Angle of loll is the angle a vessel heels to and settles at when her metacentric height (GM) is negative — she cannot remain upright and stable at zero degrees of heel.
Why It Happens
At zero heel, a ship’s righting arm (GZ) depends on GM: with positive GM, any small heel generates a righting moment that brings her back upright. With negative GM, the opposite happens — a small heel generates a further heeling moment, so the ship keeps rolling away from upright. As she heels, her waterplane widens and the centre of buoyancy shifts further outboard, and at some angle the righting arm becomes positive again. That angle — where GZ crosses back to positive despite negative GM — is the angle of loll, and the vessel settles there in a genuinely stable (if unwelcome) equilibrium.
Common causes of negative GM include a slack or partially-filled tank (free surface effect reducing effective GM), ice accretion or added weight high on the ship, loss of low ballast, or a cargo/bunkers plan that leaves the vessel top-heavy.
Angle of Loll vs a List
These are frequently confused but are different failures:
- List — caused by asymmetric weight distribution (cargo shifted to one side, an off-centre tank) with GM still positive. The ship heels to a stable angle proportional to the offending moment, on the side the weight sits. Correcting it means moving the weight back toward the centreline.
- Angle of loll — caused by negative GM itself, independent of any asymmetric loading. It can develop with perfectly symmetrical cargo and ballast, and the vessel can loll to either side — including flopping from one side to the other with no obvious external cause.
Why the Wrong Correction Is Dangerous
The instinctive response to a heeling ship — counter-ballast the low side, or bring weight up to level her — is exactly wrong here, because GM is still negative. Forcing the vessel toward upright while GM stays negative does not restore stability; it can instead cause her to suddenly flop over to the opposite side, often through a wider and less predictable angle than the original loll.
The correct sequence is to lower the centre of gravity first: fill low, centreline double-bottom tanks slowly and symmetrically to restore positive GM. As GM comes back above zero, the vessel should return upright on her own without needing to be forced there. This counter-intuitive order — fix GM before touching the heel — is exactly why angle of loll is such a heavily tested topic in DG Shipping oral examinations at every level.
Exam Relevance
Angle of loll, the GM-negative mechanism behind it, and the correct low-ballast corrective sequence are standard questions in MEO and Mate/Master oral examinations, precisely because a wrong answer here describes a genuinely dangerous action.