Prop shaft coupling fails when going hard astern

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Windfall

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Thankfully the rudder stopped the whole thing dropping into the sea and we didn't sink, but now we are trying to work out what might have happened and how we can stop it happening again.

Coupling is a Vetus bullflex on a 1" shaft and has been running problem free until yesterday. We had to apply full astern to get off a downwind pontoon when there was a loud bang and a loss of drive. Fortunately we were able to drift back onto a nearby pontoon and the boatyard managed to get us out of the water so that we could sort it.

Now we are keen to see it doesn't happen again and are looking for any suggestions as to what the problem may have been and how we fix it so that it doesn't happen again.

Any ideas or suggestions gratefully received.
 
Were the Bullflex clamping bolts greased and tightened properly, with a torque wrench, according to the installation instructions?
 
Windfall said:
Thankfully the rudder stopped the whole thing dropping into the sea and we didn't sink, but now we are trying to work out what might have happened and how we can stop it happening again.

Coupling is a Vetus bullflex on a 1" shaft and has been running problem free until yesterday. We had to apply full astern to get off a downwind pontoon when there was a loud bang and a loss of drive. Fortunately we were able to drift back onto a nearby pontoon and the boatyard managed to get us out of the water so that we could sort it.



Now we are keen to see it doesn't happen again and are looking for any suggestions as to what the problem may have been and how we fix it so that it doesn't happen again.

Any ideas or suggestions gratefully received.
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I had this happen on a delivery trip of a friends boat, in thick freezing January fog off the Owers when we picked up a pot marker line. The rudder stpped the shaft vanishing totally luckily and we were able to jury rig a line to pul it away forwards to make the rudder usable again so we could sail back to brighton from whence we came, After that any boat I haveowned had a hoseclamp or two added to the shaft to stop it sliding backwards out of the boat leaving a girt great hole..
 
I had exactly the same failure method except I was stopping the yacht and kicking her stern in on a very windy day which required a bit of power. The power was brought on smartly after engaging reverse but the engine just revved. My prop shaft was bent and it was not obvious at first. My coupling was the type where there is a white plastic centre piece (with a wavy edge) between two plates and mounting bolts to clamp the sections together; I don't know the make. The clamping bolts had sheered and they looked quite rusty. However, I found one of the engine mounts was loose so the engine was only supported on 3 mounts which have had a negative impact on coupling. I had the surveyor check the engine as part of the survey and he missed that, this happened on the day I took delivery of the yacht. Anyway, check the mounts, check the bolts for corrosion and have the shaft checked for straightness.
 
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I think these things are the same as Centaflex.

The shaft should be free of grease before it is locked in place. Though, personally, I doubt if a little grease would lead to failure, I could be wrong.

Do you have any signs of damage? There was a post, some years ago, showing a unit which had cracked in the area of the collet bolts have a look with a glass.
 
Did the shaft come out of the clamp, or did the coupling sheer? If the shaft came out then check that the end of the shaft is actually 1" and not 25mm, or is worn to less than 1". If the clamp bolts were tight and the shaft and coupling are the correct size, it should not come out just by putting it in reverse.
 
Robin said:
I After that any boat I haveowned had a hoseclamp or two added to the shaft to stop it sliding backwards out of the boat leaving a girt great hole..
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I have a plastic block clamped to the shaft, about 1/4" ahead of the gland nut, for that purpose. Fortunately it has never been needed!

Is the Vetus coupling similar to the Centaflex? If so, all that keeps the shaft engaged in the coupling is a a tapered collet that grips the shaft. The collet tightens onto the shaft as it is drawn into the body of the coupling.

I'd just repeat what others have said. Check that nothing has broken and that the clamping bolts are tightened to the correct torque and that the shaft is degreased and fits accurately into the collet.
 
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It is not the same as a Centaflex. Just a split hub and four clamping Allen screws. See p58 of Vetus catalogue for details.
 
Tranona said:
Did the shaft come out of the clamp, or did the coupling sheer? If the shaft came out then check that the end of the shaft is actually 1" and not 25mm, or is worn to less than 1". If the clamp bolts were tight and the shaft and coupling are the correct size, it should not come out just by putting it in reverse.
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Exactly, need to know what failed. I have managed two kinds of failure here (a) shaft detaching itself from the coupling, (b) the rubber flexible bit shearing. Shaft bolts now have lock nuts, and a massively over spec flexible coupling has been installed. My silly little engine dances around like buggery, which asks a lot of the coupling.
 
The problem with all "soft" couplings - once they've slipped you need the whole thing out, the propshaft machined and (possibly) the coupling face rebuilt and precisely bored out - costs about €20.
It's happened to me about 5 times in 25 years, invariably as the result of a wrap around the shaft.
Comfort yourself that it's a fail-safe, designed to prevent the expensive sound of a gearbox self-destructing.

To have the propshaft come out, shows considerable inattention, bordering on criminal negligence, by the skipper - there's plenty of prior warning signs.
 
charles_reed said:
To have the propshaft come out, shows considerable inattention, bordering on criminal negligence, by the skipper - there's plenty of prior warning signs.
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Ooh, I shall speak to the skipper when I get home tonight - he's a computer/forum phobe and won't be watching this thread himself. But as mere crew I have to say I didn't spot any prior warning signs. Could you elaborate?

It's a brand new propshaft and brand new coupling - well new with maybe 2 hours of motoring on it.

We did indeed 'gun it in reverse' but the coupling should surely withstand that? There was no evidence that the prop has bent, but we lift out for the winter mid October and won't be using the boat before that so we'll have a good look over the winter.

In the meantime Paul will be speaking to Vetus and the engineer today. Will keep you posted.

I shall be posting another thread thanking Gosport (Premier) marina staff as we were very impressed with the serivice and help we received.

thanks

Sharon
 
Sounds very much as if the clamp was not tight enough. That method of attaching the coupling to the shaft is extensively used, not just in the Vetus Bullflex, but as standard on all the small Yanmar engines. Biggest complaint is actually about getting them loose when you want to get them off if they have been on the shaft for a long time.
 
Cannot help with the problem, when it happened to me on my last boat the last owner had put a jubilee clip round the shaft "just in case" :/

As some one suggested here I know keep my "spare" shaft anode stored on the shaft between the engine and stern gland "just incase":/
 
Tranona said:
It is not the same as a Centaflex. Just a split hub and four clamping Allen screws. See p58 of Vetus catalogue for details.
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I see it now. I stand corrected.

Windfall, you can ignore post 7.

If the thing is undamaged and unworn, with the correct shaft and hub dimensions, then the most probable conclusion might be that the bolts were not tight enough. No amount of spirited work on the throttle would normally cause the thing to fail.
 
The other installation variable is how much of the shaft protrudes into the split coupling.
I would expect that it should be well past the inner set of clamping bolts.
 
earlybird said:
The other installation variable is how much of the shaft protrudes into the split coupling.
I would expect that it should be well past the inner set of clamping bolts.
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If the new shaft and coupling were made correctly, the shaft should go right to the stop inside, which is well past the clamping screws.
 
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