To reduce the resistance of the bottom of a ship by forming tap the bottom into a recessed shape surrounded by side boards, providing several partition plates between both side faces, providing partition weirs having inclined parts as longitudinal partitions suitably in several places, and charging compressed air in each partitioned section.

A plane part of the bottom of a ship is made into the recessed bottom 1 surrounded by side boards 3, and one to several partition plates 4 are provided between both side boards 3. Partition weirs 5 are provided suitably in one to several places as longitudinal partitions, and the partition weirs 5 are provided with inclined parts 6. Compressed air is then charged in each partitioned section. In the bottom at the time of smooth water, the compressed air charged in each section is filled down to the lower ends of the side boards 3, and partition plates 4, and the bottom l surface by the height of the side boards 3 through the water surface and an air layer so as not to be in direct contact with water. This part is brought into contact with air and water by sailing, and its frictional resistance is close to zero compared to the frictional resistance between the bottom and water, so that water resistance is reduced by the area portion of the bottom 1' that has surfaced.