Recommendations on metacentric height

Ideally, the stability should be assessed using the complete righting arm curve, but since it is impossible to calculate righting arm curves without the outline design, more easily determined GM values are given as a function of the ship type, Table 1.4. If a vessel has a GM value corresponding well to its type, it can normally be assumed (in the early design stages) that the righting arm curve will meet the requirements.

Table 1.4 Standard GM—for 'outward journey', fully loaded

Ship type

GM [m]

Ocean-going passenger ship

1.5-2.2

Inland passenger ship

0.5-1.5

Tug

1.0

Cargo ship

0.8-1.0

Containership

0.3-0.6

Tankers and bulkers usually have higher stability than required due to other design considerations. Because the stability usually diminishes during design and construction, a safety margin of a GM = 0.1-0.2 m is recommended, more for passenger ships.

When specifying GM, besides stating the journey stage (beginning and end) and the load condition, it is important to state whether the load condition specifications refer to grain or bale cargo. With a grain cargo, the cargo centre of gravity lies half a deck beam higher. On a normal cargo ship carrying ore, the centre of gravity is lowered by about a quarter of the hold depth. The precise value depends on the type of ore and the method of stowage.

For homogeneous cargoes, the shipowner frequently insists that stability should be such that at the end of operation no water ballast is needed. Since changeable tanks are today prohibited throughout the world, there is less tank space available for water ballast.

The GM value only gives an indication of stability characteristics as compared with other ships. A better criterion than the initial GM is the complete righting arm curve. Better still is a comparison of the righting and heeling moments. Further recommendations and regulations on stability are listed in Appendix A.1.

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