TRANSVERSE STABILITY OF SHIP : SHIP STABILITY AND THE MOVEMENT OF CENTRE GRAVITY ‘G’
- The centre of gravity of a body `G’ is the point through which the force of gravity is considered to act vertically downwards with a force equal to the weight of the body. KG is VCG of the ship.
- The centre of buoyancy `B’ is the point through which the force of buoyancy is considered to act vertically upwards with a force equal to the weight of water displaced. It is the centre of gravity of the underwater volume. KB is VCB of the ship.
- To float at rest in still water, a vessel must displace her own weight of water, and the centre of gravity must be in the same vertical line as the centre of buoyancy.
- KM = KB+BM. Also KM= KG+GM.
THE METACENTRE ‘M’ OF VESSEL
The point of intersection between an imaginary line drawn vertically through the centre of buoyancy of a floating vessel and a corresponding line through the new centre of buoyancy when the vessel is tilted.
The vertical distance between G and M is referred to as the metacentric height. If G is below M the ship is said to have positive metacentric height,and if G is above M the metacentric height is said to be negative.
- G below M
- Metacentric Height GM positive
When a ship which is inclined to a small angle tends to heel over still further, she is said to be in unstable equilibrium. For this to occur the ship must have a negative GM. G above M.
- G above M
- Metacentric Height GM negative
When G coincides with M the ship is said to be in neutral equilibrium, and if inclined to a small angle she will tend to remain at that angle of heel until another external force is applied. The ship has zero GM. Note that KG=KM.Therefore there is no moment to bring the ship back to the upright or to heel her over still further. The ship will move vertically up and down in the water at the fixed angle of heel until further external or internal forces are applied.