How do Rolling , Pitching and Yawing occur?
Rolling
When the ship is afloat in calm water, the COG and the CoB are in a vertical line. If the crest of a wave was to strike the ship on the starboard side, the underwater volume on the starboard side would increase. The CoB, being the geometric centre of the underwater volume, would shift to the starboard side.
Since no weight has been added or removed, the position of the CoG remains unaffected. The force of buoyancy, acting upwards from the new position of the CoB to starboard, would cause the ship to incline to port. The opposite happens when the crest of the wave passes over to the port side – the ship would incline to starboard. This oscillatory rotary movement from starboard to port is called rolling.
Pitching
When the ship is afloat in calm water, the COG and the CoB are in a vertical line. If the crest of a wave was to pass under the bow, the underwater volume at the bow would increase. The CoB, being the geometric centre of the underwater volume, would shift to the forward side. Since no weight has been added or removed, the position of the CoG remains unaffected. The force of buoyancy, acting upwards from the new position of the CoB ahead, would cause the bow to rise up and the stern to dip down. The opposite happens when the crest of the wave passes under the stern – the stern would rise up and the bow would dip down. This oscillatory rotary movement up & down by the bow & stern of the ship is called pitching.
Yawing
When a wave strikes the ship on one bow (say starboard), the bow gets pushed to the other (port) side. When the next trough is against the same bow (starboard), the bow gets pulled towards that side (starboard). Since the crest & trough would come alternately, the ship would tend to oscillate about the original direction in which it was heading. This is called yawing.
Ref : Ship stability at Management Level by Capt. H Subramaniam
