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→Centre of Buoyancy
=== Centre of Buoyancy ===
The centre of buoyancy (CB) is a counteracting force to gravity. It is the midpoint of the underwater volume of the boat, i.e., it is the centre point of the geometric shape of the hull. It is on the centre line of the hull, usually amidships with a vertical height just a bit more than half the draft.
The upward thrust of the CB counteracts the downward thrust of gravity. To illustrate this, float a bowl in some water. Put your finger in the centre and push down. The bowl will resist sinking and push back. Your finger is gravity. The resistance you feel is the buoyancy.
Plenty of hull area beneath the waterline lowers the CB. As a boat is more heavily loaded, increasing the draft, the CB moves lower, reducing the righting arm, and the freeboard and ultimate stability are reduced.
=== Righting Arm ===
When a boat is upright, the CB is above the CG, on the centreline. As a boat heels, the CB moves to the side in the direction of the heel. The horizontal distance between CG and CB is the righting arm (GZ). Heeling changes the underwater shape of the boat, and begins to move it toward a tipping point. As the edge of the freeboard meets the water, the outboard shift of the CB reduces and eventually changes direction as the boat heels further. This is caused by the change in the underwater hull shape. Obviously as the CB changes direction, the GZ is reduced.