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Thread: Ben Hur cab over (old skool)

  1. #11
    Team Member JohnsonM50's Avatar
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    Quote Originally Posted by stupidbaker57 View Post
    I've been down that road with respect to the motorcycle having a transmission that would require shifting. The Honda engine I have is from a cb400a. It has an automatic trans, low and high. I have the drive designed as low as possible in the boat but I am not using a belt. Since the engine will be sideways, the sprocket is off centered to starboard. (31 years in the navy and I think it's still the right side) Power comes thu a 5 inch jack shaft and transmits the power to the sprocket (see pic) via chain. This is where I can change ratios to speed up or slow down the prop shaft to better match HP to speed. Rough figures put this at 50 mph with a 27 hp engine, a 16/21 ratio running 7000revs with a 7X10 prop.
    If this doesn't figure right to you, it might be that my figures on the cocktail napkin got blury when I spilled some beer.
    In terms of what your working with seem to be within reason & workable. It will be an interesting moment when it up-shifts. I was thinking 2 stroke at 1st but guess this is a 4?

  2. #12
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    I've been thinking, and that alone could be dangerous, about the shaft angle. Even tho I measured and checked as many points as possible, there could be a chance that the shaft angle may need (may?) some change after a test flight. So be it.
    This is where it gets scarey. I have designed (cocktail napkin again) a trimable drive without U joints. Seems there isn't much room in the back for them.
    My method is the pivot the entire drive, engine and shaft to get the right angle while underway.
    The hole in the transom would only need to be 2 inches in diameter and slightly elongated up & down.
    The pictures may help. I first built a mock up from wood, then went the steel route. The shaft will be stabalized by a 4 bar system with heim joints.DSCF0005.JPG

    DSCF0006.JPG

    DSCF0007.JPG

    DSCF0002.JPG

  3. #13
    Team Member JohnsonM50's Avatar
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    The Ben Hur as a racer would have the prop a certain depth, angle & distance back. Not sure exactly what those #'s would be but guess you would want to be close. Considering the Honda & drive to be possibly lighter & its core weight farther forward Id guess you would want less aft lift. This can of course be compensated for by driver position & prop choice [bow lift prop]. Im pretty sure you'll need the thru transom point to be right on he bottom to get 'ballpark' on drive or maybe since it has to enter the water as a shaft before prop anyway even a little lower might help. That way when you adjust the angle lowering the motor to 1-3 degrees of prop shaft/bottom you wont be raising the prop too high to work. Im not trying to be a nay-sayer, just sayin that even tho its different being near what the established setup would be is probably going to be close. Then your move the whole thing plan might be able to bring it right where it should be considering the pivot being at the transom, the hole shouldnt have t be too large either. Good Luck with further planning.

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    The centerline of the shaft can't be any lower than 2 1/4 inches from the bottom. Reason being is the sprocket and chain are there.
    I'm now working on a way to raise and lower the engine cradle (max 5 inches total) electricly. I don't want hydrolics because of weight. I opened the hood on my Stude with a jack screw and a power window motor. Maybe I'll have to go that way.
    Shifting will be electric. I'll use a door lock solinoid. (push pull type)

  5. #15
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    Here's a couple of pictures of the final drive less the rudder. The trim action works smoothly with no binding.
    Next is the mounting of the engine to the cradle and installing the jack shaft. I have everything for this so I should be able to get it done in about a week or so. DaveDSCF0008.JPG

    DSCF0009.JPG

  6. #16
    Administrator Ron Hill's Avatar
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    Default Looks good: Make sure

    Make sure those transom pieces are strong, might brace them with some aluminum angles, inside.... You will be pulling hard on those two transom piece make sure they are braced. I've seen rubbers pull out of the transom and the results ain't pretty...

    What horsepower and gear ratio are you using?????

    Do you have a pilot's license?

  7. #17
    Team Member JohnsonM50's Avatar
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    It looks cool, I agree, these mount area's need to be hard points. One advantage it might have if you can get it to easily adjust on the fly for routine driving is lowering the prop to get going & raising it to cruise.

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    This drive will be trimmable on the fly. The entire engine and drive pivot at the transom. There is a total of 4 1/2 inches of travel for the prop. The inside has been braced for the extra torque that will be present. The engine is only a 27 HP 2 cylinder 4 stroke and the ratio is 16/21 like an outboard.
    Ron, How's that experimental outboard doing? I was thinking of laying a six cylinder merc on its' side and useing it as an inboard. Dave

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    That's a really odd linkage. I guess it will work, so long as the motor and torque tube aren't rigidly attached to the cradle (pivot point) inside the boat. If they were, the whole thing would bind up and the forces in the outboard links will get huge. Just seems that you've got to have some flexibility right where you may not want it. You have to support the motor but let that support be flexible or it's going to bind up. I'm curious to see how you are going to work out the rest of it.

  10. #20
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    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

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