A propulsion assembly of a ship or floating platform (1) uncommonly capable of a simple construction and reduced overall dimensions, comprises: - a hull insert (2), or trunk, forming a housing (3), or propeller case, located within a hull (4) of a ship or platform; said housing (3) being a housing watertight with respect to the hull in all the walls (5) thereof, except for an opening (6) opening externally to the hull (4) of the ship or platform; - a motor (7); - said motor (7) being arranged in said hull (4), but externally to said housing (3), and being arranged stationary with respect to said hull (4); - a thruster (8), for example, a propeller foot; said thruster (8) being suitable to be arranged externally to said hull (4) to generate a propulsion thrust on said ship or floating platform; - a telescopic driving shaft (9); - said telescopic driving shaft (9) passing sealingly through one of the walls (5) of said housing (3) to operatively connect said motor (7) and said thruster (8) and to transmit the movement from the motor to the thruster for the movement of said thruster (8); said thruster (8) being retractable within said housing (3) passing through said opening (6); - a submersible movable hatch (10) in a separate piece and completely disconnectable from said thruster (8) and said housing (3); - said movable hatch (10) being suitable to associate externally to said housing (3), passing externally to said hull (4), and sealingly close said opening (6) of said housing (3), closing said housing (3) and forming a watertight closed chamber (11) with respect to the outside of said hull (4); - a fluid evacuation device (12) for evacuating fluid (13) that is present within said closed chamber (11) so as to arrange said thruster (8), when retracted in said housing (3) closed by said hatch to form said closed chamber, at least partially in air.