In an operatively water borne vessel whose center of gravity lies on an axis extending fore to aft thereof,
- water-engaging hull-forming means defining a pair of first and second flotation chambers therewithin which are symmetrically arrayed about the aforesaid fore to aft axis of the vessel and extend on substantially parallel axes thereto in a generally horizontal plane when the vessel is disposed in the normal upright condition thereof on a body of ambient water,
- said first and second flotation chambers having first and second means enclosed therewithin, respectively, for retaining gas under pressure,
- means interconnected in a closed circuit with the respective first and second gas retention means. and operable to transfer gas retained in the first gas retention means to the second gas retention means, and vice versa,
- the first gas retention means taking the form of a resiliently flexible gas-inflatable bag which is enclosed with the first flotation chamber to occupy alternately greater or lesser volume of the chamber, depending upon whether gas is added to or withdrawn from the bag, respectively,
- means operable to control the operation of the gas transfer means in accordance with the angular orientation of the vessel about the fore to aft axis thereof,
- said control means being responsive to the vessel capsizing in the body of ambient water to cause the gas transfer means to withdraw a portion of gas from the bag and add the portion to the second gas retention means,
- liquid transfer means operable to open the first flotation chamber to the body of ambient water when the vessel has assumed the capsized condition thereof in the water, so that ambient water occupies that portion of the first flotation chamber evacuated by the bag when the portion of gas is withdrawn therefrom,
- means closing the second flotation chamber to the body of ambient water when the portion of gas is added to the second gas retention means, so that the addition of water to the first flotation chamber shifts the center of gravity of the vessel further away from the axis of the second flotation chamber and induces the vessel to rotate about the same in the direction relatively downward from the aforesaid horizontal plane,
- the control means being operable to cause the gas transfer means to return the aforesaid portion of gas to the bag in the first flotation chamber when the vessel has rotated downward about the axis of the second flotation chamber through an angle greater than 90.degree. from the aforesaid horizontal plane, so that the water occupying the first flotation chamber is displaced from the same by the bag to induce the vessel to continue rotating about the axis of the second flotation chamber in the direction relatively upward toward the aforesaid horizontal plane, and to thereby return to the normal upright condition thereof, and
- means operable to equalize the buoyancy of the respective first and second flotation chambers when the vessel has resumed the normal upright condition thereof.