Catastrophic prop failure

[solent clown]

We had a prop fail on us a few day back. motoring along the Itchen headed for newer climes there was suddenly a huge amount of vibration. No bang, just an alarming rotational thumping. Fortunately we had only gone a few hundred yards so I pur the revs right down and we limped back.
I had hoped it was just something fouling, but a boathook inspection gave no indication of anything, so I had resigned myself to puling the boat out of the water to have a look at the Cutless bearing.
However yesyerday I mananged to careen the boat over a bit so she settled just right in the mud to show her prop.
To my amazement I could see one of the blades had broken off, in a dead straight line as if someone had taken a grinder to it. Having never had a prop fail before - and a limited knowledge of prop metallurgy I am wondering if it was doomed from the day of casting or it just sheared that way upon impact. (We did not feel any impact by the way, but it was a foggy early morning so we may have not seen some obstacle in the water. Pics to follow

 

[sarabande]

Ouch !


If you had hit something, I would expect to see dings and dents on the leading edge of the remaining blades as evidence..

If it's really straight line break I wonder if the blade has been straightened with a lump hammer after a past incident, causing embrittlement / loss of ductility / stress cracking, or even had a new blade attached and with a brazing failure.

 

[vyv_cox]

I await your photos, hopefully as close up as you can get to the fracture face and in good focus. The usual cause of fractures to your description is fatigue but this is unusual in a propeller blade.

 

[RichardS]

Pics to follow

I'm looking forward to the pics .... but, the first thought I had was to recall the late Jack Bruce singing the line "If it wasn't for back luck, I wouldn't have no luck at all"

Richard

 

[solent clown]

I am going to try to get it off low tide today. there is going to be some wallowing in the mud in a flubber, nor particularly looking forward to it

 

[solent clown]

Pics

IMG_20181008_164302 by mark punksteel, on Flickr

IMG_20181008_164332 by mark punksteel, on Flickr

IMG_20181008_164340 by mark punksteel, on Flickr


Sorry they are poor quality, I was hanging off the boat upside down with no one to hold my legs!

 

[{151760}]

I don't see an anode on your shaft. It's difficult to see clearly, but I think I can detect some pink bits in the break which might indicate dezincification. No doubt someone more knowledgable will have a better idea.

 

[Pete7]

I think you will need a torch. Once you have got that prop off and cleaned it up hopefully you will find the size and pitch. Then, but not on a Sunday as they are closed, take a womble down to Faulkes Boatyard on the Hamble. They have a ISO container full of props but its dark inside hence the torch. They are stacked up on rods but you can turn them to see the sizes.

http://chandlerybarge.com/

Pete

 

[lpdsn]

Pics

I think I can see teeth marks. Was there a large white shark in the vicinity? Did you hear any music?

 

[langstonelayabout]

I think I can see teeth marks. Was there a large white shark in the vicinity? Did you hear any music?

Large shark? He's gonna need a bigger boat...

 

[solent clown]

I think you will need a torch. Once you have got that prop off and cleaned it up hopefully you will find the size and pitch. Then, but not on a Sunday as they are closed, take a womble down to Faulkes Boatyard on the Hamble. They have a ISO container full of props but its dark inside hence the torch. They are stacked up on rods but you can turn them to see the sizes.

http://chandlerybarge.com/

Pete

Yep, that's plan b if the spare I have does not fit.

 

[vyv_cox]

Pics not really good enough to be definite but it looks to me like a fatigue failure. Most likely scenario is that there was some damage, maybe caused by impact with something, that has acted as an initiator for the fatigue crack. This could have been a long time ago, e.g. years, since when the crack has been propagating (not actually intended to be a pun but is quite a good one!)

 

[solent clown]

Pics not really good enough to be definite but it looks to me like a fatigue failure. Most likely scenario is that there was some damage, maybe caused by impact with something, that has acted as an initiator for the fatigue crack. This could have been a long time ago, e.g. years, since when the crack has been propagating (not actually intended to be a pun but is quite a good one!)

Sounds about right Vyv. Better pics this evening when I pull it off

 

[fredrussell]

...and clean your bottom! Good lord!

 

[solent clown]

...and clean your bottom! Good lord!

nag nag nag

 

[Bergamot]

You said she was coppercoated - Looks pretty encrusted for coppercoat and no anode!

 

[KeithH]

I had exactly the same thing happen 2 years ago. Huge vibration on leaving the mooring in Chichester Marina -so limped back. I had expected to see a rope round the prop, but on lifting the boat, had a blade missing from the 2 blade prop. It too looked like it had been machined off. On close inspection, there was a pink tinge on the broken edge, so it looks like dezincification. The prop anode had disappeared - it was fine in the previous June - and this was April.

After discussing with the marine engineer who did the replacement (and a new shaft!) I added a clamp-on anode on the new prop shaft. It also had a small anode on the end of the prop (I think called a Beneteau anode) but that is rather small.

So far, so good, but I do recommend that shaft anode.