To reduce hull resistance, by restraining a vortex and separation, by accelerating and straightening a water flow of a hull surface generated in the stern in front of a propeller in sailing, while reducing frictional resistance of a fin itself.

The long side direction is constituted for projecting from the hull surface so as to become parallel to a line of connecting a pint S being a point of horizontally extending the axis of the propeller and a top part of the propeller, in the depth direction of a ship by substantially 15% Lpp to the bow side more than AP, with a normal in a point p1positioned by substantially 3% Lpp to the bow side more than a position of the propeller in the longitudinal direction of the ship and above by 1/6 Dp of the top part of the propeller in the depth direction of the ship, as the axis parallel to the short side of the paper strip-like fin.

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