The author considered the hydrodynamic characteristics of free-running screw propellers from the point of view of the lifting-surface theory and estimated lift-slope correction factors k ₁ (ξ') of which the lifting-line theory corresponds to this theory through the medium. Then following results were obtained : (i) k ₁ (ξ') is effected mainly by the contour and the breadth of the blades. (ii) k ₁ (ξ') is mainly due to the self-interference of the blades whereas the mutual interference is secondary. (iii) Skews have little influences on the values k ₁ (ξ').