Flex shaft coupling keeps "tripping"

[mattman]

Hi friends,

I'm having some major engine transmission problems that I've been unable to troubleshoot.

Boat: 1984 Nonsuch 26
Engine: Westerbeke 21
Transmission: Hurth
Shaft seal: PYI PSS
Flexible shaft coupling: PYI / R&D

This summer I replaced my motor mounts and the flexible shaft coupling between my transmission and propeller shaft.

Everything was going fine for many hours, until I checked below and saw a small leak where the PSS carbon flange meets the S/S rotor. The rotation of the shaft was spraying little droplets of water around the engine compartment.

I figured we could put the sail up, shut off the engine, and this would be an easy fix underway. Unfortunately, this seems to be where things went wrong, and where I'm hoping someone has suggestions.

To address the leak, I loosened the rotor set screws and used the extra leverage of a 2x4 to compress it more firmly against the PSS carbon flange. Installed new rotor set screws and went back to sailing for 2 hours.

When we re-started the engine to take the sails down, the engine started fine, went into gear just fine, but when we rev'd up to about 1500 the engine made a loud grinding noise and shut itself off. I went down below and saw the flex coupling was completely distorted and the shaft would not rotate at all in any direction. By distortion, I mean the two half-couplings on either side of the flex coupling were further apart than they were supposed to be; the prop shaft had slid back about 1/2"

I loosened two of the bolts on the prop side of the coupling and this enabled just enough rotation that I could access to other 2 bolts and free one half of the coupling. This also let the flex coupling return to its normal shape.

With the halves separated, given we were still underway, both sides would rotate freely and the prop side continued spinning through the water current. I figured in my repair I must have secured the rotor too tight against the carbon flange so I loosened it up a bit, while checking for the leak, and then put everything back together.

Once again, the engine started up, I tested both forward and reverse gears at idle speed, and everything was fine. Rev'd up to about 1500, cue the loud noises, and engine shut off. Same distortion in the flex coupling as before.

We ended up getting a tow home since I didn't want to risk any more damage. And in the slip the boat has remained.

At this point I'm not sure what might be going on, or where I should check.


Photos and videos can be seen here:
https://photos.app.goo.gl/aqiHL8QJWnjmP6k56

Unfortunately I did not get a photo of the flex coupling in its "tripped"/broken position.

The two videos at the end show the transmission engaged after the 2nd repair at idle speeds.

• Could the fact that this repair was done underway, in strong winds and strong current have anything to do with this? It certainly seems like there was additional aft pressure on the prop that I didn't account for when tightening the PSS rotor, but this doesn't really explain the first repair since the shaft was connected to the transmission.
• The engine alignment was within tolerances before this repair and running very smoothly, but I need to recheck the alignment when I get back to the boat and see if anything could have gone awry there.

Many thanks for any suggestions!

[tkeithlu]

All my thoughts go to mis-alignment of one sort or another. The soft mount is flexing until it tears itself to pieces at higher RPM. Now, where might it be coming from?

"recheck the alignment when I get back to the boat and see if anything could have gone awry there."

"when we rev'd up to about 1500 the engine made a loud grinding noise and shut itself off."

The forces you have described (stopping the engine from 1500 RPM) are certainly enough to have shifted your engine on its mounts. They are also enough to have messed up your cutless bearing, such that the shaft does not stay in alignment as the RPMs go up. The forces are enough to have bent your propeller shaft, such that you will never get alignment. Your propeller may be damaged, such that you are rotating an un-balanced weight.

So, start with the boat in the water and the shaft separated fro the engine.

Move the shaft sideways/up and down - how much play? Is the cutless bearing keeping it in line?

Turn the shaft - how much run-out? Is the shaft bent?

Align the engine to the shaft - Get it to 0.001" if possible, but certainly under 0.01". The soft mount is there to take up some vibration and tiny misalignment only. Be sure that the engine center line is not displaced above/below left/right, as well as parallel.

When you think you have it right and have eliminated issues like a bent shaft or bad cutless bearing, start the engine, put it in gear at idle, and look for any possible wobble. If you can feel (be very careful touching it, touching with a drinking straw is safer) or see any vibration, it ain't right. A new possiblity enters here - You might have a bent propeller. Gradually increase RPM while watching it and being sure that you do not
1. Wash your neighbors into the concrete seawall beside them
2. Pop your mooring lines and exit your slip, T-boning the megayacht across the marina from you.

This can be a rewarding investigation. You know that something in that assembly is whacko, and need on explore it systematically to come to the "Got it" momnet.

I hope it isn't a "OH SH*T!" moment instead.

[Nicholson58]

The seal makers have a specific spring compression. Exceeding that could damage the seal. It seems doubtful that the seal has anything to do with the shaft rotation.

I would start with a snorkel to look at the prop and shaft. I propose the seal is a red herring and there is something wrong below. Perhaps you ran into debris that is wrapped on the shaft or buried in the Cutlass. Look for witness marks on the hull, rudder, strut if you have one.

I would replace the coupling spider. Get a spare. The one you have has been overworked, damaged. It may look OK but it may now be failing at lower torque. Will your repair job operate at lower rpm? It may all be OK but the spider is shot.

On the leaking coupling, they don’t last forever.

[a64pilot]

I think you have busted motor mounts and the engine is moving. Either the engine is moving or the shaft is, well actually the shaft is of course but if the engine is moving it’s moving the shaft too, or the shaft is moving in it’s coupling