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Thread: Ben Hur cab over (old skool)

  1. #21
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    Quote Originally Posted by stupidbaker57 View Post
    Imagine this,,,,,, A fat kid on a seesaw, (engine) and a little girl (prop). My engine will be mounted on the cradle. The prop tube is also mounted on the cradle inside the boat. To submerge the prop deeper in the water, the fat kid goes up. I'll be raising it up and down via a motorized jack screw. So far this project has cost less than $500. (not counting the 2 Ron Hill props and beers) Dave
    You are missing what I am trying to say.

    I already understood that you were going to attach the torque tube to the cradle, but when you attach your torque tube to to the cradle the linkage will bind up because you have two extra links that make the linkage rigid.

    Correct me if I am wrong, but I beleive you are planning to put the pivot point of your cradle near the bottom of what was the motor board inside the hull.

    If the torque tube is attached to the cradle with a rigid mount, then the shaft must pivot about the inner pivot point, near the bottom of what was the motor board. If the two links that go forward to the transom aren't in line with that inner pivot point the linkage becomes rigid. It can't move without putting high forces in the links. In a car suspension there are ball joints that allow angular movement of the upright, but your upright is rigid, it is welded to the torque tube, so no angular motion is present in that section.

    Right now your linkage will move, only because the torque tube isn't firmly attached to the engine cradle. If you attach the engine to the cradle and leave the torque tube free, you will need a coupling that will allow the shaft to move relative to the engine. You will need both axial, radial and angular misalignment capability in the coupling to make it work. If the engine is rigidly mounted to the shaft, then you will need to have some type of mount that will allow it to move relative to the cradle. Neither is a good solution.

    You should probably go through the range of motion with a scale drawing to see what is moving where, but right now the kinematics are such that it will bind up unless there is a lot of slop in the joints. You have Heim joints in the links now and that doesn't allow for any slop.

    In order to make it work the transom mount links need to have their pivot axis on the same line as the cradle pivot. If they don't you have a static system and it won't move unless something bends or breaks.

    I would simply get rid of the transom links and design a structure that holds the torque tube without them. Keep your pivot to one axis and it will work. Otherwise you are just going to have problems.

  2. #22
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    Everything in the picture is tight, shaft tube and all. The cradle assembly moves freely from the lower starting off angle all the way thru to the full speed trimmed out position with no binding any where. I understand triangleation where as pivot points must line up or binding will occur.
    I might be building this in a garage and using a drill press as a bridgeport, and a radial arm saw as a surface planer, with a cocktail napkin for a drafting table, but I think I have done what I planned to do without any trouble.

  3. #23
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    Quote Originally Posted by stupidbaker57 View Post
    Everything in the picture is tight, shaft tube and all. The cradle assembly moves freely from the lower starting off angle all the way thru to the full speed trimmed out position with no binding any where. I understand triangleation where as pivot points must line up or binding will occur.
    I might be building this in a garage and using a drill press as a bridgeport, and a radial arm saw as a surface planer, with a cocktail napkin for a drafting table, but I think I have done what I planned to do without any trouble.
    If your holes are loose enough and the range of motion is small, and the mounts aren't too rigid, you can get away with it. More power to ya!

  4. #24
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    I finally got the jack shaft parts from the machine shop . They've been there for over 2 months. In that time I've built 2 rowboats and sold them, one "cocktail racer" (for sale) and got a 10" Atlas lathe from a friend for free and rehabed it to working condition. Now I can do my own lathe work.
    My steering is going to be "push pull" useing PTO cables conected to the steering wheel.
    Now that I'm getting near the end of this monster, I have another plan in my head for the jet ski engine I have, but first I must draft it out on a cocktail napkin.
    Those who say it can't be done should stop bothering those that are doing it.

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