Angle of Loll

The angle of loll is the angle at which a vessel with a negative metacentric height (GM less than zero) comes to rest on her side in a state of stable—though unsafe—equilibrium. A vessel with negative initial stability is unstable at zero heel and will fall to one side until, at the angle of loll, the righting lever curve crosses zero and the vessel finds a new equilibrium. The ship will roll between the angle of loll to port and the angle of loll to starboard, but will not return to the upright. The angle depends on the degree of negative GM and the shape of the underwater hull; it may be only a few degrees or may be severe enough to cause cargo shift or flooding. The chief mate recognises loll by the vessel's sluggish rolling and reluctance to return upright, combined with a negative GM shown in the loading computer. The immediate danger is that the vessel may loll to the side where free-surface liquids shift, deepening the negative GM further and accelerating the heel. The corrective action is to lower the centre of gravity by adding ballast into double-bottom tanks on the low side first—adding water to the high side ballast tank could cause a violent and dangerous rollover—or by discharging high cargo weight. The IMSBC Code and the ISM Code require that any loading condition that risks producing negative GM be identified and corrective action planned before the vessel leaves port.