduction in RPMs might put the engine back into the peak of the TQ curve and allow it climb back up to the HP peak. It would be an interesting experiment but the engine would have to be built for the split to capitalize on any benifits available.
    So many projects, so little time

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    Senior Member H20MOFO's Avatar
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    If you already have the proper cut impy in you boat....one that is perfect for your power curve. Then add a "over drive"....you will slow down....period. Maybe you could put way to small of an impy in.....and come close....but why would you want to.
    Another Hot Boat refugee

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    Or use the right impeller for top speed runs, then have a of for cruising.

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    Senior Member Boat 405's Avatar
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    Opposite..... spin a huge impeller at a slower rpm than the motor letting the motor turn where ever it makes peak power......
    Boat 405.

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    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by widowmaker View Post
    Opposite..... spin a huge impeller at a slower rpm than the motor letting the motor turn where ever it makes peak power......
    Good Info here !

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    I think Widowmaker is on the right track here. Run a big impeller for max efficiency at cruise. Then switch gears (underdrive the pump) to get the engine in the correct RPM range for max power absorption and boat speed.

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    "Need For Speed" Gearhead's Avatar
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    What Widowmaker is talking about is the way the Head Over Heels boat runs the gearbox. I have one of the gearboxes from Paul Prange as we ran it in the Pro Gas boat of Robbie Barrett a few years ago.

    With the HP development that has taken place in NHRA Pro Stock, these engines are producing a high horsepower at high RPM. The RPM is many times in excess of 9000 RPM. We cannot get an impeller to have any efficiency when cut to the point it would turn 8500 to 10,000 RPM. However, what can be done is to use an efficient impeller size like an "A" then use an underdrive planetary like an Owens unit. this will allow the engine to be turned in the RPM of the engine peak efficiency, while spinning an impeller in a peak efficiency RPM. For an example, we were running Robbie's engine at approx 8500, but turning the "A" impeller around 7000 RPM.

    A single planetary drive, not a gear change operation.

    Gear

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    My misunderstanding.

    So, do does anyone know if some backyard genius ever rigged anything similar to my misunderstood configuration (big impeller/2-speed box)?

    It would (obviously?) have to be pretty compact & lightweight...

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