To provide a hydrofoil, which is especially a rudder for a vessel, capable of reducing either or both of harmful influence of vortex forming in the flow direction behind this hydrofoil and harmful influence of Karman's vortex street.

In order to reduce vortex formation, at least one projection 30 is arranged on a rear edge 20 usually composed to be flat or oval of the hydrofoil 100. Projecting an object from a surface of the rear edge can avoid especially formation of a pair of vortexes, which is necessary for forming the Karman's vortex street, rotating to the opposite directions from each other on both the side surfaces of the hydrofoil. At least one projection composed to be a hard inelastic object which does not bend is more suitable. This makes flowing behavior of the hydrofoil constant, with respect to especially generation of vortexes.