(a) Angle of Loll
The angle of loll is the angle at which a ship with an initial negative metacentric height (negative GM) comes to rest in still water. The ship is unstable in the upright position and therefore heels to either port or starboard until it reaches a position of neutral equilibrium.
At the angle of loll:
- GM = 0, or equivalently KG = KM.
- The ship is in neutral stability and remains at rest at this angle in calm water.
- If the ship is further heeled beyond the angle of loll by an external force, it may become unstable and capsize.
Conditions Under Which Angle of Loll Occurs
An angle of loll occurs when:
- The ship has an initial negative GM, making it unstable in the upright condition.
- This may result from:
- Excessively high centre of gravity (KG).
- Large free surface effect due to slack tanks.
- Improper loading or shifting of weights.
- Since the upright position is unstable, the ship heels to one side until sufficient form stability is developed, resulting in neutral stability at the angle of loll.
Corrective Actions
- First, determine whether the vessel is suffering from a list or an angle of loll.
- For safety, always assume it is an angle of loll and take corrective action accordingly.
- Calculate the contents of all tanks and identify any slack tanks, as these contribute to free surface effect.
- In a listed condition, always try to lower the centre of gravity (CG) by discharging ballast from the high side first, where appropriate.
- Begin ballasting the low-side tanks, preferably using small tanks to minimise free surface effect during filling.
- Do not ballast the high-side tanks first, as the additional heeling moment may be sufficient to cause the ship to capsize.
- Gradually fill the centre (midship) tanks, followed by the corresponding tanks on the opposite side.
- The ship should gradually return to the upright position. If not, continue ballasting other tanks using the same method until adequate positive stability is restored.
Important: During an angle of loll, never ballast the high-side tanks first, as the resulting heeling moment can rapidly shift the vessel to the opposite side and may lead to capsize.