Prop failure during two week cruise- seeking advice

[cutsurfer]

So I was hoping not to have to create another post quite so soon, however on a planned two were cruise of the Warwick ring a rope was fouled in our prop. Earlier in the week I had to get into the weedhatch and cut it away ( it was a multi stranded flexible rope) this tooo hours but finally we were able to continue. However at the bottom of the Aston flight in Birmingham we lost reverse, then finally lost forward.

 

As there was nowhere to Moor down that flight my three crew and I managed to walk the boat painstakingly up the twenty odd locks to cambrian wharf side crt point.

 

I've had a look in the weedhatch and the prop is turning totally independently of the shaft ( which still engages both directions with the engine).

 

The nut also feels loose and the split pin remains in and intact, there is also a very loose washer between the bolt and the entrance through the hull and some remnants of rope.

 

I've been advised it may be removing the pin tightening the bolt and trying to realign a 'wedge?' that is needed.

 

I'm just trying to work out if this is something that sounds like an out of water job,or if I may have to ask Sherborne wharf to tow us into their dry dock whenever possible

 

Any info or advise ie hugely appreciated

[nicknorman]

We are moored just round the corner, I'll pop round and talk to you about it in a few minutes.

[Mike Tee]

Well that's got to be quicker than RCR!!

[WotEver]

Cool

[MtB]

Given the key that goes into the keyway in the shaft that locks the prop to the shaft is normally a piece of steel about 1/4" square, it is vanishily unlikely that it has broeken or been sheared.

 

I'd say it has been missing from day one when the prop was fitted. This would mean the prop easily loosens on the shaft in the event of a prop foul, which appears to be exactly what has happened.

 

So basically, remove the split pin and tighten the nut to lock the blade back onto its taper and complete your journey, then dock the boat at your leisure to remove the prop and find a suitable key.

[nicknorman]

OK so the prop was quite loose, maybe 1/2mm of play between the washer and the prop boss which made me suspicious about whether the tapers matched (since it is a lot of wear to suddenly appear. Anyway we got the prop off. The key is brass, and sheared. The slots are quite wide so perhaps designed for brass? The two halves of the key came out easily. There is some damage to the mating surface on the prop taper but that's probably tough in the short term!

 

The nut wasn't castellated but just a normal nut with one hole drilled through, so the split pin could only be inserted in one of two rotational positions 180 deg apart - not good!

 

I've advised the OP to try to get a new key and a castellated nut, put it all back together doing the nut up REALLY tight and hope for the best! Oh and I think the nut is brass, is this normal? I'd have thought it would normally be steel.

 

Here are the bits as extracted:

 

 

There's some sort of fibrous washer between the prop and the boat, not sure if it some escaping packing or what. The packing is very thin with not much adjustment left, although not leaking significantly at the moment I've advised the OP to get the packing replaced in the not too distant future. You can perhaps see the scoring and the width of the keyway in this photo?

 

Edited July 30, 2016 by nicknorman

[MtB]

Wow that must have been one helluva stop to shear that key! I doubt it would have sheared had it been steel though.

 

I think the fibre just looks like residue from the rope foul.

[WotEver]

Good work Nick. Could the 'fibrous washer' be the remains of the rope that OP had to cut off?

Mike beat me to it.

[nicknorman]

Good work Nick. Could the 'fibrous washer' be the remains of the rope that OP had to cut off?

Yes could be, it was sort of stuck to the face of the prop but maybe just melted onto it with friction.

[WotEver]

Wow that must have been one helluva stop to shear that key! I doubt it would have sheared had it been steel though.

Might have been rattling fora while and developed a fatigue crack.

[nicknorman]

Wow that must have been one helluva stop to shear that key! I doubt it would have sheared had it been steel though.

 

I think the fibre just looks like residue from the rope foul.

 

I suspect the nut was loose for a while and with chattering, the key started to fail over time and eventually gave way under the extra load of the rope foul.

[Sir Nibble]

Perhaps the key was too thick and prevented the prop seating on the taper as it should throwing all the load on the key rather than forcing an interference fit.

[Keith M]

Prop nuts are normally brass

 

Keys can either be steel or brass

 

keith

[David Mack]

Is there any way the two halves of the key can be bodged as get-you-home measure? E.g using something else to pack either side of half the key? Also can you drill holes through the other two pairs of faces of the nut to give a better chance of doing it up tight?

[nicknorman]

Perhaps the key was too thick and prevented the prop seating on the taper as it should throwing all the load on the key rather than forcing an interference fit.

 

Yes something like that I think. Bearing in mind how much play there was once the key sheared (the nut not having turned) I don't think it can have been properly on the taper. Either that or whoever last fitted it had to back of the nut nearly 180 deg to get the split pin in. Of course now the taper surface on the prop is so scored, it is never going to be an interference fit again!

[bizzard]

It seems the prop developed a lot of play when the key sheered which points towards the key being too proud and not allowing the shaft and prop tapers to tighten up snug with the full tapers in contact. Its very important to file a key to match so this doesn't happen to just under the thickness needed.

The nut and key is very likely hard admiralty brass, gun metal or manganese bronze. But yes a castellated nut is whats needed and a monel metal pin is better than stainless steel.

Edited July 30, 2016 by bizzard

[nicknorman]

Prop nuts are normally brass

 

Keys can either be steel or brass

 

keith

 

Thanks Keith, I had a feeling ours was steel but maybe not.

[Sir Nibble]

 

Yes something like that I think. Bearing in mind how much play there was once the key sheared (the nut not having turned) I don't think it can have been properly on the taper. Either that or whoever last fitted it had to back of the nut nearly 180 deg to get the split pin in. Of course now the taper surface on the prop is so scored, it is never going to be an interference fit again!

No chance of lapping it?

[Keith M]

 

Thanks Keith, I had a feeling ours was steel but maybe not.

 

It would all depend on supply the prop shaft to the boat builder

[nicknorman]

Is there any way the two halves of the key can be bodged as get-you-home measure? E.g using something else to pack either side of half the key? Also can you drill holes through the other two pairs of faces of the nut to give a better chance of doing it up tight?

 

The boat is a couple of hundred yards from Sherborne Wharf so I'm hoping the OP can get bits there. I think it would be sensible to get something for a new key whilst he is in central Brum rather than bodging it and having it fail in the middle of nowhere. Especially as the taper surface is quite scored and doing it in the water through a rather small weedhatch is quite a faff!

No chance of lapping it?

 

Could that be done in the water? I've no experience of lapping brass, it is a funny material! I would have thought it would need to go in a lathe to at least get the worst high spots off. But as I said, I don't really know what I'm talking about!

Edited July 30, 2016 by nicknorman

[Keith M]

Is Sherborne wharf not closed for redevelopment

 

But I think they are close by not sure where

 

Keith

[nicknorman]

Is Sherborne wharf not closed for redevelopment

 

But I think they are close by not sure where

 

Keith

 

There is some redevelopment going on but the OP had been in touch with them before I pitched up. They have a mechanic available but not until Monday at the earliest, so I guess they are still open for business and they certainly have plenty of their own boats to maintain.

[cutsurfer]

Hi All thankyou very much for all replies and Nick for being an absolute hero! I am.poppong down to a place called keeleys to engineer a key on Monday if Sherborne hasn't got one and i'll do it through the weed hatch and hope this will get me home. Because the rope that tangled was my old multi strand climbing rope i suspect the dynamic stretch in the rope may have stopped the prop and then pulled it the other way ( but that's just my guess). Thanks again for everybody's help on this!! there is now hope

[tosher]

When I bought my new prop from Crowthers they provided a new key, nut & split pin but not a washer. I asked about this and they said they do not recommend fitting a washer so do not provide one. I fitted the new prop without a washer and it's never moved or given a problem in 6 years.

[bizzard]

Hi All thankyou very much for all replies and Nick for being an absolute hero! I am.poppong down to a place called keeleys to engineer a key on Monday if Sherborne hasn't got one and i'll do it through the weed hatch and hope this will get me home. Because the rope that tangled was my old multi strand climbing rope i suspect the dynamic stretch in the rope may have stopped the prop and then pulled it the other way ( but that's just my guess). Thanks again for everybody's help on this!! there is now hope

For the correct depth of the new key you will need to measure the depth of the key slot on the shaft end and the key slot in the propeller. This can be done with the depth measuring probe on the handle of a vernier gauge. The new key will probably need filing down to those combined measurements to make sure its not proud plus a touch more to make sure.