To enhance course keeping performance of an enlarged ship.

Fins 3 and 4 are provided in a bilge part within the range of 15-25% of a ship length looking from the after perpendicular AP toward the bow side and in another bilge part within the range of 15-25% of the ship length looking from the fore perpendicular FP toward the stern side, almost perpendicularly to both outboards of a hull 2 and along the surface streamline of the hull 2. In addition, the height of a bilge keel from the hull surface is set for 10-20% of the thickness of a boundary layer.