A method for the maintenance of a ship or platform propulsion assembly during the floating thereof, comprising the steps of: providing a hull insert, or trunk, forming a housing, or propeller case, located within a hull of a ship; providing for said housing to be watertight with respect to the hull in all the walls thereof, except for an opening opening externally to the ship hull; arranging a motor in said hull externally to said housing, avoiding that the position thereof with respect to the hull is changed; providing a thruster, for example a propeller foot, which is suitable to be arranged externally to said hull to generate a propulsion thrust on said floating ship or platform; providing a telescopic driving shaft passing sealingly through one of the walls of said housing operatively connecting said motor and, in a preferably, but not necessarily, removable manner, said thruster, to transmit the movement from the motor to the thruster for the movement of said thruster; said method provides the further steps of: retracting said thruster within said housing passing through said opening, preferably, but not necessarily, avoiding that it projects externally to said hull; lowering externally to the hull a submersible movable hatch in a separate piece and completely disconnected from said thruster and from said housing, by means of removable hanging cables; associating said submersible movable hatch externally to said housing and sealingly closing said opening of said housing, closing said housing and forming a watertight closed chamber with respect to the outside of said hull containing said thruster; evacuating possible fluid, for example, sea water, which is present within said closed chamber even when the hull or platform are floating and partially submersed, so as to arrange said thruster at least partially in air. providing a hull insert, or trunk, forming a housing, or propeller case, located within a hull of a ship; providing for said housing to be watertight with respect to the hull in all the walls thereof, except for an opening opening externally to the ship hull; arranging a motor in said hull externally to said housing, avoiding that the position thereof with respect to the hull is changed; providing a thruster, for example a propeller foot, which is suitable to be arranged externally to said hull to generate a propulsion thrust on said floating ship or platform; providing a telescopic driving shaft passing sealingly through one of the walls of said housing operatively connecting said motor and, in a preferably, but not necessarily, removable manner, said thruster, to transmit the movement from the motor to the thruster for the movement of said thruster; retracting said thruster within said housing passing through said opening, preferably, but not necessarily, avoiding that it projects externally to said hull; lowering externally to the hull a submersible movable hatch in a separate piece and completely disconnected from said thruster and from said housing, by means of removable hanging cables; associating said submersible movable hatch externally to said housing and sealingly closing said opening of said housing, closing said housing and forming a watertight closed chamber with respect to the outside of said hull containing said thruster; evacuating possible fluid, for example, sea water, which is present within said closed chamber even when the hull or platform are floating and partially submersed, so as to arrange said thruster at least partially in air.