The moment of statical stability, commonly referred to as the righting moment, at any given angle of heel is found by:
which results from the buoyancy force (Bf) (being equal to the ship’s displacement (Wf)), acting on the end of the lever GZ, which pivots about G.
The righting moment at any angle of heel represents the instantaneous ‘value’ of the ship’s ability to return to the upright, expressed in tonnes-metres, when the ship is in ‘still water’ conditions and is momentarily at rest i.e. acceleration forces as the ship rolls are ignored.
CALCULATION OF THE MOMENT OF STATICAL STABILITY
In triangle GZM:
Note: The above formula for GZ can only be used for small angles of heel which is often considered to be any inclination of the ship up to approximately 10°.