Rudder Shafts

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.


Back in the office after a week in Alaska. My room was number 5200. How cool is that?
I got the new composites construction manual into Pagemaker/Indesign and about 90% done. After about a week it will be in the do-no-harm stage and I can send it out on CD to KHSD composite boat builders. I can’t polish it until weather gets colder. Too many epoxy projects to do first.  What, $20 as a CD to not yet builders?  With free update when I finish in October, and deductable from plans in future?
The new CM construction manual is about 99% there. Is there any interest in seeing it as a print manual? Those would not have the hot links, but it would be a book. I have to see what the printers want for it and how much all-color photos hurts. Let me know.