The Southern Cross 31 is a pretty solid boat, with circumnavigations to its credit. While it has a cored hull, the hull is very solid, provided it hasn't had any water penetrate the core.
BTW, from my calculations, the SA/D should be more like 13.25 or so, rather than the 12.55. I am basing this on the following specifications:
LOA 31', LWL 25', Beam 9.5', Displacement 13600, SA (Main + 100% Jib
) 472 sq. ft. The main and 100% Jib
areas were calculated based on the I, J, P, and E measurements of the boat, which are 36.5, 15.5, 31, and 12.2 respectively. Mainsail area is approximately (P*E)/2 or (31*12.2)/2 or 189.1 sq. ft. 100% Jib
are is approximately (I * J)/2 or (36.5 * 15.5)/2 or 282.9 sq. ft.
These numbers are probably a bit conservative, since they assume a mainsail with no roach area whatsoever, which is somewhat unusual today. BTW, the numbers for the SC31 are from the sail specifications section of the MauriProSailing website. The displacement, LOA, LWL and Beam are from several different sites, which all had them listed the same.