Ship’s Stability

Stability of a body can be defined as its tendency to its state of equilibrium after having suffered a momentary disturbance from its equilibrium position. If the body returns to its original position, it is said to possess positive stability with respect to its original position. If following the disturbance the body remains in its newly displaced position, it is said to be in neutral equilibrium. If the body continues to displace further from its original position after the disturbance is removed, it is said to possess negative stability with respect to its original equilibrium position. See the three stability conditions of ships in the figure below.

Stability of Ships During Dry docking

When a vessel enters dry-dock, it has a trim and hence the keel makes an angle with the dock bottom/keel blocks. The dock bottom may be slightly sloped to help draining of the water when the dock is emptied. However the keel block top may be horizontal.

As the water level falls due to pumping after the vessel has entered the dock, the keel of the ship touches the keel blocks. Vessels are usually trimmed by the stern and if so, the stern end of the ship touches the keel blocks first.

When a ship has just touched the keel block, the position is as shown in the sketch below.

P = upward force excerted by the keel block

M1 = metacentre of waterline W1L1

G = centre of gravity of the water-borne ship

W = displacement of water-borne ship

W-P = water-borne displacement at waterline W1L1

GM1 = virtual metcentric height of the ship at waterline W1L1

When ship’s stern just touches keel block, the force of buoyancy would be less than the weight of the ship, since the ship would be supported by the combination of buoyancy and the reaction of the bottom (P). This scenario is same as removing a mass from the ship, where ship’s stern touches keel block. The result is shifting center of gravity (G) upwards. This reduces metacentric height (GM).

So it is evident that there is a reduction in metacentric height during docking, when ship just touches keel block. This reduction must be taken into consideration to ensure that a positive GM is there during the entire dry docking period.