The metacentric height (GM) can be determined approximately by means of a rolling period test. It is performed by measuring the rolling period of the vessel.

 

Stiff vessels have a high metacentric height (GM) and a short rolling period; while vessels with a low metacentric height (GM) have a long rolling period. 

 

The rolling period test is a frequently used method to determine the stability of smaller vessels. Rolling period test can be performed at any time and the crew can perform it themselves.

 

The test can be performed in the following way:

1. Mooring lines should be slack and the vessel should not be too near the harbour.
2. The vessel is made to roll.
3. When the vessel’s roll is sufficient (approximately 2 - 6° to each side) the vessel is allowed to roll freely and naturally.
4. The time is taken which it takes the vessel to go approximately four complete oscillations. (One complete oscillation will have been made when the vessel has moved right across to the other extreme side (i.e. starboard) and returned to the original starting point).
5. The time in seconds (T) for one oscillation is found by dividing the number of oscillations made with the total time.

If the calculated value of T, in seconds, is less than the breath of the vessel (B), in metres, it is likely that the initial stability is sufficient, provided that the vessel carries full supplies and fishing gear and has high freeboard. 

 

As the vessel’s supplies decreases the rolling period (T) becomes longer since the vessel’s centre of gravity (G) becomes higher and the metacentric height (GM) becomes larger. In such circumstances it is recommended that the calculated value of T, in seconds, should not be more than 1.2 times the breath of the vessel (B). 

 

This method is not applicable to vessels with a hull shape that dampens the rolling, for example vessels with large bilge keels or vessels of an unconventional design, such as high-speed fishing vessels.