Rudder of Beneteau First 47.7, 2001

[matelot]

to reply semi seriously for once tranona, I have at the back of my mind a vague memory of an article about carbon shafts and failure mechanisms. Something to do with bearings I believe. But I cant find it on the net so its probably a false memory.

Have done a fair bit of sailing on a Freedom with unstayed carbon fibre masts and been very impressed by their strength when abused. Wondered many times why all boats arent built like that - we dont use wires to hold plane wings any longer, do we?

 

[Twister_Ken]

Although carbon will, ultimately, shatter, until that point it flexes.

Does that mean that if there were a non-ultimate impact on the rudder, a carbon shaft might flex and recover, whereas a stainless shaft would bend, and stay bent?

 

[Tranona]

I am not sure that SS shafts bend. Modern designs on spade, or even partial skeg rudders are short, well supported and large diameter. The same is true of carbon fibre.

If either are likely to flex or bend then that would be seen as a weak link, whereas the weak link is probably the blade itself. In other words in a collision/impact the lower part would absorb the shock and break before the shaft.

Deep spade rudders, however constructed are vulnerable, but from observation (of a small sample) it is either blades or bearings that get damaged - not shafts pulling out and allowing water in.

 

[matelot]

No. They both flex until they reach a certain point and that point should ideally be the same for both because thats what the designer should design to. In other words the rudder should withstand a certain level of impact without damage whatever its made of.

After that point (and just ignoring any built in safety margin), carbon fibre fails catastrophically, stainless steel bends. So jammed steering or no steering? Same overall result, just a different mechanism.

 

[Tranona]

Just been re-reading the Megawatt (Hanse 371) report to see if that sheds any light. However the break there was a fatigue fracture right at the top of the stock so the whole rudder and stock came out allowing water into the boat.

The report does, however contain the ABS standards for the design of rudders and stocks, but the maths is far beyond me!

 

[Twister_Ken]

As I recall Megawatt, hadn't an autopilot arm been attached in a non-approved way, creating a stress point which lead to the fatigue situation?

 

[Tranona]

Yes, you are right, although the report was not unequivocal about this being the direct cause of the fracture. The owner was pretty convinced - even though the boat had done two transatlantic crossings with the autopilot, but the metallurgists suggested that the initial fracture could have been caused by a manufacturing fault or something being trapped inside the autopilot arm - not enough evidence to be clear either way. However, there were no reports of any failures on other stocks of the same design, so the favourite was the autopilot arm , which was a different design from the one used on other similar boats.

Life is never simple!