A ship's propulsion unit such as a ship's azimuthing propulsion unit, wherein the propulsion unit comprising a shell structure arranged below a hull of the ship and immersible at least partly in water so that the shell structure is at least partly surrounded by water, and an electric motor for rotating a propeller outside the shell structure, wherein the electric motor having a stator and a rotor for rotating in the stator, wherein the electric motor is arranged in a motor housing section of the shell structure so that the stator of the electric motor is form-fitted into a cylindrical section of the motor housing section of the shell structure, wherein the cylindrical section having a cylindrical outer surface, and wherein the shell structure includes a support section having an lower end connected to the motor housing section of the shell structure and an upper end connected to the hull of the ship, further defined by a closed gas circulation cooling system containing gas and comprising gas circulation means for circulating gas through channels in the electric motor, by a closed liquid cooling system having an inner space containing liquid, by a gas-liquid-heat exchanger in fluid connection with the closed gas circulation cooling system and in fluid connection with the closed liquid cooling system for exchanging thermal energy between gas circulating in the closed gas circulation cooling system and liquid in the closed liquid cooling system, by the closed gas circulation cooling system, the closed liquid cooling system, and the gas-liquid-heat exchanger being parts of the propulsion unit, by the closed liquid cooling system being arranged fully outside the hull of the ship, by the closed gas circulation cooling system being arranged fully outside the hull of the ship, by the gas-liquid-heat exchanger being arranged fully outside the hull of the ship, by the closed liquid cooling system being in the form of a closed liquid circulation cooling system containing liquid and forming said inner space containing liquid and provided with liquid circulation means for circulating liquid in the inner space of the closed liquid circulation cooling system, by the inner space of the closed liquid circulation cooling system being partly limited by a part of the cylindrical outer surface of the cylindrical section of the motor housing section so that liquid flowing in the inner space of the closed liquid circulation cooling system is in direct contact with said part of the cylindrical outer surface of the cylindrical section of the motor housing section for exchanging thermal energy between the electric motor arranged in the cylindrical section of the motor housing section and liquid flowing in the inner space of the closed liquid circulation cooling system via said part of the cylindrical section of the motor housing section between the electric motor and liquid circulating in the inner space of the closed liquid circulation cooling system, and by the lower end of the support section of the shell structure being connected to the motor housing section of the shell structure so that the cylindrical outer surface of the cylindrical section of the motor housing section of the shell structure of the propulsion unit partly forms the outermost surface of the propulsion unit.