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 inchpounds 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 34 to 36 of the Leeson book)
Regards,
Pete
KS4XG

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