View Full Version : Any reason not to do this? Prop shaft coupler.

02-27-2011, 03:23 PM
As usual, my ongoing, never-ending work on my sailboat consists of one step forward, half a step back.

So yesterday I very easily - almost too easily - pulled the prop off the shaft, pulled the shaft and stuffing box out of the stern tube and pulled out the cutless bearing. Easy peasy.

So today I took the stuffing box apart and went at taking the prop shaft coupling off the end of the shaft, so I can clean it all up (it was pretty rusty).

The square-head set screw on the side of the coupling was so rusty it snapped off when I twisted it with a wrench.

I drilled out the center of the set screw, so that I would at least be able to get the coupling off the shaft.

That coupling was STUCK. Long story short, I used up probably half a cylinder of MAPP gas heating it up, half a can of Kroil, and I managed to snap off one of the arms of one of my gear pullers (well, it is a cheapo Taiwanese one from Horror Freight, so no big surprise there - and no big loss either).

Once I had the coupling off, I went to use the next size drill to slightly enlarge the hole I had drilled through the broken set screw, so I could extract the remains of the screw and clean up the threads in the hole. The drill, of course, snapped off in the hole. And, of course, it's a cobalt drill.

Further efforts at removing the firmly wedged, broken off tip of the cobalt drill resulted in another snapped-off tip of a smaller cobalt drill being lodged firmly right beside it.

Yes, go ahead, I hear the snickering now.


I'm thinking the hell with it, I'll just leave them in there, and drill a new hole, 90 degrees away from the old one, tap it and stick a new set screw in there.

Any reason why not to do that and buy a new prop shaft coupler instead? I've found a couple sources for them, but I'd just rather not spend another $70 if I don't have to.

Bob Adams
02-27-2011, 03:55 PM
Not that it matters in my mind, was the screw over the key? Anyway, I'd put a little epoxy over the boo boo, fair it and paint it, and go ahead and put a hole on the other side. BTW, I usually dimple the shaft where the set screw goes so you don't have to tighten the bejebbers out of it.

02-27-2011, 04:06 PM
Bob - the set screw is 90 degrees away from the key. And yes, there is a dimple in the shaft where it currently is. I figured I'd make a new dimple in the new location, and put a little threadlock on the set screw for extra measure.

I just couldn't think of any negative aspect of doing this and wanted to check, since there are so many people here who know so much more than I do about boat mechanics - you never know; for all I know someone would write back, "NO!! Don't do that!! Under no circumstances must you ever do that and here's why!!!" or something...


Roger Long
02-27-2011, 04:38 PM
Re-using a coupling is a bad idea, especially in the condition it sounds like that one is in. Good thread recently on shaft couplings at SBO by one of their resident experts:


02-27-2011, 04:57 PM
Thanks for the link, Roger!

Okee dokee, so I'll go ahead and spring for a new one.

Next question is finding a good shop that knows how to do the fitting and facing properly...

02-27-2011, 05:13 PM
if you can put the coupler in a vice clamped to the bed of a drill press take a carbide drill and break the flutes off so you are left with a carbide shank that is tap drill size or a little smaller

clamp it so the carbide shank will go straight into the center of the hole and run the drill press as fast as you can and plunge the carbide shank in with pressure

it will turn cherry red and melt its way through

this can be done with broken taps too

John B
02-27-2011, 05:25 PM
You must have the engine out to be able to pull the shaft with the coupling on it .
If you don't fix the coupling issue now ,then once everything is installed , you won't be able to remove it to work on the stuffing box if you need to. It should all be assembled with whatever the local anti seizing grease goo is so you can do whatever is necessary in different situations. ( its called Rescue steel here)

For example, I carry a spare dripless seal because its a rubber bellows type which potentially could get damaged in an odd or extreme situation. Because I know I can remove the coupling while the boat is in the water, I know I can swap that out if I really had to.
another example: I blew my gearbox last year. To remove it and reinstall it in the water , I needed to drop the coupling to get clearance.

02-27-2011, 05:30 PM
That would drive me nuts to see each time I looked :) Look for a tap removal place , they can burn it pretty cheap . One piece of advise to prevent drills from grabbing and braking . Take your drill and grind a little flat on the leading edge parallel with the center line of the drill . Still cuts good , but stops it from pulling or screwing into the part .

02-27-2011, 05:35 PM
That would drive me nuts to see each time I looked :) Look for a tap removal place , they can burn it pretty cheap . One piece of advise to prevent drills from grabbing and braking . Take your drill and grind a little flat on the leading edge parallel with the center line of the drill . Still cuts good , but stops it from pulling or screwing into the part .

you can take the keenness of by rubbing the cutting edge on a stone, that will keep it from grabbing

or you can use the down stop to limit the down travel in steps

wizbang 13
02-27-2011, 05:49 PM
"split coupling" is what you may need. I did.

02-27-2011, 05:53 PM
Based on Roger's link above and some other sites and went and read after seeing Roger's info, I'm just going to buy a new coupling rather than mess around with the old rusty one. Seems the safer bet and they're not horribly expensive. What the heck, it's only money!! And I'm spending a whole mess of it on the boat this Spring, so what's another $60-70?

wizbang 13
02-27-2011, 06:25 PM
A split costs more n that.

02-27-2011, 07:53 PM
Here's what I would need. (http://www.getaprop.com/content-product_info/product_id-7690/atomic_four_solid_coupling_3_410.html) $70.

I found the same one elsewhere a few bucks cheaper, but now I'm not finding it again. Gotta keep Googling...

02-27-2011, 09:47 PM
Whatcha got, a sailboat or powerboat?....I sailed a long, long way with no engine. Made for fewer holes in the boat.

02-28-2011, 07:55 AM
Yeah, it's a sailboat, but I'm not ready to go quite that purist yet.

02-28-2011, 10:28 AM
Here's the coupler I used on my boat when I replaced the engine. Of course I had to redo just about everything.


This cost around $200 or so. It's easier to install and remove than the heat shrink flange you had. However, I still had shaft vibration problems. I redid the alignment and it seemed to be within tolerance with a feeler guage (usually 0.001 inch per inch of flange diameter). But I got out on the lake and ran at various speeds. I had to adjust the front motor mounts to lower the front of the engine ever so slightly until the vibrations disappeared.

My impression with this unit is that it doesn't require turning in a lathe at a machine shop to flatten the face. But then again I also got a new shaft to go with it.

wizbang 13
02-28-2011, 11:35 AM
That's the type of split I'm talking about. It won't get stuck on the shaft.

Bob Adams
02-28-2011, 12:43 PM
Replace it if you want, can't hurt. I still say it would be fine.

02-28-2011, 01:08 PM
Yeah, I've done more research last night and today and have concluded it really is the preferred and most advisable course of action to replace it. From what I've read, it does seem there is a valid and good reason for the ABYC standard requiring a press fit and then facing the flange on a lathe.

For the $100 or so bucks it will cost, it seems well worth it, rather than trying to make the old one work and having the end of the shaft snap off out in the Bay or something.