Air cushion vehicles

An A.C.V. having cushion sealing means at least partly constituted by an inflatable skirt structure comprising an inflatable bag member constructed from flexible impermeable sheet material attached to rigid body structure of the A.C.V. along attachment lines that are spaced apart and one of which is situated outboard of the other so as to enclose with said rigid body structure a space, a flexible impermeable web member having a first edge attached to the rigid body structure and a second opposed edge attached to the bag member along respective attachment lines on the rigid body structure and the bag member that are spaced between and substantially parallel with the attachment lines of the bag member to the rigid body structure and whereby said space enclosed by said bag member and said rigid body structure is divided into inboard and outboard compartments with the volume of said outboard compartment being disposed substantially outboard of the rigid body structure of the A.C.V. when the bag member is inflated, first duct means communicating between a source of pressurized air on said A.C.V. and said inboard compartment, second duct means communicating between said inboard and outboard compartments, third duct means communicating said outboard compartment to ambient, first valve means co-operating with said second duct means and adapted for closing said second duct means, and second valve means co-operating with said third duct means and adapted to control venting of said outboard compartment to ambient, said first and second valve means being controllable to vary the inflation pressure of said outboard compartment and thereby to effect changes in the geometry of the flexible skirt structure.