A method for the maintenance of a ship or platform propulsion assembly includes providing a hull insert forming a watertight housing located within a hull of a ship except for an opening. A motor is in the hull external to the housing and a thruster is external to the hull. A telescopic driving shaft sealingly passes through one of the walls of the housing to operatively connect the motor and the thruster to transmit movement to the thruster. Moving the thruster includes retracting the thruster into the housing through the opening. A separable submersible movable hatch is lowered by removable hanging cables. The submersible movable hatch externally engages the housing and sealingly closes the opening to close the housing and form a watertight closed chamber. Fluids within the closed chamber are evacuated even when the hull is floating and partially submersed, with the thruster at least partially in air.