The study deals with developing small and cheap autonomous underwater vehicles, AUV, entrusted of extended manoeuvrability for surveying and docking missions, with accurate of path tracking and attitude keeping, capable of travelling at low speed, down to proper depth, to carry out the given tasks with proper autonomy and without relevant impact on the surroundings. The to day available AUVs, generally, make use of several propellers, distributed around the body, to achieve the desired mobility with full attitude control. The solution requires the closure of multiple loops between sensors and actuators, highly cross-coupled and dependent on dynamics non-linearity and marine surroundings disturbances. The prospected solution considers a properly shaped vehicle, to grant minimal drag for lowering power consumption. A single rear propeller has in charge propulsion and manoeuvring. The propeller can be oriented around two axes, say, pitch (for heaving) and yaw (for veering); the screw (hydrodynamic) torque is balanced by variable tilt fins, either, by counter-rotating vanes. A three degrees of freedom parallel kinematics robotic wrist provides joint path-and-attitude selection and twist compensation, by driving the propeller assembly through three independent actuations. The build-up makes use of a duct-restricted propeller, with externally driven pitch-and-yaw bending and twist to counteract the screw torque by reactive fins, either, continuous rotation. The balancing wile assures best performance, in the reactive mode, during steady surveying missions, while the local hover or docking is better accomplished by active counter-rotation. The paper discusses the basic design steps by example manoeuvres carried with virtual checks on a digital mock-up.

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