Changes

The righting moment is a force generated by the righting arm (GZ). The righting arm is the transverse distance between the centre of gravity (CG) and the centre of buoyancy (CB).<ref>A Best Practices Guide to Vessel Stability, Guiding Fishermen Safely Into the Future, Second Edition, United States Coast Guard, http://www.uscg.mil/hq/g-m/cfvs/</ref> Hopefully this will become clearer as you read on.

The centre of gravity (CG) is the point inside the hull where the downward force of gravity equals the weight of the boat, i.e., its displacement. It is the midpoint of the mass. Keeping weight low in the hull lowers the CG. A low CG increases stiffness, i.e., resistance to heeling and capsizing. That’s why engines are mounted low, ballast is put in the keel; and heavy superstructures or loads on deck are bad. Makes you wonder about dinghies on the boat deck.

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.

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.

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.

Righting Moment = GZ*D