Abstract: The disclosure relates to a ship's propulsion unit including a shell structure arranged below a hull of the ship and an electric motor for rotating a propeller. The propulsion unit can include a closed gas circulation cooling system containing gas and include a gas circulation device for circulating gas through channels in the electric motor, a closed liquid cooling system having an inner space containing liquid, and a gas-liquid-heat exchanger in fluid connection with the closed gas circulation cooling system and 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.