Often, increases in stresses are effectively to the proportion to powers of 2, 3, or 4. Shock loads vs. simple sustained loads have huge effect differences. Doubling one thing might effectively mean 4, 8 or 16 times different stresses or forces.

Friction or adhesion within imbedments goes through different stages as one loads up an assembly: an elastic periods (when it will give, but bounce back) to a plastic period (where it will rebound less, and less) until it is effectively parted.

Because there are so many different combinations and sequential elements, a lot of the design parameters arrived at by a combination of analysis and experience, trial testing, and just adding safety factors on critical members.

This is made more challenging because composite materials and assemblies aren't uniform in their restraining tension vs compression. Some materials have variations in performance depending on all kind of things such as uniform mixing, temperature, cure time, etc.

Our, say, 32,000 pound boat has a very deep rudder with a 6" diameter carbon fiber rudder tube. It's design and capacity is way beyond the simple proportion of displacement differences or certainly length differences.

In the case of the Alpha 42, I think the issues described of one rudder having its connection to the actual rudder body by torsion is troubling. The other rudders bending is also troubling -- this wasn't a case of a collision with a fixed object at speed. In the first case the interface of the tube would seem to have internally failed inside the rudder. In the second, the tube itself's inadequacy in bending would seem to be revealed.

Examples, if the issue was limited to the tube diameter...You could solve the problem of a tube being too weak one of two ways: decease the rudder size; or increase the rudder tube. At speed, you can live with a smaller control surface but will not have the ability to control the boat at slower speeds. The there is the issue that making little course changes vs dramatic ones dramatically increase the forces. "Hard Over!" When you tack, or a sal gets loaded up is a dramatic increase in stresses.

The diameter of tube (with adequate wall thickness) has more to do with its strength in bending than whether it's simply solid or hollow. The ability it restrain torque is also not simply proportional.

Boats big windows, ports, etc leak a bit on occasion. Frame trim getting washed away is maybe a different story ;^)))