Abstract: The invention relates to a ship propulsion system comprising a nozzle body and a propeller enclosed by said nozzle body; the nozzle body has an inner surface which forms a flow channel, and further an outer surface; a constriction is situated between the inlet plane and the outlet plane of the flow channel; as seen in an axial sectional view, the inner boundary surface of the flow channel has a first contour between the inlet plane and the constriction, and a second contour between the constriction and the outlet plane; the constriction is situated at a distance of 0.25 to 0.55 of the length of the nozzle body as measured from the inlet plane, preferably 0.3 to 0.4.