Prop Shaft Alignment

[Stu Fyfe]

First trip out this summer and the 'shaft-saver' (4" rubber attachment to the prop shaft) shears off in Sesuit Harbor. The Tow-Boat/US guy started drooling as the $$$$ flashed before his mind. We hoisted sail and brought her back without assistance. Now my question. Just how exact must the coupler be attached to the engine?. I'd been told that the two plates must be flush to within thousandths of an inch. This is a seven hp diesel engine and the RPM's will be low. So just how exact must I be?

[RGM]

Propulsion shaft/coupling alignments should be done "within thousanths of an inch". They need to be in alignment radially and axially (face and periphery). Having some misalignment with a flexible type coupling and low horsepower should not result in sheared bolts or coupling material. Did this thing actually part? Sounds like you had some type of impact event. Check the prop/shaft for evidence of such (nicks,dings, gouges, line or burn marks from line). Are there lots of lobster or crab pots (traps back East) around?Have this checked out by a reputable marine mechanic. Be around when this problem is solved if you can, it may prove to be educational. Good luck

[Bob Cleek]

What RGM said!

[NormMessinger]

Right!

--Norm

(Not that I am that much of an expert on the subject but I thought it would be a record if three of us in a row gave the same advice.)

[Stu Fyfe]

The shaft saver sheared off where the rubber meets the metal plate at the coupler. This happened when I was docking and shifted into reverse. The prop didn't hit anything. Here's what I think happened. The engine is 15yrs old. I rebuilt it two years ago and re-installed with what I thought was accurate alignment. It wasn't. The shaft saver was able to make the adjustment for two years but eventually the unit weakened. When I reinstalled the new coupler yesterday, I noticed that the plates didn't match up well at all. So now everything is lined up well. Just got back from two hours under power and everything seems fine. Now I'm ready to head to the show in Rockland. I'm leaving on July 7 with overnight stops in Provincetown, Gloucester, Biddeford and Damariscove. If anybody else is sailing up, drop me a line. Maybe we can rendevous along the way

[dale o]

That shiftin' into reverse can put a lot of torque on the coupling and shaft. Better the rubber shaft saver sheared off rather than your prop shaft or the output shaft. It did one of it's intended jobs ("the weakest link!). That's one of those spare parts you should have handy in your tool or parts box. Good luck and best wishes!

[Allen Foote]

First off, pat yourself on the back for using the shaft saver. The process of alignment has to do with moving the engine to met the fixed shaft. The shaft should be spinning freely through the cutlass bearing at 1 end and the packing gland at the other. The engine (being the movable object) should be adjusted (brought to) the shaft. Most out measurement, should be thousandths of an inch. This is actually easisly attained if your motor mounts are in good shape....ergo the problem most common. Motor mounts that are rusted and frozen in place. Take the time to do this right and put antisieze on your mount nuts and bolts ect.

[Bayboat]

The previous comments are right on. It's a case of the tail wagging the dog, the shaft & coupling being the tail and the engine the dog. The engine has to be shifted about until its coupling meets the shaft coupling, which means the mounts have to be worked easily and the bed has to be firm. The final alignment should be done after the boat is launched, since just about any hull will change shape a bit when it hits the water.

[Stan Derelian]

Just to throw out a suggestion that no one seems to have made. After a number of hours of use, you should undo the coupling and recheck the alignment. I've had it change.