40" jackshaft on twin sterndrives

Discussion in 'Sterndrives' started by cudashark, Aug 19, 2007.

  1. cudashark
    Joined: Jul 2006
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    Location: South Florida

    cudashark Senior Member

    Hello Fourm,

    I am building a 33' cold molded runabout. To achieve the optimum weight I need to jackshaft the stern drives about 40".

    I have been hearing rumors at the boatyard that "jackshafts" cause excessive vibrations.

    So far no one can tell me of this who has had direct experience with jackshafts.

    Please if anyone out their in the big cybersea has had experience on this issue PLEASE respond.

    Thanks

    Ray
     
  2. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Jackshaft don't cause problems. Improper installations cause vibrations. Can you describe setup better.
     
  3. Frosty

    Frosty Previous Member

    mydauphin is correct why should it vibrate? A car has a prop shaft thats up to 6 feet!! it does'nt vibrate even with a flexible engine and the axle able to go up and down a foot. Its all in the installation.
     
  4. cudashark
    Joined: Jul 2006
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    cudashark Senior Member

    Thanks for your reply mydaupin and Frosty.

    Your setiments are mine exactly. I have been told that if the angle is more than 6 degrees then their will more than likley be vibrations.

    I have a hard time believing the rumers.

    Thanks again...

    Ray
     
  5. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    I am updating my boat presently from one-piece solid shafts to thrust-bearing with a jackshaft to cut down on alignment, noise and vibration issues. All my friend that are boat mechanics told me jackshaft system was going to be a problem same as you. But truth is it easier because alignment is less critical. That said the less angle the better, also make sure that your torque ratings for jackshafts are up to spec. I have a couple of jackshaft from trs drives, but I dont know who sells new ones. I will be installing a python-drive on mine. My cousin is distributor and getting me a break.
     
  6. Frosty

    Frosty Previous Member

    You must use 2 universal joints and make sure the yokes are opposing and not aligned. Put the UJ's at each end and incorperate a spline.

    Have the shfts made professionaly on a lathe. I have balanced them myself with jubilee clips or 2 sometimes by trial and error.

    I dont like CV joints for transmitting torque constantly in the same position. UJ's will be stronger and last, come in all sizes and are cheaper.
     
  7. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Greater angles, speed, and length exaggerate imbalance. The reason for the aforementioned U-joints at each end of the shaft is that all such shafts accellerate and decellerate during each revolution. The speed variation is caused by the first U-joint's geometry. The correction for the accelleration and decelleration is done by the second U-joint. Hence the angles to input and output at each end should be as nearly the same as possible.

    Alan
     
  8. Pericles
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    Pericles Senior Member


  9. cudashark
    Joined: Jul 2006
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    Location: South Florida

    cudashark Senior Member

    That is just what the Dr. Ordered Pericles.

    Thanks again for your advice..I'm contacting them in the morning.

    Ray
     
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