You can get a lot of torque out of the shaft. If you want to measure the same turning capability between a "barn door" (pejorative term, I apologize for) and either a hydrodynamically balanced spade rudder, you actually need a much larger surface on the barn door. That means much larger forces and loads on the shaft of the older design.

In the paper-covers-rock world that leads to two problems, either undersized full-keel-with-attached-rudders or a lot of weight and forces to deal with. In the case of balanced spade rudders, the efficiency is an opportunity to under-size for extreme loads.

I agree that having an attached (and protected) barn door is less likely to be damaged by being hooked or grounded-down. In the meantime, there are huge number of boats that have better all around performance and hold-up very well which have properly designed, constructed, and maintained spade rudders. The skeg keel units, share more with the spade designs.

The design of the system is critical, the robust retention of the quadrant/shaft/etc., construction of the rudder itself and it's bonding to the shaft, etc. On our J/Boat, the rudder extends down 6'3" or more; and, our modified fin keel is down 7'3" below the water. We have a very robust carbon fiber tapered tube that is almost 6" in diameter at the top. This is from our the J/Boats boat's standard specs

STEERING
• Balanced high aspect rudder constructed with unidirectional glass.
• High strength custom engineered and molded carbon fiberglass rudder shaft, to provide maximum strength in case of collision.
• Tides Marine “Strong” self-aligning rudder bearing bearing.
• Edson 66" Diamond series aluminum wheel with padded elk hide covering.
• Edson pedestal top with stainless guard, 5” Ritchie binnacle compass and throttle control.
• Gear ratio of 1.2:1 for finger-tip steering control and to facilitate installation of self-steering devices.
• 8” Edson steering sheaves with needle bearings and guards.
• Aluminum anodized emergency tiller.
• Tripod anodized aluminum rudder stop.
• Edson tiller arm bolted to rudder shaft in
• Removable platform in front of rudder quadrant to maximize storage and protect steering cables from loose gear.