strength is strength, and loads are loads. The strength is reliably calculable. The loads are more elusive. However water is a pretty well behaved fluid, and the rudder cannot develop much more load that max lift coefficient (usually this is more that the drag coefficient even if the rudder is being dragged sideways). The unknown variable is what the instantaneous flow velocities are around the rudder.

There was a Swan built with load cells embedded throughout the structure as part of a Finnish university study. If you search for "SailLab" you may find the paper. Rudder loads were measured and found to exceed the elastic limit of the installed stock, which was a bit stronger than the ABS/ORC recommendations. It has been pointed out by a friend of mine that capturing short duration peak loads is nearly impossible on a sailboat, so this was probably not the worst case. There were a few other surprises in the paper as well, having to do with slamming loads and keel grounding loads.

Two things that bother me about a rash of recent boat abandonments: keels and rudders falling off. There aren't many things less convenient to lose on a boat than the keel and rudder. And it is generally easy and cheap to insure that these are strong enough for insure that the rest of the hull will be in small pieces at the point these items are lost. I have noticed that many NAs are not well versed in structural engineering, perhaps they should ask for help.