Prop shaft problems

Discussion in 'Mechanic's Corner' started by Stacey, Nov 7, 2021.

  1. Stacey

    Stacey Member

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    Last time we tried taking the boat out (26' Macinaw) the prop shaft fell out of the transmission coupler.

    A few rusty set bolts (is rustoleum rust disolver ok for boats??) later, + sanding the deformed edge of the shaft, + a new key that I'm sure is the wrong material (where can you buy keys from and should they be stainless??) We were able to push it back together...

    Now, having little to no idea what we're doing, I'm wondering how to know if we've done it right? Any opinions if this sounds right (video link below)?
    Most importantly, is it really just 2 set screws that hold the shaft in? There's still play turning the shaft back and forth before it engages the key, I can't tell if the set screws are holding on to anything..

    Also should the shaft be put in further? Idk how to add pics here but you can still see part of the key. It looks like we got it to where it was before though.

    https://youtube.com/shorts/b4bap7Evp3c?feature=share
     
  2. Stacey

    Stacey Member

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    Picture
     

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  3. Ship

    Ship Well-Known Member Supporting Captain

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    First and foremost there should be no Play between the coupler and the shaft.Bronze key stock is fine. It should be probably quarter inch key stock. Where the two set screws contact the shaft, the shaft should be dimpled to accept the heads of the set screws.
     
  4. Ship

    Ship Well-Known Member Supporting Captain

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    The shaft should be all the way in the coupler. So it's smooth to the front side. Btw a new coupler is about $55.00.
     
  5. Stacey

    Stacey Member

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    Ok but where do I get bronze key stock?

    Ace only had hardened steel, other hardware stores don't carry the size I need (1/4 ain't it, see pic of old key next to 1/4)

    No marine stores had key stock..

    Coupler seems fine just maybe need to get the set bolts in further?
    I didn't want to also learn how to replace the coupler lol.. 5 more rusted bolts and took us 4 days to get out the 2... are there seals or anything besides just unbolt / bolt on new??
     

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  6. Gary S

    Gary S Well-Known Member

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    A coupler is also a press fit to the shaft, something like .005 interference if I remember right. Usually just heat it up in an oven or bbq to help get it on to the shaft. You also will have to look into why it came loose-normally caused by engine alignment being off. Where are you located?
     
    Last edited: Nov 7, 2021
  7. Ship

    Ship Well-Known Member Supporting Captain

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    The press fit can be done with a slight bevel on shaft and a dead blow hammer but the coupling needs to be disconnected from the tranny. To remove the 4 bolts that connect it, just grab with vice grips or pipe wrenches and twist the bolts until they shear off. Or grind them off with a grinder/cut off blade. The key stock can be had on line at most marine parts stores.
    https://shop.hamiltonmarine.com/ine...?mode=searchstore&search[searchfor]=key+stock
     
  8. crhodes

    crhodes Well-Known Member

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    Want to emphasize what Ship said in post #3. "Where the two set screws contact the shaft, the shaft should be dimpled to accept the heads of the set screws". You may need to slide the coupling up or down the shaft to align the dimples with the set screws holes. (Look through the threaded set screw holes when sliding the coupling.)

    In 1975 was working on a sailboat that the shaft had been replaced. They forgot to drill set screw dimples. Somehow I was able to drill set screw holes by had without messing up the threads but don't know how... Would not recommend drilling with coupling on. Mention this as it is possible you shaft may not have dimples (but it should).

    Google "1/4 brass keystock" and you will find several sources if you don't have a local marine or industrial supply house.
     
    Last edited: Nov 7, 2021
  9. Mistress

    Mistress Well-Known Member Staff Member Supporting Captain

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    That isn't in all the way. Shouldn't be that much if any key slot visible.

    Keystock should be bronze or stainless. Brass isn't happy in salt water.

    You have a Globe drive saver. I'm wondering if someone shortened the shaft to accommodate the drive saver and didn't re-dimple the shaft.

    The press fit is very minor. Like 0.0005". An alternative is a split coupler.

    Ideally the coupler is faced on a shaft to provide parallelism. I've done it both ways and haven't found it all the sensitive in our applications.

    Those aren't the stock set screws. Mfg used square headed set screws that were drilled and safety-wired so they can't back out.

    If your shaft isn't dimpled, it's pretty simple to add them. Just use the tap drill bit for the screw size. The female threads will pilot the bit with a bit of care. Set screws have a bit of a cup that bite into pilot drill size.

    It's kinda "funny". Many here have had extreme difficulty removing the coupler and had to cut the coupler off. First I've heard of one removing itself. That tells me something wasn't done right by someone in it's past.

    Maybe this will help:

    https://stevedmarineconsulting.com/wp-content/uploads/2019/06/ShaftCouplings179FINAL-PBB.pdf
     
    Last edited: Nov 8, 2021
  10. Ship

    Ship Well-Known Member Supporting Captain

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    I noticed from Mistresses post a possible drive saver. IF the bolts do not go all the way through the drive saver, DO NOT shear them off ! As recommended earlier. Normally the couplers are through bolted with a head and nut. Some drive savers have only bolts that are threaded into one side and are two sided with the bolts offset from the opposite side.
     

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