Flexible coupling

Discussion in 'Inboards' started by Nasher, Jan 11, 2015.

  1. Nasher
    Joined: Apr 2005
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    Location: Pompey, UK

    Nasher Junior Member

    The little project I'm working on at the moment involves a small inboard engine and shaft drive prop.

    I've never had the engine fitted to the boat so have no original fittings.

    I've sourced some aluminium to machine up a flexible coupling, and will be aligning the engine output shaft and propshaft really well.

    However today when looking at it I had a thought.

    The propshaft and engine output shaft are only 5/8in in diameter.
    The engine itself is very low power and torque, a 4.5HP 2-stroke, and it will drive a small 8x7 prop.

    So the thought occurred, that considering the low torque/HP, and the good alignment I'll achieve, why don't I just join the two shafts with a length of thick wall rubber hose pushed over and clamped to each shaft with a small gap between the shafts inside the hose.

    What do you think?
    Would it last very long?
    Am I being daft?

    Nasher.
     
  2. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Hmm....low power so it should work to rotate the shaft.

    What about the thrust of the prop shaft .?
     
  3. Nasher
    Joined: Apr 2005
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    Location: Pompey, UK

    Nasher Junior Member

    Hi

    There is no facility to transfer the thrust from the prop to anything other than the engine in the original design, so bearing in mind the low power I was hoping that would be handled through the hose too.

    I Have a piece of old fire hose that if I can source a new piece of might be ideal.
    It has a bore just smaller than the shaft, but has a wall thickness of nearly 1/2in with several layers of reinforcement.

    Nasher
     
  4. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The idea isn't bad at all, Dremel uses it in its 25.000 rpm "Mototool".
    With good alignment the hose should be able to handle the thrust as well, at least for some time....
     
  5. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Perhaps clamp a catcher piece of hose around the shaft aft of the coupling , to catch the shaft.

    If you were haul'n *** down the river, baseball cap on backwards, throtle to the floor ....a submarine surfaces in front ...you throw her into full reverse and you might poop your prop and shaft out the rear end.

    To compensate for thrust you might be able to insert an appropriate length , diameter rubber or other material plug into the rubber hose coupling to absorb thrust
     
  6. gdavis
    Joined: Dec 2014
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    Location: belfast,maine

    gdavis Junior Member

    Hello Nasher, well, I've tried that hose trick. I've built 4 of those Asian long shaft outboards, the first one was a 5 hp briggs. I thought the same thing your thinking so I used a piece of the best hose I could find, that stuff with the stainless mesh in it. It worked for about 5 mins, bummer. So me being me I had to try it again, of course it broke again. My next try was a steel sleeve with set screws, that worked for a while but then I noticed the shaft was starting to crack. The shaft was about 7'6" long so there was enough flexing to be a problem. Next I used a universal joint, wa la that's was the ticket and I was off down the river again. So that may work for you though the shaft does have to be pretty close to aligned. That prop sounds a little big for a 4.5 2 stroke, might it be a 4 stroke? I know most 4 stk engines can handle the thrust loads no problem. What is your engine? Make and what it came out of or what are they used for?. The last long shaft motor I built was 23hp briggs, yow, you want to see some torque. That too had a universal joint, I kept it oiled up to help prevent rust. The universals are pretty affordable at your local car parts store. Maybe out there is some sort of super duty nasa designed stronger than anything reinforced hose just waiting to be tried. Anyhow I hope this info helps ye a bit..................have fun......g
     
  7. Mikeemc
    Joined: Nov 2014
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    Location: South Carolina

    Mikeemc Junior Member

    1965 Pontiac tempest spigettie drive shaft.
     
  8. Nasher
    Joined: Apr 2005
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    Location: Pompey, UK

    Nasher Junior Member

    Thanks for the replies.

    The engine is a 1940’s British Motor Boat Co. Mariner 4.5HP Horizontally Opposed 2-stroke twin.
    The crankshaft is vertical as it’s more usually found powering their Britannia outboard, but for military inboard purposes it was fitted with a right angle gearbox which incorporates a mounting plate and cooling water pump. I’m assured it has a thrust bearing built in to transfer the load to the boat, which hopefully I’ll find when I strip it down to check it over a bit better. It has no other gearbox, or clutch, so the prop spins all the time the engine is running, like an old seagull.

    The boat it’s in is effectively a Riveted aluminium construction canoe of 18’6”.
    The surviving data on them does specify an 8x7 prop, and I have a period image showing the engine installed in the boat, but the coupling between engine and shaft is hidden under the exhaust. There is only @6in between the output shaft and the sterntube, which in the image is mainly full of stuffing box.

    This weekend I’ll take some pictures as it’s an interesting piece of kit, but should be able to find the image of the engine in the boat this evening when I get home.

    Nasher.
     
  9. Nasher
    Joined: Apr 2005
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    Location: Pompey, UK

    Nasher Junior Member

    OK, so I've managed to cobble together some images of what I'm doing using photos I took a while back, historic images I've collected through research, and blatantly stealing an image from somewhere else.

    Image1
    This is the same as the engine I have, but the gearbox is on 180 deg out.

    Image 2
    This is where it fits. Ignore the hastily made up seal on the shaft, that was made to get it watertight and see if it floated or sank.

    Image 3
    Historic image of the engine in there.
    The shaft seal/stuffing box can clearly be seen, I am going to recreate this from scratch.

    The rest of the images are of the 'Canoe' complete including a historic image of one of the same type in all it's glory.
     

    Attached Files:

    1 person likes this.
  10. gdavis
    Joined: Dec 2014
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    Location: belfast,maine

    gdavis Junior Member

    nasher, Well that's a cool project, what a great looking engine. Opposed cylinders! It does remind me of a b seagull. Where did you ever find that? Do you have the lateen sail also? So it looks like you only have room for keyed flanged couplings, Most good marine supply outfits should be able to help you with the flanges and being so small they shouldn't break the bank. Or you could try like a industrial machine parts shop. I'm not so sure aluminum would be strong enough. bronze or steel would do. I know I could get them here if you can't find them there. That transfer case definitely looks rugged to me, what an awesome rig, looks like it's going to fun......adios..g..
     
    Last edited: Jan 13, 2015

  11. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    That is unusual and cool!
     
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