Causes & Remedial action of Parametric Rolling
Parametric rolling is a phenomenon whereby a ship which is pitching moderately in bad weather suddenly experiences very heavy rolling, without any warning. The roll angle can increase from 2 or 3° to over 30° in just two or three cycles.
Causes:
When the bow is down during moderate pitching, and the ship has rolled slightly to one side (say to starboard), the sudden Immersion of the large flare causes the restoring buoyancy force to push the bow upwards and to the other side of roll (portside). The opposite happens during the next pitch of the bow downwards. Within a few cycles, the angle of roll would suddenly and become very large without any warning.
To sum up, parametric rolling is the result of synchronism between the pitch, roll & wave periods.
Remedial action:
Do not panic. Change heading to bring the wind broad on the bow to break the synchronism that has caused parametric rolling. Come back to the original heading gradually, if necessary. Otherwise, if time & space permit, steer a zigzag course spread over few hours, until the weather abates.
