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what happens when someone doesn't assemble it correctly
An inspection revealed that the prop shaft had slid out of the coupling – the whole of the key way and the end of the shaft were visible. When I went into reverse, the reverse thrust on the prop had pulled the prop and its shaft back and they were brought to rest by the back of the rudder, hence the rudder being jammed. The collet, tapered ring and locking ring all seemed tight but yet the shaft was detached.
The coupling is a Vetus Type 6 dating back to 1985. The shaft is held in by a taperlock collet with enough grip on the shaft to transmit the fore and aft propulsion force. There is an option for a roll pin in the design.
According to Vetus UK:
1 Though there is provision for a roll pin, fitting it is optional
2 For Starry Night's engine bhp, the optional roll pin is insufficient on its own to transmit the forces involved
3 Provided that the parts are properly assembled, there is enough grip on the shaft provided by the taperlock to transmit the fore and aft propulsion force without a roll pin
4 Provided that the parts are properly assembled the coupling should stay connected to the prop shaft indefinitely.
Another expert in this coupling advised that a roll pin wouldn't be a bad idea as our engine's 72 BHP rating is towards the upper limit of the coupling's design. There was a hole in the end of the shaft with what looked like the remnants of a roll pin inside it but no other broken parts anywhere visible.
Once Starry Night had been towed to the nearest yard and had been lifted, all became apparent. In putting the coupling back together, the engineer discovered why it had come apart, and it’s a miracle that it has lasted so long; the tapered ring was installed back to front and there was no grip being provided by the taperlock! The last time anyone had disturbed the coupling would have been when we had a new stern tube fitted about five years ago.
Having taken some basic measurements and done some scaling off photographs it looks as though the tapered ring was not properly engaged on the taper of the collet.
It was time to consider how the dimensions of the parts must be arranged for the threaded locking ring (c-ring) to tighten onto the tapered ring without "bottoming out" on the shoulder (see picture): the smaller diameter of the tapered ring must be smaller than the smaller diameter of the collet taper to stop the tapered ring from travelling too far up the collet. Then it is not possible for the tapered ring to engage at all with the taper of the collet when it is the wrong way round as it will not go past the shoulder. So perhaps there had been a roll pin and this had been doing all the work until it broke.
This rather puts paid to the claim by the yard that fitted the stern tube/shaft log, that the taper lock would work just as well with the ring the wrong way round, if not better! I don't think so!
So new roll pins and new coupling rubbers were obtained and a few weeks later the coupling was opened up for inspection. I was determined to get at the truth.
When the coupling was disassembled the engineer found a piece of a roll pin in the propshaft and a piece of a roll pin remaining in the collet. The roll pin was incomplete. There were no loose fragments of roll pin inside the coupling. A previous search of the bilges had revealed no pieces of roll pin. So it can be deduced that the remnants in the collet and shaft pre-dated the re-assembly in 2004, and that at this time a new roll pin had not been installed, otherwise, following fracture of the roll pin, the remaining parts of the roll pin would be found somewhere in the vacinity, and most probably inside the coupling.
A new roll pin was fitted as additional security and while the coupling was apart a set of new coupling rubbers were fitted.
Key components of the Type 6 coupling. Note the hole for a roll pin. In this picture the rings are still tight onto the collet but the amount of collet exposed looks the same as the whole of the taper visible in the next picture. The gap between tapered ring and coupling housing is about 27mm.
Here are the two rings before the prop shaft has been pulled back a bit further to enable the tapered ring to be removed and turned round. From a dimension analysis we can deduce that the tapered ring was up against the shoulder of the collet (between threaded part and tapered part) and never engaged on the taper itself: see text for explanation.
Here is what it looks like with the tapered ring the right way round and everything torqued up. The gap between ring and coupling housing is now just 15mm.
The new roll pin. Notice that it is in fact made of two elements, the outer one being the roll pin and the inner a solid pin.
A fresh set of coupling rubbers still in their shrink wrap packet.
Copyright © 2009-2010 Mike Hawkridge