Question 1

What is metacentric height GM?

Accepted Answer

The metacentric height GM is the primary measure of initial stability. GM = KB + BM - KG, where KB is the height of the center of buoyancy, BM = I_WP/V is the metacentric radius, and KG is the height of the center of gravity. A positive GM means the vessel is stable; negative GM means it will capsize. IMO A.749 requires GM ≥ 0.15 m.

Question 2

What is the GZ righting lever curve?

Accepted Answer

The GZ curve plots the righting lever GZ against heel angle φ. At small angles GZ ≈ GM×sinφ, but at large angles hull geometry causes nonlinearity. The angle of maximum GZ and the angle of vanishing stability (GZ=0) are key design parameters. IMO criteria require maximum GZ ≥ 0.20 m at angle ≥ 25°.

Question 3

How is BM calculated for a box-shaped vessel?

Accepted Answer

For a box barge, the waterplane second moment of area is I_WP = L×B³/12. With displaced volume V = L×B×T×Cb, the metacentric radius BM = I_WP/V = B²/(12×T×Cb). Wider beam or shallower draft increases BM and improves initial stability.

Question 4

What are the main IMO A.749 stability requirements?

Accepted Answer

IMO Resolution A.749(18) requires: righting lever area 0°-30° ≥ 0.055 m·rad; area 0°-40° ≥ 0.090 m·rad; area 30°-40° ≥ 0.030 m·rad; maximum GZ ≥ 0.200 m at angle ≥ 25°; angle of maximum GZ ≥ 25°; initial metacentric height GM₀ ≥ 0.150 m.

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