Design and Performance of Bow Thrusters

This paper concerns the hydrodynamic forces and moments produced by a bow thruster. Several broad problem areas are discussed and the extent of present-day knowledge indicated. These include general duct arrangement, duct shape, and impeller design. A step-by-step design procedure is outlined that permits the selection of a practical bow thruster. This procedure is described for a minimum number of operational requirements; e.g., single bow thruster, a specified turning rate when the ship is dead in the water, and a duty cycle that requires thruster operation at ahead speed for control capability in canals, harbors, and other restricted waterways. At the present time conventional circular transverse bow thrusters dominate the field of maneuvering propulsion devices (MPD) with respect to units installed. The literature on bow thrusters is replete with experimental and analytical data concerned with performance information and design criteria. Since many of these data can be generalized it is believed timely to review and tie together this information in one paper. The performance of some bow thrusters probably could have been improved if certain information and knowledge had been available during their design. This is especially true with regard to duct size and the importance of the free-stream velocity in relation to the thruster outflow velocity in determining the total body force. Many types of thrusters have been installed and proposed for consideration and development. Included are: single and multiple units installed near the ship's bow and/or stern, axial-flow propellers (embracing fixed pitch, controllable pitch, contrarotating), cycloida] propellers, and jet configurations such as ejector, ram, etc. In order to keep the present paper of reasonable length, emphasis is placed on the hydrodynamically applied forces and moments due to a single bow thruster duct with a single fixedpitch propeller (impeller). The combined action of multiple thruster units or coupling with rudder action is not considered. Such factors as wind, water current, ship motions, etc. that require a knowledge of ship particulars and ship response are not within the scope of the present paper. However, ship rotation rates that have been used satisfactorily in the past for bow thruster installations will be introduced. The paper presents and discusses: performance factors or parameters which describe or aid in the evaluation of thruster performance, the extent of present knowledge and design criteria as related to configuration arrangement, duct geometry, propeller design, added resistance at ahead s~ip speed, and interaction of thruster jet flow with the mainstream. Particular details encompassed in this paper are recommendations or criteria for the following design quantities: duct immersion, duct diameter, duct length, duct lip radius or shape, propeller hub-pod and fairwater effects, propeller blade shape, and propeller pitch-diameter ratio. A step-by-step design procedure which permits the selection of a practical bow thruster is outlined. This procedure is described for a minimum number of operational requirements; e.g., single bow thruster, a specified turning rate when the ship is dead in the water, and a duty cycle that requires thruster operation at ahead speed for control capability in canals, harbors, and other restricted waterways.