Abstract: The invention concerns a hull and a submerged propeller unit demountably fitted to the hull and continuously rotatable through 360.degree. of azimuth relative to the hull to give full thrust in any required direction. An aperture in the hull is surrounded by a fixing flange to which extend a drive input shaft for the propeller unit and at least one steering drive shaft. At least one steering drive motor is mounted in the vessel and is connected to the steering drive shaft to effect rotation thereof. A gearbox casing of the propeller unit fits in the aperture with a circumferential flange releasably bolted to the fixing flange. A stem depends from said casing and is rotatably supported therein at its upper end to act as a steering tube for a propeller rotatably supported at its lower end. A steering gear wheel in said casing through which the stem passes and to which it is secured effects rotation thereof.