I share concerns expressed about copper antifouling and aluminium.
However we do not know what antifouling is used on the Arcona.
It is  clear on the Arcona web-site that the vessel has a spade rudder
The video referenced in the PBO article shows failure well inside the hull, just below the steering quadrant.  No corrosion is visible.
Assuming the rudder shaft is constant diameter along its length, the most highly stressed point is at the lower end of the lower rudder bearing, normally right at the hull.
The failure is NOT at this location, making the cause a mystery to me (a retired mechanical engineer with an aluminum boat with a rudder configuration very similar to the Arcona) 

Dick,
I’m not sure if you have seen the video of the rudder stock failure but it shows a broken aluminium shaft immediately below the steering quadrant inside the lazarette. I can dig out the video if you haven’t seen it.
Simon

+x

 

captmarklp - 24 Apr 2024

I wish I had more money to send these folks !!  I, sadly and probably very inconveniently, want to know more about their vessel and its maintenance as I see from the online specs of the vessel that it may have had a Jefa rudder system -- which is a wonderful product but for when an errant swipe of copper antifouling goes onto the aluminum shaft.  I have managed a boat for more than a decade with this brand of rudder system and it is great and admirable quality yet I do what is suggested -- I drop the rudder out of the boat annually to inspect.  I do this without fail as the 1st time I dropped it, I found significant erosion of the rudder shaft in the section above the lower bearing but would be commonly wet when the boat moves.  The rudder fits tightly to the hull and there's a temptation to antifoul it right to the very top.  As a caution, I have painted out a "no go" area around the shaft's entry hole and with a no copper/no metal product.  I feel like a jerk asking for detail because the boat was completely lost for the rudder doing harm to boat's integrity.  

+x

 

captmarklp - 24 Apr 2024

I wish I had more money to send these folks !!  I, sadly and probably very inconveniently, want to know more about their vessel and its maintenance as I see from the online specs of the vessel that it may have had a Jefa rudder system -- which is a wonderful product but for when an errant swipe of copper antifouling goes onto the aluminum shaft.  I have managed a boat for more than a decade with this brand of rudder system and it is great and admirable quality yet I do what is suggested -- I drop the rudder out of the boat annually to inspect.  I do this without fail as the 1st time I dropped it, I found significant erosion of the rudder shaft in the section above the lower bearing but would be commonly wet when the boat moves.  The rudder fits tightly to the hull and there's a temptation to antifoul it right to the very top.  As a caution, I have painted out a "no go" area around the shaft's entry hole and with a no copper/no metal product.  I feel like a jerk asking for detail because the boat was completely lost for the rudder doing harm to boat's integrity.  

More for those interested. Dick Stevenson, s/v Alchemy
https://www.pbo.co.uk/news/arcona-460-sinking-prompts-investigation-by-arcona-yachts-85886

I wish I had more money to send these folks !!  I, sadly and probably very inconveniently, want to know more about their vessel and its maintenance as I see from the online specs of the vessel that it may have had a Jefa rudder system -- which is a wonderful product but for when an errant swipe of copper antifouling goes onto the aluminum shaft.  I have managed a boat for more than a decade with this brand of rudder system and it is great and admirable quality yet I do what is suggested -- I drop the rudder out of the boat annually to inspect.  I do this without fail as the 1st time I dropped it, I found significant erosion of the rudder shaft in the section above the lower bearing but would be commonly wet when the boat moves.  The rudder fits tightly to the hull and there's a temptation to antifoul it right to the very top.  As a caution, I have painted out a "no go" area around the shaft's entry hole and with a no copper/no metal product.  I feel like a jerk asking for detail because the boat was completely lost for the rudder doing harm to boat's integrity.