Stable equilibrium
A ship is said to be in stable equilibrium if, when inclined, she tends to return to the initial position.
For this to occur the centre of gravity must be below the metacentre, that is, the ship must have positive initial metacentric height.
Figure hows the ship inclined to a small angle. The position of G remains unaffected by the heel and the force of gravity is considered to act vertically downwards through this point. The centre of buoyancy moves out to the low side from B to B1 to take up the new centre of gravity of the underwater volume, and the force of buoyancy is considered to act vertically upwards through B1 and the metacentre M. If moments are taken about G there is a moment to return the ship to the upright.
Unstable equilibrium
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.
Note how G is above M.
Figure shows a ship in unstable equilibrium which has been inclined to a small angle. The moment of statical stability, W GZ, is clearly a capsizing moment which will tend to heel the ship still further.
A ship is said to be in stable equilibrium if, when inclined, she tends to return to the initial position.
For this to occur the centre of gravity must be below the metacentre, that is, the ship must have positive initial metacentric height.
Figure hows the ship inclined to a small angle. The position of G remains unaffected by the heel and the force of gravity is considered to act vertically downwards through this point. The centre of buoyancy moves out to the low side from B to B1 to take up the new centre of gravity of the underwater volume, and the force of buoyancy is considered to act vertically upwards through B1 and the metacentre M. If moments are taken about G there is a moment to return the ship to the upright.
Unstable equilibrium
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.
Note how G is above M.
Figure shows a ship in unstable equilibrium which has been inclined to a small angle. The moment of statical stability, W GZ, is clearly a capsizing moment which will tend to heel the ship still further.
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