Ski's are moving through and on completely different media, transitioning in and out of incompressible and completely compressible media (not to mention completely different PHASE STATES of media), having different parts of their structures in different physical media at once, being dynamically changed in attitude and altitude relative to each other, etc. It isn't just an extreme example, it is an absurd example. With absolutely no relevance to catamaran rudders in any way.

Yes, catamarans definitely experience lots of different and changing forces. They are designed to not only resist those forces in some places, but to actually accommodate them in others. They twist and rack for sure, but are designed for this. You are very much over-analyzing these forces and putting way too much emphasis on them. Particularly when it comes to the rudders. Perhaps you have just recognized this difference between mono and cat and are thinking about it for the first time. But you are bringing the wrong eye and mindset to the issue.

Here is what I find most amusing about all of the "expertise" in this thread (besides the fact that few of the "experts" actually have any catamaran sailing or design experience) - NOBODY ACTUALLY KNOWS IF THE SHAFT BENT OR HOW IT HAPPENED!

We know that the webbing inside one rudder broke so that the rudder spun free on the shaft. We know that one guy without any technical knowledge at all looked over the side of the boat in open ocean in rough weather and said that the other rudder was bent to a large angle.

We don't, however, know if that rudder even had a bent post. It is possible that rudder also had broken webbing and had broken the skins and was jammed at an angle with a perfectly straight shaft. Or that the webbing could have partially broke and put that rudder at a stalled angle that caused the shaft to bend - and this would have happened even if the shaft was 3". Or that it was jammed up into the hull at a large angle of attack that only looked like it was bent to an untrained eye under a dynamic platform in large seas.

Until that boat is recovered and the rudder situation determined for real, we don't know what happened, if the shaft is bent, or even if the shaft was a component of the problem. We have no actual knowledge of what was wrong and how it went wrong on a one-off build unrelated to any other boat at all.

Except for the "experts" here, that is.

Now, having said that, I do believe there was scantling, design and build issues with those rudders on the Alpha 42. I also think 1.5" shafts were under-sized for it. However, I think you and others are way too focused on that shaft diameter (let alone the loading dynamics on two foils in the water). I don't think that was a major factor in their problems. I think the welding was bad, the webbing poorly designed, and the rudder design itself seemed to have way too much leading edge area.

Did anyone else notice those rudder shapes and the position of the posts in it?

Mark