I see that my 53/57 cruising catamaran now in the South Pacific, the Lil Explorers, has trashed a rudder. I understand they have done a repair by now. It looks in the picture like it absorbed a lot of kinetic energy. Note to future self. Keep the board down deeper than the rudder at all times. Its much easier to fix.
That was a 3″ OD shaft with .75″ wall on the plans. It moved. You wonder why I shake my head at those other designers who think 1/8″ wall is fine? My boats are designed to dry out on a tide flat with no rudder damage.