A marine ducted propeller jet propulsion apparatus comprising a propulsion system comprising: an intake section; an impeller section positioned downstream of the intake section, a discharge section positioned downstream of the impeller section, the discharge section comprising a steering nozzle assembly; a passage extending from an intake opening of the intake section formed in a bottom surface of a hull at one end to an outlet of the steering nozzle assembly at the other end; a mass of the internal working parts of the propulsion system being accommodated within the passage; whereby the passage has an axial cross-sectional area, the axial cross-sectional area of the passage being adjusted throughout the passage by increasing the external dimensions accordingly and/or by increasing the radius of the axial cross-sectional area to accommodate for a volumetric mass of the internal working parts so that the passage is capable of converging a passing water mass on a volumetric basis while maintaining a solid state character of the passing water mass as the passing water mass is driven through the passage from the intake opening of the intake section to the outlet of the steering nozzle assembly, whereby, in use, the axial cross-sectional flow area substantially and regularly decreases from the inlet to the outlet without resistance of the mass of the internal working representing a restriction or obstruction to the flow, characterized in that, the impeller section comprising an impeller housing, comprising an interchangeable blade assembly impeller, said interchangeable blade assembly impeller comprising an impeller hub and impeller blades,wherein the impeller blades are fixed along an outwardly tapered convex surface of the impeller hub,said interchangeable blade assembly impeller being suitable for allowing the changing of the individual impeller blades on the interchangeable blade assembly impeller if damaged, and suitable for allowing the changing of the pitch of the impeller blades for different applications,the impeller section comprising a diffuser/ confusor housing, said diffuser/confusor housing comprising an interchangeable diffuser/confusor blade component, leading edge blades and diffuser vanes,said interchangeable diffuser/confusor blade component being suitable for allowing the changing of leading edge blades to the diffuser so to replace such leading edge blades if damaged, and for allowing the changing of the pitch of the leading edge of the diffuser vanes to meet the needs of the trailing edge velocities of the interchangeable blade assembly impeller. an intake section; an impeller section positioned downstream of the intake section, a discharge section positioned downstream of the impeller section, the discharge section comprising a steering nozzle assembly; a passage extending from an intake opening of the intake section formed in a bottom surface of a hull at one end to an outlet of the steering nozzle assembly at the other end; a mass of the internal working parts of the propulsion system being accommodated within the passage; whereby the passage has an axial cross-sectional area, the axial cross-sectional area of the passage being adjusted throughout the passage by increasing the external dimensions accordingly and/or by increasing the radius of the axial cross-sectional area to accommodate for a volumetric mass of the internal working parts so that the passage is capable of converging a passing water mass on a volumetric basis while maintaining a solid state character of the passing water mass as the passing water mass is driven through the passage from the intake opening of the intake section to the outlet of the steering nozzle assembly, whereby, in use, the axial cross-sectional flow area substantially and regularly decreases from the inlet to the outlet without resistance of the mass of the internal working representing a restriction or obstruction to the flow, characterized in that, the impeller section comprising an impeller housing, comprising an interchangeable blade assembly impeller, said interchangeable blade assembly impeller comprising an impeller hub and impeller blades,wherein the impeller blades are fixed along an outwardly tapered convex surface of the impeller hub,said interchangeable blade assembly impeller being suitable for allowing the changing of the individual impeller blades on the interchangeable blade assembly impeller if damaged, and suitable for allowing the changing of the pitch of the impeller blades for different applications,the impeller section comprising a diffuser/ confusor housing, said diffuser/confusor housing comprising an interchangeable diffuser/confusor blade component, leading edge blades and diffuser vanes,said interchangeable diffuser/confusor blade component being suitable for allowing the changing of leading edge blades to the diffuser so to replace such leading edge blades if damaged, and for allowing the changing of the pitch of the leading edge of the diffuser vanes to meet the needs of the trailing edge velocities of the interchangeable blade assembly impeller.