With an unmanned underwater vessel having a pressure hull, drive assembly and at least one ancillary device arranged on the pressure hull, for example a propeller protection apparatus (15), the at least one ancillary device (15) is fixedly connected to a lifting body (16), which has a lower density than the density of the water, in order to make it possible to replace the ancillary devices or to arrange additional ancillary devices on the pressure hull without changing the trim of the underwater vessel as a result. The density and volume of the lifting body (16) are selected such that the lifting force acting in the water on the ancillary device (15) and the lifting body (16) compensates for the force of gravity acting on the ancillary device (15) and the lifting body (16).