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Centre of gravity and centre of buoyancy

The weight of a vessel is distributed along its length, acting downwards over the entire structure.

However, we consider all the weight to be acting vertically downwards through one point which we call the centre of gravity (G). Consider a plank of wood, placed on a fulcrum, a seesaw if you like. Moving the plank back and forth you will find the point where the plank is balanced; this point is the centre of gravity. If the plank is perfectly even through its length, the centre of gravity will be exactly in the middle, If it is not even, G will be in such a position that the weight on one side will balance the other.

Note: the triangle in the diagram above represents the fulcrum or pivot point.

A ship is the same: all the weight is assumed to act downwards through the centre of gravity, G.


Having considered the weight of a vessel, we now look at the buoyancy opposing that weight. The hull of the ship is supported by water along its entire length.


Just as the weight of the vessel was assumed to act downward through the centre of gravity, the buoyancy force is assumed to act vertically upwards through a single point as well. This point is known as the centre of buoyancy (B). This centre of buoyancy is the centre of the underwater part of the vessel’s hull.


The two forces, weight and buoyancy, are equal and opposite. For a vessel floating at an even keel or upright, G and B are in the same vertical (centre) line. This will be true if we consider the vessel lengthways (longitudinally) or across the vessel (transversely)


The relative positions of the centre of gravity and the centre of buoyancy will determine the stability of the vessel, as we will see in the next part.