(a) Identify the main factors, which effects the rolling period of a vessel.
1. The period of roll varies inversely as the GM, the larger the GM the shorter the rolling
2. The period of rolling varies directly with the radius of gyration. In other words larger the
radius of gyration the larger the period of roll.
3. The period of roll will change when weights are loaded, discharged or shifted, since both
the GM and the moment of inertia will be effected.
4. The amplitude of the roll does not affect the period of roll.
(b) Explain the term synchronous rolling and describe the dangers if any associated with it.
This occurs when the natural period of roll is equal to the apparent period of wave. When this
occurs the wave gives the ship a push each time she rolls (like a swing) causing her to roll
more and more heavily. This effect is known as synchronous rolling.
1. Possible danger of capsizes.
2. Cargo shifting due to heavy rolling.
3. Possible cargo damage and structural damage, personnel injury.
4. Dangers of free surface effect.
5. Possible machinery / Nav. Aids damage.
6. Ship is more vulnerable if engine break down occurs.
(C) Describe the action which may be taken by the ship’s officer when it becomes apparent
that the vessel is experiencing synchronous rolling.
1. Alter course this will alter the apparent period of the waves, an alteration of course towards
the is likely to be particularly effective, as it reduces the apparent period of the wave.
2. Alter speed (effective if the area not abeam).
3. Change GM or distribution of weights aboard the vessel by ballasting/deballasting / shifting