n j moreover ballasting fore and aft may be used for carrying extra fuel for extreme travel requirements in this way fuel may be added to match mission requirements a quick attach fuel transfer line may be used for shifting ballast for and aft and to supply fuel to the power module the propulsion subsystem in addition to propulsor shaft units 122 123 also includes appropriate water inlets 128 special features such as gear reduction impeller diffusers reverse gearing counter rotating propellers strakes etc are matters of design choice for the skilled designer the steering subsystem follows the propeller 122 and may be easily implemented using vertical steering vanes 124 these are housed within the exhaust shroud of the propeller housing to ensure they are protected from damage and that there is adequate structural strength the resist the control forces however other rudder or fin structures may be used and may be controlled by any of a variety of maritime systems like electrical linear actuators bell cranks hydraulic or pneumatic systems etc reverse thrust may be achieved by fully closing vanes channeling the thrust to the sides and forward thrusters may also be used for close quarter maneuvering and anti.