Beyond the tube itself, there is the issue of fixed restraint and tube attachments (How much bending, etc.)

As for "massive", we have a 6" diameter tube. Where it's hollow, the thickness on the drawings is 1" on each side; and, the length from the bottom of the bearing on the tube supports to the top of the rudder is about 2", or less.

I think that mASSive is a hyperbole that should be avoided here. Simply looking a a profile of the boat, it has lateral forces that are imposed by water driven effect on the hulls which are much greater than just it's displacement would infer. To have control surfaces that are meaningful (i.e., area of the rudder, and it's depth) require the ability to handle the extreme "torquing-twisting" and bending.

If you took a Hobbie Cat and made it three times as long - and 30 times (or whatever) the displacement/projected area (or whatever), then you'd be muliply the rudder tubes on the little Hobbie Cat by more that three, by a long shot.