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Roll is ship's motion around its longitudinal axis and it happens for every ship at sea when it's not drived by its propulsion and a ship has other motions as well , like its motion around transvese axis and so . But heel is when a ship is not upright and tilted around its longitudinal axis by an angle which should be avoided and most of the time happens when mass distribution of ships hull or equipments is not symetrical .
Heel describes a temporary inclination generally involving motion, such as wind or turning while roll indicates periodic inclination from side to side such as wave action.
Heel is a deviation around the longitudinal axis (static). Roll is a movement, tilt, around the longitudinal axis (as it's a movement, roll isn't static).
Heel angle represents constant floating angle of vessel, which must be zero. At different heel angles you have different heeling moments. For Ex. If you have STB5 degrees of heel it means ship was loaded more heavily on STB and at this angle you have zero heeling moment to zero the ships heeling angle.
Rolling is motion of ship regarding the dynamic conditions. Rolling angle is zeroed by heeling moment in even heel loaded conditions.
