Why should uppers be thinner than lowers?

[cmm]

I wish there were more evangelistic operators in the yachting industry - I'd love my engine installer to become evangelistic about using stainless fasteners in my bilge.
Thanks, I fully take your point, Brion, and re-reading your earlier post "the lowers should be sharing less than half the total load", I interpret that to mean a pair of fore and aft lowers working together share less than 50% of the total load on one side of the boat, and so each lower carries less than 25% of the load (and therefore need not be as strong as the 3/8" upper shroud).

Although it hasn't been said in the thread, I presume Matt's Valiant has Upper and Intermediate shrouds, and that the "total load" referred to by Brion is the sum of the Upper, Intermediate and Lower loads.

Martin