my prop shaft detached from the engine, can someone explain

[Shearwater]

...what happened as I wasn't able to watch the guys repair it.

The key dropped out of the keyway into the bilge so I was able (just) to replace to and push the shaft into the back of the motor to get me home (with 3 youngsters aboard, none of them mine!!). I noticed the end of the shaft to be threaded; if this is lefthanded (?) does that imply banging the engine into reverse puts an excessive unwinding torque onto the shaft and should be avoided? Tks

 

[MoodySabre]

If it is threaded then that implies a nut is needed to hold the end of the shaft in the coupling. Take the coupling off and see if there is room for one (or one sitting there!). The shaft will either be held by the nut or, if it is a split coupling, by bolts squeezing the coupling tight on the shaft. The key will stop it turning in relation to the coupling but not hold it in.

 

[saxonpirate]

Is the end of the shaft tapered. Most tapered shafts are keyed with a nut.

 

[Searush]

A jubilee clip (or 2 for balance) on the shaft can stop it slipping out (leaving a big hole in the back) or jamming the rudder if it should ever come loose again.

As to how it fits, just ask the guys who fixed it, they are unlikely to be upset, more likely to be pleased you are interested in what they did.

 

[Neil_Y]

If it didn't just come loose it is possible debris wrapped around prop has pulled the shaft out. It builds up between the prop and bearing carrier and can exert a huge force trying to pull the shaft aft.

We see this as one of the common problems on fishing boats that get debris on prop, on the fishing boats it usually pulls the gearbox off rather than seperating the coupling.

 

[Bosun Higgs]

Take a drill down to the boat and drill and tap a hole for a bolt though the side of the coupling and into the shaft. Easy to do

 

[Shearwater]

Thanks all. As to "ask the guys that fixed it" I will, but they speak Catalan and I speak Spanish so one man's widget is another's floblange. I certainly like the idea of tapping a thread through engine flange and shaft...dare I ask why the manufacturers don't do this? It makes me think this is something I ought do (or have done) with my next boat so it never happens again. The problem with boating is that once you have invested in preventing one incident from happening, you have simply shortened the odds that something else will happen sooner. Yus, I know, that's boatin' !

 

[Shearwater]

going into reverse in a rush (err Panic)

BTW can I repeat my question... does slamming the engine into reverse while in forward motion put overly excessive forces on the motor/transmission????

 

[MoodySabre]

BTW can I repeat my question... does slamming the engine into reverse while in forward motion put overly excessive forces on the motor/transmission????

Yes it does - the shock is taken on the splines on the gearbox input shaft. Let the engine revs drop to tick over first. If your engine is late 1980s or earlier then this can be an issue. An issue that can be fixed. Read about it here. I did my 1988 gearbox a couple of years ago and it needed it! The DB Marine conversion was then £400. A thread on the subject here.

 

[ianat182]

If you do go to the drilling a hole through the shaft and coupling process be aware that you will need a Cobalt drill, constant revs/speed and pressure ,to be successful, and needs to be across the shaft and not through the new woodruff key that has been fitted.

But first, examine the clamping system used to hold the shaft in the coupling; loose cotter bolt(s) in the coupling clamping the shaft end may not have been tightened fully-
sometimes these are allen socket headed and hard to fully tighten to a correct torque - I hate the things!

ianat182

 

[Norman_E]

BTW can I repeat my question... does slamming the engine into reverse while in forward motion put overly excessive forces on the motor/transmission????

Yes, as MoodySabre says. In addition I have seen a boat that lost its propeller from the same cause. It is not just the weak splines on certain old Volvo gearboxes that can be damaged that way. The practice puts big and potentially damaging loads onto the clutches inside any gearbox, and it is always best practice to pause in neutral long enough for the prop to stop spinning before engaging the opposite direction, particularly if the engine has been running fast. Not so much of a problem if you were only at tickover going forward, and need a burst astern to stop the boat.

P.S. I fit an anode ahead of the shaft gland as a backstop against the shaft falling out if it is ever detached from the gearbox.

 

[Pagetslady]

If you are determined to put a hole through the coupling and shaft ( this is not really a job for a hand drill, as its very easy to run off the centre line and, you will end up putting a hole out of centre of the shaft, if its a large shaft then I guess it wont matter but then it will be a big job with a hand drill.) I would put a simple nut and bolt rather than tapping a thread. I have a volvo 2003 with a 25 mm shaft this has a coupling with pinch bolts and a 6mm bolt through the coupling and shaft with a nut and washer on the end, this was obviously drilled by hand before I bought the boat I imagine by Westerly, as they will have installed the engine, and the bolt will only fit one way so the hole is of centre however I have had the boat 17 years and had the gearbox and engine apart several times and never had a problem yet anyway, I did carry out the gearbox convertion mentioned earlier by DB marine some 5 years ago and have done 3000 hours since. If your shaft has a thread on the end I assume there must be a nut inside the coupling flange and with a key in the shaft the nut must have come undone unaided as the shaft would not be able to rotate in the coupling, it sounds to me as if the shaft as been assembled with out the nut at some time or at least with out any form of lockind tab. This system should not require a retaining bolt. If you can I would have the flange off, if it is held with Allen bolts you can get an allen key to fit, which will attach to a half inch square socket set, this will make undoing them much easier. Check to see if there is a nut on the end and if so does it have a locking tab or split pin to secure it.
Mike