Ryan,
now I went through the numbers (didn’t double-check...). If the boat has a
Disp = 3970kg, Beam = 2.667m and I use a guess factor of 0.20, the 'guessed' righting moment should be
Mr = 3970kg x 2.667m x 0.20 = 2118kpm
When using the given max righting arm =1.352ft = 0.412m the righting moment will be
Mr = 3970kg x 0.412m = 1635kpm ( = 77% of my guessed Mr)
My way of guessing the righting arm tends to result in slightly high values of Mr, but I guess it’s better to be safe than sorry, and if I manage to make a mast which doesn’t add more than 3% to the boat’s displacement, I am happy enough.
Now, if I didn’t screw up entirely, and the sigma of that 6061 alloy is 250MPa, the yield strength of that mast should come out at My = 4777kpm, which is over twice the MR, even using my conservative method.
The lower part of that mast therefore appears to me to be strong enough.
Arne
PS: Are we still talking about one of these rigs?