States of Equilibrium ships Stable Equilibrium, Unstable equilibrium,Neutral equilibrium


For a body to float the centre of gravity and centre of buoyancy should be equal and opposite and acting on the same vertical line . Depending on the positions of the centre of gravity and the centre of buoyancy the states of equilibrium of a floating ship is classified into three namely,

. Stable Equilibrium
. Unstable Equilibrium
. Neutral Equilibrium

1. Stable Equilibrium
In this type of equilibrium the position of the centre of gravity(G) of the ship will be below the transverse metacentre(M). Transverse metacentre is the point of intersection of lines of action of buoyancy force in initial upright condition and at subsequent heel angles. As the ship is heeled the centre of buoyancy (B) shifts to B1. Therefore a lever is formed which helps the ship to come back to its upright condition after being heeled. This is called the righting lever. The righting lever(GZ) is the seperation between the vertical lines passing through Centre of gravity (G) and the shifted buoyancy (B1). The moment which makes the ship to come back to its upright condition is called Righting Moment. So if the ship is heeled to an angle it comes back to its upright condition.This type of equilibrium is called the Stable equilibrium. In stable Equilibrium the GM(Metacentric Height) is always positive.

2. Neutral Equilibrium
In this type of equilibrium the centre of gravity (G) and the transverse metacentre (M) coincides at the same point. This is a very dangerous situation onboard. As both are at the same point there is no seperation between the vertical lines that is, there is no righting lever. Hence, once the ship heels to an angle it continues to remain at that angle . This equilibrium is called the Neutral equilibrium. When the ship is in Neutral equilibrium some unwanted cargo shifts may cause the unstable Equilibrium. In Neutral equilibrium the GM is zero as both G and M are on the same point.

3. Unstable Equilibrium
In unstable equilibrium the position of centre of gravity (G) is above the Transverse metacentre (M). As the ship heels to an angle centre of buoyancy B shifts to B1. This creates a lever , but in the opposite direction . When the ship is heeling to an angle this lever makes the ship heel further more until the ship finds a stable Equilibrium ( G below M).If stable equilibrium is not reached, this will continue and will result in the capsizing of the ship. Therefore the lever created is called Capsizing lever. This type of equilibrium is called Unstable equilibrium. The GM at Unstable equilibrium is less than zero or negative.