For the folks that enjoy knowing how to do this stuff yourselves:
Section modulus(of a hollow cylinder) = pi*(D**4 - d**4)/32D
Where D is the outer diameter and d is D minus two times the inner diameter,
"*" means multiply and "**" means raise to the power of (and pi is 3.1415).
If you divide the section modulus into the inch-pounds of force on the mast
you get the bending stress which must be less than the yield strength of the
material to avoid damage.
(From pages 3-4 to 3-6 of the Leeson book)
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