As long as the load of a ship remains stable, G is fixed (relative to the ship). For small angles, M can also be considered to be fixed, while B moves as the ship heels.
The metacentric height (GM) is a measurement of the initial static stability of a floating body[1]. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.