GZ curve and critical axis

I think that in the case of various designs of semis and other floating platforms, the critical axis for GZ may not always coincide with the critical axis of the waterplane inertia, which is why calculations of GZ for these vessels are made about different axes of heel, and the results are plotted to find the axis which gives minimum GZ.

Later work on the stability of semis and other unconventional vessels suggests the need to have software capable of not only assuming the vessel is free to trim and heel as usual, but also free to twist about a vertical axis as well. Because of the rather unconstrained nature of vessel motion, one winds up with results which form rather complicated three dimensional surfaces. In general, the combination of motions which requires the least change in energy determines the critical axis, or axes, of motion.

For example, see the papers listed and archived at:

http://www.gustomsc.com/index.php/zoo

particularly the paper presented at Stab2009 on energy build-up.

This later paper presents a more visually-oriented discussion of the results obtained from analyzing a floating body in a more general fashion:

http://www.shipstab.org/files/Proce...ance/01_Session_Offshore/ISSW_2013_Santen.pdf