PURPOSE: To effectively utilize the space in a hull by providing water-tight compartments on the both sides of a flat-box type hatch cover whose both ends are held on the edge member of said hatch, providing connection between both compartments through a water-tight duct in which a certain resistance is formed, and using said hatch cover also as an anti-rolling tank.

CONSTITUTION: Water-tight compartments 2A, 2B are formed, through vertical partitions 1A, 1B, on the both sides of a flatbox type hatch cover 10 whose both ends are held on the side coaming of a hatch edge member. Both compartments 2A, 2B are filled with a definite quantity of water while they are connected together through a water tight duct 3. In this duct 3, buffle plates 4 as a resistance and openings 5 are provided, adjusting the phase of water which reciprocates between both compartments 2A, 2B, forming an anti-rolling tank. Thus, the hatch cover can be also used as an anti-rolling tank, enabling the effective use of the space in the hull, e.g., to attempt increase in cargo loading capacity.