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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.
=== Ballast Stability ===
Ballast is weight added to a boat below the waterline to counteract the effects of weight above the waterline. <ref>https://en.wikipedia.org/wiki/Ballast</ref> Without the ballast a vessel witll be very tippy and happier upside down.
On a sail boat, ballast must counteract the lateral forces on the sails. Without this a sailboat will lay down in the water and capsize.
Ballast is usually placed in the keel, which acts as a lever, so you don't need as much weight below as above. The keel is filled with a high density material, such as concrete, iron, or lead. By placing the weight as low as possible (often in a large bulb at the bottom of the keel) the maximum righting moment can be extracted.<ref>https://en.wikipedia.org/wiki/Sailing_ballast</ref> However, this will increase the Roll Moment of Inertia, which can be avoided by distributing weight instead of concentrating it.
Adding excessive ballast will make the roll motion more aggressive and less comfortable. Extra ballast will reduce the roll angle but the return will be snappier with a higher roll acceleration and more conducive to seasickness. <ref>http://www.kastenmarine.com/beam_vs_ballast.htm</ref>
A ballast tank, found on larger vessels, holds water to balance the boat. Water can be pumped from side to side to counteract rolling. On large cargo ships travelling empty water can be pumped in to lower the centre of gravity and keep the propeller and rudder submerged.
=== Static Stability ===
Static stability determines the angle of heel under constant wind or wave conditions. Factors that increase static stability are heavy displacement, low centre of gravity, and a centre of buoyancy that shifts outboard quickly when the boat heels. Boats with wider beams exhibit more static stability (stiffness) and less dynamic stability.