A double-hulled vessel has a ballast draft line previously set at an empty state thereof. The double-hulled vessel of the present invention comprises an outer shell formed on bottom and side portions of the vessel; an inner shell formed on bottom and side portions of the vessel within the outer shell; an air tube positioned between the inner and outer shells; and a seawater tube positioned between the inner and outer shells, wherein seawater holes are formed in the outer shell to be connected to the seawater tube, the seawater tube is capable of containing seawater to substantially fill a space between the inner and outer shells extending from the bottom portion of the vessel up to the ballast draft line of the side portion of the vessel when the vessel is not loaded with cargo, and the air tube is capable of containing air to substantially fill the space between the inner and outer shells of the bottom and side portions of the vessel when the vessel is loaded with cargo.