Abstract: A trim system for a submersible comprises a ball control system and a hydraulic control system. The ball control system includes a tubular ball transfer line 33 substantially through the length of the vessel, a plurality of high-density, solid balls 34 within the ball transfer line 33 which are moveable to alter the weight distribution, a coil 1 at each end of the ball transfer line 33 as a reservoir for the balls 34, brake device 3 for temporarily fixing the position of the balls and sensing device 5 for determining the location of the balls. The hydraulic control system includes vehicle control unit 27, power transfer unit 13, manifold 2, compensator 20 and various valves, filters 17 and quick disconnects 12 for moving preselected balls through the ball transfer line 33.