How much stronger is a solid stainless rudder shaft compared to a tube? Lets see. I can’t attach excel files but I can put the equations here so you can copy and past into excel.
A tube equation for section modulus SM, for bending calculations is =(0.098168*((E3^4)-(E4^4)))/E3 where the OD is at E3 and ID is at E4.
Moment of inertia (I) formula for deflection calculations is =0.04908*((E3^4)-(E4^4)) again same for OD and ID.
For a solid tube put zero or near zero in for the ID. Oddly enough none of my references have a simple solid section properties equations, so I do this.
For section area, to compare weight, use =(6.2832)*(E3/2)*(E5) where E3 is ID again and E5 is wall thickness.
My thoughts on rudder shafts are that the fattest, thickest tube that will fit easily into a 13% section, with the top bearing about at the same distance from rudder top as from the center of effort, will be right for sailing multis. It will also meet ABS ORY. The one exception is sailing cats with big engines. A pair of caterpillar 500 hp diesels bent anything this side of a solid shaft some years ago and took us to school.