The monohull example would be considered *seriously* under-ruddered if deep keeled. No responsible NA would draw that kind of rudder unless required by the shallow draft design brief. Cats are almost always shallow draft, either with stub keels or retracting daggerboard and so the rudders must be made either a shallow compromise, or retractable. Retractable is expensive to build correctly and isn’t often done except for small cats. Sure you can steer with a small rudder. But not as well as a deep one.

But those rudders are still under designed: simple assumptions, 2’ depth by 1’ chord for 2 sq ft, uniform lift distribution. At 15 knots the dynamic pressure of water is about 625 lbs/ft^2, so 1625 lbs at a lift coefficient of 1.3. 12” moment is 19,500 in-lbs, well above the 11,500 in-lbs yield point for the 1.5” shaft. It is then no surprise that they failed, they were designed to do so. They were near yield* at 10 knots.

2” shafts would have had a modest safety factor of about 40% but would still be well exceeded on the odd 20 knot surf. If it were my boat I would want at least 2.5 or 3” rudder posts to have an acceptable safety margin. There is clearly room in the rudder section for that.

(One small side comment: provided the section is correctly chosen, a trapezoidal planform is just as good as the swoopy shapes that some designers seem to prefer. If you happened to get the swoopy shape exactly right - very expensive analysis required - you will get a couple of percent less drag but no more lift. You are more likely to end up worse off, than better.)

*I take the yield point of 316/304 stainless steel to be about 35KSi. Some NAs seem to use a figure like 70 or 80KSi. Stainless in its normally supplied state will bend at 35KSi, work harden as it deforms, and finally fracture at 70 or 80Ksi. This is why you sometimes see the ultimate tensile listed as 80KSi. However engineers usually require that any critical structure be designed to use the material within its elastic limits (if not its generally lower fatigue limit). For SS, the so called 0.2% yield point is around 35KSi - this is the stress that will cause permanent elongation of 0.2%, the normally accepted definition of yield. A bent rudder is a failed rudder, in my opinion.