This is the angle that the vessel is presumed to heel given a force of 1 pound per square foot on the sails, assuming they were all sheeted flat amidships. It is an approximation only, and is based on the upright stability characteristics of the vessel. Being a very common and easily done calculation, it is highly useful as a preliminary tool for comparing one vessel to another in terms of a boat's relative power to carry sail. ... Of course, these “predictions” only consider the boat at rest in calm water -- they do not predict how the boat will behave when it is in motion in a seaway. Nevertheless, these are the standard means of comparing one boat to another, and for such purpose they are highly valuable. So if your boat heels differently when in motion than the equations predict, it is to be expected!