Abstract: A capsize prevention device for a multihull vessel which may be mounted to an existing vessel or be integrally manufactured therewith. The device includes first and second tubes disposed longitudinally along the lower portion of the keel of the vessel through which seawater flows during the sailing of the vessel. Each of the tubes includes forward and rearward watertight valves. When the valves are open, water is free to flow through the tubes. When the valves are closed, water will be trapped within the tubes, thus adding the mass of the trapped water to the keel. The mass of the water within the tubes serves to prevent capsize about one of the outriggers of the vessel.