that does seem to be a weak spot in many steering gears. A lot of them depend on a small piece of key stock. For example a 1/4 x 2" SS key on a 3" shaft shears at around 1200 ft lbs torque. There are some pretty big boats depending on that or less. If it gets the tiniest bit of play then it will begin to work, and fail at a lower value.

With my spade rudder and carbon post, the connection gets more difficult to design. I came up with this and built it. The torque is taken by the ball bearings clamped into partially machined sockets in the two mating pieces. That allows it to be tightened to eliminate play. Secondary safety is the rectangular shape which would steer the boat if it had to. It was all machined from 316, the lower part bonded into the carbon post with Plexus. That part has to be small enough (and the tiller arm demountable) so that the rudder post can be dropped out through the roller bearings. Discounting any contribution from the bolts (not shown), I think it is good for around 12,000 ft-lbs. The rectangular tube sticks up through a few inches to accept an emergency tiller socket.