Angle of loll
As we know that KM increases as angle of heel increases until it reaches a maximum value at some large angle of heel. As an unstable vessel heels over more and more, it may happen that, at some angle of heel, KM increases sufficiently to equal to KG. The vessel would then be in neutral equilibrium and the angle at which this happens is called the angle of loll.
In other words, when an unstable vessel heels over to progressively increasing angles of heel, it may happen that, at some angle of heel, the COB may come vertically below the COG. The vessel would then be in neutral equilibrium. The angle of heel at which this happens is called the angle of loll.
Remedial Action
1) Press up all slack tanks.
2) Run up SW into the DB tank which has the smallest moment of inertia about its centre line. If this tank is not on the centre line of the ship, then on the lower side first, and after it is full, its counter part on the higher side.
3) Repeat action 2 with another tank and so on until the ship becomes stable.
4) If discharging or jettisoning deck cargo, do so from the higher side first, then from the lower side. If using ship’s own gear, due allowance must be made for the shift of COG, of each sling of cargo, from the upper deck to the crane head during the operation.
Justification for the above action:
At the angle of loll, any existing free surface effect must be eliminated/minimised first. FW or HFO may require to be transferred internally such that the tanks finally remaining slack are those with the smallest moment of inertia about the tank’s centre line.
While running up ballast into a DB tank, FSE would be created. This must be kept to a minimum.
The necessity to fill up the tank with the smallest about its contre line is, therefore, vital. So also, the necessity to fill up only one tank at a time. If the tank being ballasted is not on the centre
line of the ship, but on either side like No: 2 P and No: 2 S, then fill up the lower side tank first.
One might think as if filling up the higher side tank would produce better results but it is not really so. The ship can loll to either side, If after the higher side tank is run up, wave action caused the ship to toll over to the other side, the ship would topple over and the momentum of flopping over will carry the inclination well beyond this. Since the GZ formed near the angle of loll is very small, the ship would heel over to the other side much more and take a very long time to return to this angle of inclination. If during this time:
(a) any openings went underwater &/or
(b) a wave struck the ship adversely and/or
(c) any cargo shifted, the ship may capsize.
By filling up the lower side tank first, the inclination would increase a bit at first, but this would be gradual and would last only until the tabk on the other side is also run up. The same line of reasoning is applicable when considering discharge or jettision of cargo from the upper deck.
If the ship is in calm waters, such as inside a dock, the possibility of flopping over to the angle of loll on the other side may not be there. In such a case, ballasting the higher side tank or discharging deck cargo from the lower side may prove more effective and immediate.
