Synchronised rolling
When the period of encounter of wave is the same as the roll period of the vessel and when waves are experienced from beam of vessel it is called synchronised rolling.This situation can be avoided by altering course
Synchronous rolling motion
Large rolling motions may be excited when the natural rolling period of a ship coincides with the
encounter wave period. In case of navigation in following and quartering seas this may happen
when the transverse stability of the ship is marginal and therefore the natural roll period becomes
longer. Its more pronounced in beam seas
Parametric rolling
When the rolling period is equal to or twice the wave encounter period. When the crest is in amidship the ship rolls the maximum n also the gm is reduced to minimum. The most dangerous is when the ship pitches twice for every roll.
When the vessel pitches into the crest the buoyancy increases and pushes the ship on to other side causing violent roll, n this coupled with reduced gm when the crest is amidship, can cause the ship to capsize.
Action. Ship speed and course should be reduced in such a way that the above condition is not met.
For ship's loading condition with high GMs, i.e. shorter natural roll period say around 10 second, the waves that could cause the pitching period around 5 seconds are usually not very high and therefore unlikely to initiate parametric rolling in head/stern seas. However, the synchronous roll may occur
in beam seas. So the way to reduce the roll is by heading into the sea.
For ship's loading conditions with low GMs, i.e. long natural roll periods, parametric roll in head/stern seas of moderately high sea state is likely. But synchronous roll in beam seas is unlikely. Containers are more prone for parametric rolling
MMD / MCA ORALS MASTER EXAM
