But difference in the strain on the hulls is probably a lot more than you'd expect.

Now I didn't use the edging of the skis as you go into a compression turn off the top of a mogel and slide down the otherside, etc. as an analogy. But it is analogous. You do know it's happening because you are sensing it yoursel AND because if you kept your weight even, edges can't be set the same all through a single or double fall line course, etc. There are all kinds of different stresses which are being distributed througout a cat's system. Sometimes these stresses are momentarily greater, then distributed so as to equalize them; and sometimes, it's more consistently unequal.

A classic example of unequal, is when one hull becomes significantly raised and the other hull (hopefully) remains in the water. You don't need to bring the windward hull completely out of the water -- just raise one enough to move the rudder into the disturbed surface boundary zone. That position and affect of zone would vary with speed, seaway, etc. You avoid that condition for a variety of reasons, especially in a cruising cat...

Moreover, I think the rudder failures of the cat in question is supportive of the proof of signficant differences in forces on each rudder:

If you accept that the the same specific incident caused BOTH rudders to fail, then why such different failures unless the forces were different? One seems to have torsionally seperated from it's post (so that it now freewheels, lonely in its isolated floating condition); and, the Second bent while still held hard-over by the autopilot so that it is locked in it's jamed in its ture or against the hull. Was the quality control so different in their construction? or did the experience very different forces?

Even if the rudders failed at different times, one survived and the other was "toast". It would appear that the disengaged torsional failure was first, but you could make a case that it might have been otherwise. if the forces on the rudders were that same -- why not the same result?

If the bending failure was first, then you would conceed that it's likely that the forces are different because the SS tube's performance in bending would be a lot more consistent because it's not as much a composite experience. That would be "proof" of significantly different forces.

Of course a rudder will do it's job until the lateral forces begin to overload its ability efficiently to keep the boat on track and the boat will slide. If the rudder is designed properly, it will just lose effectiveness and not get "broken". Of course the resisting of "slippage" is not limited or principally the function of the rudder, especially THOSE proportioned rudders -- it's just a minor component of the whole hull and keel underbody system.

But I believe that there are differences in forces between the two rudders on a Cat; and, those differences become material in a robust seaway and strong winds. Properly designed and constructed rudders with strong steering linkage systems probably make that less noticable -- but the difference in foreces are still there.

I would suggest that there are all kinds of different stresses which are being distributed througout a cat's system. Sometimes these stresses are momentarily greater, then distributed so as to equalize them; and sometimes, it's more consistently unequal. You have a frame which is seeing all kinds of dynamic loading from above and below water conditions.