Shaft coupling

Discussion in 'DIY Marinizing' started by Rangerspeedboat, Jun 14, 2009.

  1. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    I'm building a boat thats going to use a 4 hp vertical shaft lawn mower engine as its power source.

    I'm going to make an inboard by mounting a mercury 4.5 lower unit to the bottom of the boat with the shaft running up and connecting to the motor. I have setup a test mounting board that has the same demensions as the one in the boat.

    My question is how do I connect the shafts together? I have seen many options on this forum, and many couplers on McMaster Carr
    http://www.mcmaster.com/#shaft-couplings/=2bf1v1

    The engine has a 1" shaft and the lower unit has a 7/16" shaft.

    I have thought about many couplings, from spiders to chains but heres an idea that I just came up with. the engine has a threaded hole inside the shaft. I could drill that out to 7/16" and just slip the lower unit shaft inside. This would be the simplest and cheapest way, I would just have to drill a Dead Straight Hole.

    What do you more experienced guys think about this?

    Here are some pics
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    Heres one of the boat.Its not quite done yet, this is an old pic it now has the bottom skin on and 1 side skin.
    [​IMG]

    I think a spider coupling would work great, but I want some input on the idea.

    BTW the motor doesnt run, yet It has a broken head bolt and needs a cast iron flywheel. Flywheel is on ebay, head bolt fixed with an EZ out.
     
  2. mark775

    mark775 Guest

    I think that the shaft would break at that point. I also think I don't like your boat, Sorry. You can get plans pretty cheap, these days. Good luck.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Any attempt to couple the two shafts directly will end in disaster because both the engine and the Merc. have their own bearings but are not part of the same object. Even if you have a machine shop available with high precision tools you would not succeed.
    The only way to do this is with a flexible coupling like a rubber disc, a reinforced hose or an universal joint.
     
  4. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    Ok so a direct coupling is no good. I need something that will have some "play" and allow misalignment. I believe that a flexible spider shaft coupling McMaster Carr #6408K14 would work. This style of coupling allows angular and parallel misalignment.

    mark775-Whats wrong with my boat?
     
  5. gonzo
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    gonzo Senior Member

    A belt drive that multiplies the rpm may work. A lawnmower engine doesn't turn as high as an outboard
     
  6. gonzo
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    gonzo Senior Member

    You will also have to be very careful not to run aground and when you pull the boat out of the water
     
  7. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    Thanks gonzo,
    Belt drive would be a little hard, its already difficult, but by removing the govenor on the engine I can get close to the RPM of an outboard.

    As for running aground, I'm going to be in a bay or a deep bayou. I'll have a trailer that has the boat high enough where the lower unit doesnt hit the ground, when the boat floats off the trailer, I know that there is enough water.

    I'm worried mainly about the torque from the 1" shaft traveling to the 7/16" shaft.

    Any other ideas, anyone?
     
  8. mudman
    Joined: Mar 2007
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    mudman Junior Member

    Would a lovejoy work? Would it work like a flex plate?
     
  9. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    :confused:

    Please explain what your talking about, I'll google it though ;)

    Edit: is this what your talking about?
    [​IMG]

    Thats what I mean by spider coupling.
     
  10. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    A spider coupling might work, if you can find one that's fairly tolerant of misalignment.
    Any sort of direct connection will need nearly rigid engine mounts, though, to keep it in line. If you want soft mounts, you'd need a coupling that's much more tolerant than most spiders. Like a flexible hose, or a belt drive, or maybe a couple of CV joints off an old front-drive car.
     
  11. Rangerspeedboat
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    Rangerspeedboat Senior Member

    I had planned on using a rubber washer to reduce vibration, its not very thick maybe 1/16". I think that would reduce the vibration "some" yet still keep the engine rigid enough.
     
  12. mudman
    Joined: Mar 2007
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    mudman Junior Member

    Sorry, spider coupling, lovejoy, same thing. I just always called it a lovejoy.

    Anyway, a guy down here builds a boat with a "mud unit" that uses a 100 hp car engine. He refuses to use flexible motor mounts, and instead keeps it rigid with steel supports bolted directly to the boat. He claims that the flexible mounts cause mialignment in his design.

    He's using 100 hp engines, so 10 hp shouldn't be a problem using rigid mounts. Although he is using 4 cylinder engines....much smoother than your 1 cyl. Beef it up, and make sure the hull can handle the vibrations too.

    We use the lovejoy's like a flexplate on home built generators. This is because every time a piston fires, power is transfered to the crank. So power at the shaft actually pulses in an internal combustion engine.
     
  13. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    Thanks mudman.

    I decided to go with a 4 hp engine instead of the 10hp, mainly because I got a lower unit from a 4.5 really cheap. I can hop up the engine, add on some parts for more HP.

    If this works then I will hunt down a 9.9 lower unit and use my 10 horse briggs.
     

  14. Bob Eames
    Joined: Sep 2008
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    Bob Eames Junior Member

    ANother way to couple these together would be to use a "rag" joint from a late mode pickup steering column. It will take the force and the misalignment, and should hold out for a pretty good service life.

    Good luck!

    Bob
     
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