What is stable equilibrium, unstable equilibrium and neutral equilibrium?

There are 3 conditions of stability of a ship:
  • Stable Condition
  • Unstable Condition
  • Neutral Condition

A ship is in a stable condition of stability if, when heeled by an external force in still water to a small angle of inclination, it returns to the upright when the force is removed.

Consider a ship that is floating upright in still water, with the positions of G and B as shown, where the initial transverse metacentre (M) is above G.
i.e.  GM = KM – KG; which has a positive value.


The ship is now heeled by an external force to a small angle of inclination.


Consider the lines of action of weight force (Wf), acting downward from G, and buoyancy force (Bf), acting upwards from B1 through the initial transverse metacentre (M). The ship will want to return to the upright condition when the external force is removed.

In this stable condition the righting lever GZ is acting to upright the ship.
G is below M; initial GM is positive.