Jeffrey,
I certainly don’t dare to make very strong rules here. From the early days of western junkrigs there have been two schools of thought:
Hasler’s and McLeod’s work resulted in very stout wooden masts. Even after I had trimmed their scantlings down a bit, those masts came out very strong (and heavy).
On the other hand, there were the junkrigs from Sunbird Yachts with masts from Needle Spars. These masts were terribly thin, and could be seen bending and whipping even under moderate loads. The photo below, of a Sunbird 27, was nicked from a brochure dated 1983. Nevertheless, these masts held up, at least for a number of years, when a few of them suddenly came down.
The lower section of the mast of my IF, Ingeborg is Ø150 x 5mm with a yield strength of about 250MPa. That gives a max bending (yield) moment of about twice the righting moment of Ingeborg. This appears to work fine, without anything near the ‘Needlespar bend’, even when sailing Ingeborg on her ear. However, I only do coastal and inshore sailing. When writing Chapter 6b about the aluminium-wood mast, I therefore suggested one should go up 50% in strength if one plans for offshore cruising. I can only guess on this. However, I also keep the weight of the mast in mind. Ingeborg’s mast only adds 2.5% mass to the boat’s. The last hollow wooden mast I used on my Albin Viggen, Malena added about 5% to her displacement. The extra inertia was noticeable in a headsea, but I would not call it a show-stopper.
As for deciding the scantlings for taking a full roll-over - I have no idea - I don't do rollovers...
Arne
