To a large extent, you'll have to buy what the manufacturers are offering you, but you won't want to go to less than 4in diameter at the top - a mast that whips around at the top is not good, but for a 10in mast, 4in top diameter is OK, and a reasonable compromise between weight and windage on one hand, and flexibility on the other. Some of the flagpoles go down to less than that, and need trimming off. 

Scott Dufour wrote:

Regarding the mast size - who here can help me out with the math behind validating our rough guess of 10"?  What are we thinking for taper, assuming that I could order whatever I liked?

This is Eric Sponberg's tech article on designing an unstayed mast for carbon fiber. Lots of math. I thought the most important eqns are 10 and 11 in appendix A.

The basic idea is to match the mast load (in ft-lbs) at the deck with full sail up and say, a 30 deg heel to the bending moment of a round section (in your case) aluminum mast plus a safety factor.

David Tyler's intuition is spot on. For your boats 600 ft sq sail, in 12 knts wind speed and sail's CE 23 ft abv the water line (my guesstimate)  I get a 10,000 ft-lb bending moment using Sponberg's methods. A 10" OD x 1/4"Wall, 6063-T6 aluminum mast with 21,000 psi compressive strength in theory will withstand a 32,000 ft-lb bending moment which is about 3X more than required.

Huh. Is the CoG of a boat with a junk rig lower than the CoG of a Bermudian?  Or are you just saying that the mast, though heavier, has a lower CoG?  Intuitively I would think that both the CoG and CoE on a junk conversion is going to be higher than its Bermudian original.

Regarding the mast size - who here can help me out with the math behind validating our rough guess of 10"?  What are we thinking for taper, assuming that I could order whatever I liked?

Thank you for the advice, David.  That's a big mast, isn't it?  Is that pushing the limit for weight, do you think?

I hope this is not a hijack, but a continuation of the intent of the thread.  I'm looking for feedback on mast requirements for Mudita.  She's 5567 kg displacement

I've got this table of data - but what does that mean in an 14m aluminum tapered mast with about 1.5M of bury?   I have a source here in the US for flagpoles and lightpoles, but they were iffy on giving me a quote without full specs. 

Heel(deg)     RT Arm(m)    Rt Mom(Nm)

25                 0.39            21523

30                 0.44            24136

60                 0.62            33591