It might be noted that the point of maximum righting moment, where the couple GZ is largest, is at deck edge immersion. Further heeling results in a decrease in the righting moment, which is the displacement times GZ. While the vessel is still stable and possesses righting moment, that tendency to right is diminished with every degree past deck edge immersion. Obviously the size and shape of deck structures as well as the flooding of cockpits, etc...only come in to play at these large angles of inclination.
Initial stability, as measured with an inclining experiment, is a measure of the vessel's stability at small angles of heel where hull form and freeboard are much less a factor in ultimate stability. Initial stability will give us the sense of whether the vessel is tender or stiff as well as the resultant motion in a seaway.
High freeboard, whatever it's other detractions, does impart a large angle of heel prior to deck edge immersion and the resultant decrease in stability. Significant dead-rise to the underwater hull form raises the center of buoyancy as well which will provide a greater righting moment at large angles of heel. Flared bows will also increase stability at large angles of inclination. To the extent that beam plays a role in stability, it is perhaps best understood to be one of more rapidly vanishing stability after the point of deck edge immersion. A beamy, high initial stability boat that feels quite stiff may fail to offer significant stability much past the angle of deck edge immersion or, perhaps better stated, after deck edge immersion the beamy vessel may experience rapid decreases in positive righting moment and start to feel very tender rather quickly.
As a result, I am somewhat puzzled with the fascination with beamy, plumb bowed boats, although open to enlightenment.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.