A vessel propulsion system having a suction casing (4) configured with a suction inlet (4a) opening at a vessel bottom (1b), a suction flow path (4b) inclined to rearwardly ascend from the suction inlet (4a), and an impeller chamber (4c) formed horizontal, and disposed at a bottom of a stern, a delivery casing (10) connected to the suction casing (4) and submerged under a draft of the stern, and a set of forward and reverse rotatable axial flow blades (8) disposed in the impeller chamber (4c) of the suction casing (4).