Tugboats used in harbors, rivers and in offshore activities, require efficient propellers to adequately tow or push the ships. An additional factor required to tugboats is the change of the direction of thrust, in order to ensure high manoeuvrability. Currently, these services can be carried out by making use of vertical axis propellers, such as the thrusters Voith Schneider or Schottel, which allow to redirect the thrust to 360°. A new vertical axis propulsion system with orbital paddles (called Bivortix system) is currently under review and in the experimental stage; this system is constituted by a pair of contra-rotating impellers, which provide directional thrust to 360°, allowing in each case a centered thrust, avoiding parasitic components. This system has been patented worldwide and so far have been made two working prototypes, which were tested in the towing tank of the University of Trieste and at the Vienna Model Basin (SVA). A boat, named Proteus, has been built and the first prototype has been installed and tested at full scale; the results obtained showed very advanced capabilities concerning the maneuverability, the stop and crash tests and the good efficiency concerning the thrust. The efficiency results obtained were better than expected and the prototypes are currently under optimization.
