A ship or floating platform propulsion assembly, comprising - a hull insert, or trunk, forming a housing, or propeller case, located within a hull of a ship or platform; - said housing being a housing watertight with respect to the hull in all the walls thereof, except for an opening opening externally to the hull of the ship or platform; - a motor; - said motor being arranged in said hull, but externally to said housing, and being arranged stationary with respect to said hull; - a thruster, for example, a propeller foot; - said thruster being suitable to be arranged externally to said hull to generate a propulsion thrust on said ship or floating platform; - a telescopic driving shaft; - said telescopic driving shaft passing sealingly through one of the walls of said housing to operatively connect said motor and said thruster and to transmit the movement from the motor to the thruster for the movement of said thruster; - said thruster being retractable within said housing passing through said opening; - a submersible movable hatch in a separate piece and completely disconnectable from said thruster and said housing; - said movable hatch being suitable to associate externally to said housing, passing externally to said hull, and sealingly close said opening of said housing, closing said housing and forming a watertight closed chamber with respect to the outside of said hull; - a fluid evacuation device for evacuating fluid, which is present within said closed chamber, so as to arrange said thruster, when retracted in said housing closed by said hatch to form said closed chamber, at least partially in air. - a hull insert, or trunk, forming a housing, or propeller case, located within a hull of a ship or platform; - said housing being a housing watertight with respect to the hull in all the walls thereof, except for an opening opening externally to the hull of the ship or platform; - a motor; - said motor being arranged in said hull, but externally to said housing, and being arranged stationary with respect to said hull; - a thruster, for example, a propeller foot; - said thruster being suitable to be arranged externally to said hull to generate a propulsion thrust on said ship or floating platform; - a telescopic driving shaft; - said telescopic driving shaft passing sealingly through one of the walls of said housing to operatively connect said motor and said thruster and to transmit the movement from the motor to the thruster for the movement of said thruster; - said thruster being retractable within said housing passing through said opening; - a submersible movable hatch in a separate piece and completely disconnectable from said thruster and said housing; - said movable hatch being suitable to associate externally to said housing, passing externally to said hull, and sealingly close said opening of said housing, closing said housing and forming a watertight closed chamber with respect to the outside of said hull; - a fluid evacuation device for evacuating fluid, which is present within said closed chamber, so as to arrange said thruster, when retracted in said housing closed by said hatch to form said closed chamber, at least partially in air.