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→Roll Stability
Initial stability defines the angles of heel that are normal to a vessel's operation. This is also the static stability. This is usually between zero and 15 degrees of heel. A wide-beamed boat heels less (has greater stiffness), and is more comfortable. But a narrower-beamed boat has more ultimate stability. Ultimate stability is the angle of vanishing stability.
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) [28]. <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.
=== Centre of Gravity ===
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.
A good amount of freeboard will improve both the maximum righting moment and the limit of positive stability. Too much freeboard will make the boat tippy by raising the CG. Adding ballast to make the boat stiffer reduces the freeboard and reduces the zone of positive stability. Adding ballast to the flybridge, as recommended by one magazine, is absolutely crazy.
== Roll Period ==
The roll period of a boat is an excellent indication of its stability. The lower the roll period, in seconds (s), the more stable the boat. The boat will be more uncomfortable but will have greater resistance to capsizing. The roll period is based on the moment of inertia, waterline length, and beam.