A ship is in an unstable condition if, when heeled by an external force in still water to a small angle, it continues to heel further when the external force is removed.
Consider a ship that is floating upright in still water with G and B as shown. The initial transverse metacentre (M) is below G.
i.e. KM – KG = GM; which has a negative value
The ship now heels to a small angle of inclination.
Consider the lines of action of Wf and Bf. They are acting in such a way to cause the ship to heel further over. GZ is a capsizing lever!