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Propeller Shaft Spinning #17442
09/04/17 03:26 PM
09/04/17 03:26 PM
Joined: Mar 2017
Posts: 27
Annapolis
JPM Offline OP
Senior Member
JPM  Offline OP
Senior Member
Joined: Mar 2017
Posts: 27
Annapolis
So, I replaced an engine a week or so ago. The new (used but new to the boat) engine runs just fine. I need to make a few tweaks to idle speed, etc but I was able to motor out to the race course this weekend. However, on the way back in I experienced a vibration that was similar to a prop that wouldn't open. I had the main up so I had the crew go head to wind and I went over the back on the swim ladder and found that the prop would open easily. When I checked the shaft at the coupler, I noticed that the shaft would spin freely within the coupler and there were small metal filings on the sole of the boat beneath the coupling.

I am new to owning a boat with an inboard engine (previous boat was a J24) so I'm a bit out of my comfort zone. It would seem to me that the key or the set screws would be a likely source of failure but I know nothing about how to inspect this. Is this something I can do with the boat in the water or do I need to haul the boat?

Also, is it possible that the engine and the shaft weren't aligned when I did the engine swap? I dropped the new engine on the old motor mounts and it seems like things lined up OK....but again, I'm new to this...

Any help would be appreciated...

Last edited by JPM; 09/04/17 03:49 PM.
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Re: Propeller Shaft Spinning [Re: JPM] #17443
09/04/17 05:20 PM
09/04/17 05:20 PM
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Russ Atkinson Offline
Senior Member
Russ Atkinson  Offline
Senior Member
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
At the rear of the transmission is a "hat coupler" mounted to the stub shaft of the transmission. Attached to that is another coupler that holds the propeller shaft. That shaft coupler is attached to the transmission coupler with 4 bolts. The shaft coupler is attached to the propeller shaft with two square head machine screws and a keyway. The square headed screws are locked to the shaft coupler with two nuts.(FYI, none of the parts screws or nuts related to the shaft coupler are stainless - get some lubricating oil and bandaids for the skinned knuckles you will most likely have)
Finally, there should be a stainless wire running thru the square nuts that further prevents them from backing off the shaft.

If the shaft is turning in the coupler then one of three things probably happend
The shaft has moved aft enough that the key has fallen out. Or, somehow you have sheared off the key, or during assembly of the new engine the key was never put back in when the engine was changed

If the shaft is turning free in its coupler then it can fall out. Usually the only thing keeping it in the boat is a sacrificial zinc mounted on the shaft behind the strut. (Gambler a J/30 out of Sandusky, Ohio had this happen and nearly sunk. The shaft came out of the boat). Big hole in boat where shaft comes thru lets in a lot of water

Metal shavings could be from the shaft spinning against the bottom of the square head bolts. Could be from the shaft spinning in the coupling and wearing either the shaft or the coupling or from a sheared key

Fixes, 1st, remove the coupler from the shaft, see if there is damage to the shaft or coupling (if the screws have cut a groove in the shaft, it won't hurt anything). 2nd, get a key and put it back together 3rd, align the engine. Use feeler gage to measure distance between shaft coupler and transmission coupler (.003 max runout). Loosen the front engine mounts only. Adjust left to right and up and down

Re: Propeller Shaft Spinning [Re: JPM] #17444
09/05/17 11:58 AM
09/05/17 11:58 AM
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Russ Atkinson Offline
Senior Member
Russ Atkinson  Offline
Senior Member
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Additional info. You can do all the work described earlier with the boat in the water. You won't have a lot of room 2-3" max to move the shaft aft to remove the shaft coupler from the shaft. Also, all nuts and bolts are metric. Don't know the size of the keyway- again, probably metric.
There is one thing that may keep you from doing the work with the boat in the water. If there is a sacrificial zinc mounted in front of the strut it may prohibit you from moving the shaft far enough aft to remove the coupler.

Worse case, if you can not slide the shaft back to get the coupler off the shaft, Perhaps you can get a short key into the coupler/shaft.
Then tighten the two square head screws, lock them with the nuts, Wire the two square nuts, align the engine to the shaft and deal with it at the end of the season or in the spring.

Next spring, think about re stuffing the stuffing box and checking the cutlass bearing

Re: Propeller Shaft Spinning [Re: JPM] #17445
09/05/17 05:07 PM
09/05/17 05:07 PM
Joined: Apr 2007
Posts: 486
Chicago, Il. USA
D. Bartley Offline
Governor at Large
D. Bartley  Offline
Governor at Large
Joined: Apr 2007
Posts: 486
Chicago, Il. USA
On my 1980 boat, I believe the key is 3/16 brass. NOT steel. The key is a fusable link... it's supposed to break if something keeps the prop from turning while in gear... like a line wrapped around the prop. As Russ said, another way for it to fail is if the two bolts holding the shaft came loose. You might find the key on the floor below the prop.



Dennis Bartley
Planxty, s/n 23994
hull 205
Re: Propeller Shaft Spinning [Re: JPM] #17448
09/06/17 03:41 PM
09/06/17 03:41 PM
Joined: Mar 2017
Posts: 27
Annapolis
JPM Offline OP
Senior Member
JPM  Offline OP
Senior Member
Joined: Mar 2017
Posts: 27
Annapolis
I replaced the key (3/16 brass) and used the feeler gauge to adjust the run out. I had to lift the front of the motor to get the gap closed. Fired up the engine and put her in forward while tied to the dock. Ran very smoothly. No vibration at all!

Thanks for the help!

Re: Propeller Shaft Spinning [Re: JPM] #17450
09/06/17 06:24 PM
09/06/17 06:24 PM
Joined: Apr 2000
Posts: 669
Maryland, USA
Bob Rutsch Offline
Governor at Large
Bob Rutsch  Offline
Governor at Large
Joined: Apr 2000
Posts: 669
Maryland, USA
Had this problem myself on the way home Sunday. Key slid aft out of the key way. This has happened before so I had put tape on the key which made it easy to find. Not sure why it is sliding out but seems to always happen near the dock when in slow forward or reverse.

Re: Propeller Shaft Spinning [Re: JPM] #17451
09/06/17 10:02 PM
09/06/17 10:02 PM
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Russ Atkinson Offline
Senior Member
Russ Atkinson  Offline
Senior Member
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
The reason that the key is coming out is that the shaft is not far enough into the coupling or that the keyway itself is too short.
The cutter that cuts the keyway groove is radiused. The back of the keyway groove has that radius in it. The keyway should slide back till it runs up against that radius and should not go any further. If the key slides back and falls out before it reaches the radius then when the boat is on the hard, loosen the two square head bolts and slide the shaft further into the coupling. Or buy a longer key, slide the coupling forward on the shaft until you can insert the key, then re-position the coupling on the shaft

To an earlier post, I have a 1" shaft. I measured the keyway, it is .225 inches wide

Re: Propeller Shaft Spinning [Re: JPM] #17452
09/07/17 01:11 PM
09/07/17 01:11 PM
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Russ Atkinson Offline
Senior Member
Russ Atkinson  Offline
Senior Member
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Speaking first hand, it's very unlikely that the key will shear to protect the shaft, strut or other engine components. Shear strength of brass is 34100 psi. A 3/16" key x 1" long will take almost 6400lbs of force to shear the key. A line will break, the strut or shaft will bend or the engine will stall before the key shears. Given that my key is .225", the force to shear the key is almost 7500lb. I bent the strut and shaft and the engine stalled when a line wrapped around the shaft Ugh. All is fixed and original key is still functioning.

The key is actually there to drive the shaft. The two square head bolts are there to keep the shaft from sliding out to the coupling.

Bob the probable cause of your key coming out at slow RPM's when shifting in and out of reverse at the dock is most likely that the coupling is loose on the shaft. At low RPM there would be very little load on the key. Shifting from forward to reverse can cause a slight back and forth rotational torquing of the coupling on the shaft. Add a little engine vibration, shaft wanting to move fore and aft depending on prop rotation and at some point the key moves. If the length of the key is shorter than the distance from the back of the coupling to the back radius of the keyway- the key will eventually fall out

Re: Propeller Shaft Spinning [Re: JPM] #17453
09/07/17 01:36 PM
09/07/17 01:36 PM
Joined: May 2007
Posts: 2,603
Portsmouth, RI
Rhapsody #348 Online content
Past J/30 Class President
Rhapsody #348  Online Content
Past J/30 Class President
Joined: May 2007
Posts: 2,603
Portsmouth, RI
On Rhapsody I dimpled the shaft under the coupling bolts just to make sure the shaft wouldn't slide. This was done after I had a problem with the shaft sliding back on the coupling.

Re: Propeller Shaft Spinning [Re: JPM] #17454
09/07/17 02:21 PM
09/07/17 02:21 PM
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Russ Atkinson Offline
Senior Member
Russ Atkinson  Offline
Senior Member
Joined: Sep 2000
Posts: 379
Rockwood, MI, USA
Did you drill through the coupling or pull the shaft?

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