When your shaft collar/coupler start slipping


Last year I got stuck in some serious sea weed and the outgoing shaft from the gearbox slipped in the shaft collar. Since the. I don’t have 100% surface contact between coupler and shaft.

It’s the shaft on the gearbox that’s “scared” and I can’t take it out. My plan is to drill a hole through the coupler and into to the shaft, can remember is there’s a groove in the shaft already, and place a locking screw in there.

Any other/better ideas?

I just verified that I have a neugart with keyway on the output shaft. I could use that to lock the screw.

Or use a coupler and shaft with a key slot, the you won’t have slip there. To prevent damage to the gearbox in case of something blocking the prop, a shear bolt in the propeller could be used as the weakest link. Was thinking of that already, I just don’t know from what material to build it. Should be strong enough for normal operation and break if the prop gets blocked.

Hi! Use a shear pin from a bow thruster. Works perfect. I even have the prop from the bow thruster as backup.

Thanks, how do you know it works? I mean I have a 3mm stainless steel shear pin to link my alu prop with the shaft. The question is whether the pin or the gearbox is weaker, should I block the prop at speed (with seaweed, wood or whatever obstacle). Maybe a brass pin would be better. Need to find out how to calculate the needed material strength. I also need to figure max shear force under normal operation and add a safety margin, maybe multiply by 2 or 3.

It has to be shear pin material I think. Normal stainless is to soft and won’t break. Very hard to tell. From my experience the coupler is the weak link.

What you could do is to make the prop shaft with a few small splines so that it will have less grip to the coupler compared to the shaft coming from the gearbox. If the prop get stuck you might have to replace the prop and shaft buts that nothing compared to rebuilding the entire unit.

Maybe a hard polymer shear pin could pe used… I don’t know if it’s brittle enough though

yes or brass, that seems the material used for smaller outboard motors to save the gearbox when touching the ground with the prop.

loctite 648 should do the job

Not very scientific but I used a cheap Chinese M3 stainless(304?)machine screw as sheer pin with the neugart 40 gearbox. It snapped in three pieses with seeweeds in the prop. It saved my gearbox😊, but had to swim back to shore.
Would also recommend loctite 648 if a keyed shaft is not an option.

Thanks for your suggestions, I made some from a 3mm brass rod and M3 brass threaded rod. I’ll start with the weaker one which would be the threaded one and see if it holds normal operation.

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I tested both, started with the threaded rod, which was too weak and eventually broke under normal use. I replaced it with the 3mm brass rod and this one seems to hold the shear force.