BUOYANCY

By Archimedes' principle, every body immersed in a liquid receives a direct upward thrust from below that is equal to the weight of the volume of liquid displaced. This thrust is due to the presence of the liquid and is called 'hydrostatic thrust'.
In reality, Archimedes' principle is valid for all bodies that are immersed in a fluid. This means that this principle applies as much to bodies immersed in a liquid, such as boats, as to bodies immersed in a gas, such as airships.
Therefore, a body floats when the hydrostatic thrust acting on it equals its weight force. A body, on the other hand, submerges when the hydrostatic buoyancy is less than the weight force. Both the weight force and the hydrostatic thrust act in a vertical direction, but while the hydrostatic thrust acts from bottom to top, the weight force, on the other hand, acts from top to bottom.
When a boat increases its weight, which happens, for example, when passengers board, its hull, i.e. the structure that constitutes the boat's outer 'shell', increases its 'draft'. In other words, its hull, i.e. the immersed part of the hull, 'displaces', i.e. displaces, a greater volume of water and thus increases the hydrostatic thrust until a new balance between hydrostatic thrust and weight force is established. Thus, a floating boat 'displaces' an amount of water equal to the weight of the boat itself.
The weight force can be considered to be applied at a point, called the centre of gravity or 'centre of gravity'. Hydrostatic thrust is also a force and can be considered to be applied at a point, called the centre of thrust or 'centre of hull'.
The hull is also called the 'centre of gravity' because it is responsible for the main contribution to buoyancy. The part above the waterline, which, to give you an idea, defines the boundary between water and air, is called the 'dead work'.
The 'waterline length' is that length between the perpendiculars to the waterline at the bow, the front of the boat, and at the stern, the rear. Waterline length should not be confused with 'overall length', which is the maximum overall length of the hull.
The 'buoyancy reserve' is proportional to the 'freeboard' of the hull, i.e. the distance between the waterline and the deck. More precisely, the buoyancy reserve is the difference between the total buoyancy due to the volume of the hull and that generated by the volume of the hull under full load. The buoyancy reserve could be said to be the ability of a hull to take on 'weight' without sinking.
However, the buoyancy reserve should not be confused with the 'carrying capacity' of a boat, which instead consists of its 'displacement'.
Displacement' refers to the weight of the hull expressed in tonnes, while 'tonnage' refers to the volume of a hull that is intended for merchant use. Tonnage is a measure that is only used for commercial purposes and one should not be misled by the fact that it is expressed in 'tons', because, in reality, one ton of tonnage refers to a volume of 2.83 cubic metres.
Gross tonnage' refers to the overall volume of the hull, while 'net tonnage' refers to the volume of cargo and passengers.