A recent video on "sailing Delos" shows how they help a cat that had bumped one of its spade rudders and was flooding: the shaft was bent so no steering and at the front of the rudder the hull skin was pulled off and at the back the skin was pushed in. Stuffing cloth strips from under the hull and in the hull stemmed most of the inflow but this was only possible in the perfect calm of the marina where the accident happened at low speed. The safety lesson was to only trust skeg mounted rudders.

I understand the Melbourne proa has stocky rudders with lower leverage on bending a proportionally thicker shaft. A vital precaution. Would it be possible to calculate roughly what speed this rudder could take on in an under water collision?

The only vulnerability of the Denney rudders would be sidewise where the rudder attachment should give way more readily than the wall of the hull to which it is attached. In addition that wall tear would need to rip further down to the waterline to become dangerous. 

Any other insights ?