A submarine or other marine vessel has a payload launch system with an impulse tank (7) connected via a straight water inlet tube (20) to an inlet orifice (3) in an outer casing (1) of the vessel. A payload launch tube being a torpedo tube (8) extends through the impulse tank (7) to the outer wall (1), and also extends through a pressure hull (4) of the submarine to terminate at a rear door (9). The torpedo tube has an inlet valve (10) within the impulse tank (7). In order to provide pressurised water to the torpedo tube (8) to launch a torpedo (25), an impeller (19) is mounted in the impulse tank which pumps water into the impulse tank (7) to develop a pressure which is transmitted to the torpedo tube (8) when the valve (10) is opened. The impeller (19) is driven by an electric motor unit (21) which is connected to a power supply (22) by cables (30) which pass through the pressure hull (4). The impulse tank (7) and the motor unit (21) are between the outer casing (1) and the pressure hull (4) and the impulse tank (7) is preferably spaced from the pressure hull (4).