Longitudinal stability

Longitudinal stability is the ship’s resistance to changes of trim, governed by the longitudinal metacentric height GML, which equals KB plus BML minus KG. Because the waterplane’s longitudinal moment of inertia is large, BML and GML are on the order of the ship’s length, so longitudinal stability is far greater than transverse: ships essentially never capsize end over end. GML drives the moment to change trim one centimeter, MCT1cm = (displacement * GML) / (100 * L), the working quantity for predicting trim from a weight shift. The concern in practice is not loss of longitudinal stability but controlling trim within the limits set by draft, propeller immersion, and the load line.