Here's the basic definition from the linked article: ----- The free surface effect is a mechanism which can cause a watercraft to become unstable and capsize. It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior approximates that of liquids — to move in response to changes in the attitude of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea states caused by waves and wind acting upon the craft). ----- That certainly sounds like the description of the phenomenon in *practical* terms. But for anyone interested in what is the basic thing going on in math or physics, it's *so vague* that it's not much help: "The free surface effect is the tendency for [lots of things] to move in response to [lots of other things] [caused by motion of a vessel] [which itself may be caused by ... lots of things]" —Dan ----- ... free surface effect: https://en.m.wikipedia.org/wiki/Free_surface_effect ..... -----