TRIM AND STABILITY

Every hull can move around a trio of axes: longitudinal, transverse and vertical.
The oscillatory movement around the transverse axis is called 'pitching', the movement around the longitudinal axis is called 'rolling', and the directional movement around the vertical axis is called 'yawing' or 'rolling'.
Attitude' refers to the position that a boat assumes. Stability' refers to the ability of a vessel to regain an equilibrium trim, i.e. the ability of a vessel to return to its initial equilibrium position once the action of waves, wind or other events that disturb the equilibrium have ceased.
Longitudinal stability counteracts pitching, i.e. it tends to make pitching oscillations smaller and smaller until they become zero. Transverse stability, on the other hand, counteracts rolling.
One can see, therefore, how stability, especially transverse stability, plays a very important role in safety.
The stability of a boat depends on the reciprocal position of the centre of gravity and the centre of thrust. The position of the centre of gravity depends on the arrangement of all the boat's weights, including the weights of structural parts and fuel. The centre of thrust, on the other hand, depends on the heeling of the hull, which causes a change in the shape of the hull that is submerged.
The metacentre is defined as the point of intersection of the hydrostatic thrust with the plane of longitudinal symmetry of the boat. In order for a righting action, known as a 'righting torque', to occur after heeling, the metacentre must always be above the centre of gravity. If this is not the case, an action, called 'capsizing torque', is generated, which leads to the boat capsizing.
The distance between the metacentre and the centre of gravity is called the 'metacentric height'. The greater the metacentric height, the greater the righting torque will be. Thus, a boat will be the more stable the more the position of the centre of gravity is shifted downwards. In other words, to have a stable boat, it is better to place the weights as close to the plane of symmetry and downwards as possible. It is also worth remembering to always pay attention to so-called 'liquid mirrors', i.e. all those deposits of liquid substances, such as water in the bilge or in some tanks, which can all accumulate on one side of the boat during heeling.
Equilibrium is said to be 'stable' when the centre of gravity lies below the metacentre, 'unstable' when the centre of gravity lies above the metacentre and 'indifferent' when the position of the centre of gravity coincides with that of the metacentre.
A distinction is also made between 'stability of form', typical of hulls with a wide base of support on the water, and 'stability of weight', typical of sailing boats.