This paper presents theoretical formulations to evaluate the restoring moment of structures supported by air cushions at zero forward speed. The air cushion is assumed to be enclosed within a number of compartments that are open to sea. The height of the water plug within the compartments is thus an important parameter. The theory, developed from first principles, is verified against data from inclining experiments conducted on a nine-compartment air cushion supported structure at different drafts and water plug heights. The theory shows that the air cushion affects the stability in two conflicting ways: favorable decrease in the vertical center of gravity caused by the water plug effect and unfavorable enhancement of the internal free surface effect. The net effect on stability depends on the operating conditions as well as on the scale of the structure being considered. A correction factor for the metacentric height incorporating the air cushion effects is proposed.

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