frame 45 again there is a resistance cable 46 in contact with the foil of the cushion 41 which causes the foil to fail when current is passed through the cable 46 a still further exemplary release mechanism is shown in fig5 again the cushion 51 is located within the frame 52 by means of a peripheral bead 53 including a rope 54 the bead being captured by a keder edge 55 which is clipped into the frame housing 56 however in this embodiment there need not be a resistance cable instead the rope 54 may be wound round a pulley 57 and connected to a winch not shown thus when required the rope 54 is drawn by a winch and the bead 53 collapses as a result the cushion 51 is released yet another exemplary release mechanism is shown in fig6 in this case the cushion 61 is located within the frame 62 by means of a peripheral bead 63 captured by a keder edge 54 clipped into the frame housing 65 however in this embodiment a blade 66 may be provided on a carriage 67 which is arranged to be rotatable and to travel along a track 68 around at least three sides of the periphery of the cushion 61 when required cutting through the cushion foils to free the cushion 61 although the blade 66 is shown located below the cushion it.