Design and evaluation of a magnetically-geared underwater propulsion system for autonomous underwater and surface craft

This paper will cover the work performed in the Autonomous Ocean Systems Lab (AOSL) at Memorial University of Newfoundland in the design, construction and evaluation of a high performance under water propulsion system. A small 400W thruster was developed for testing and experimentation while a 3kW thruster was designed specifically for the Sea Dragon, an unmanned surface craft (USC) currently being built by the AOSL. Each thruster consists of three main components: a brushless DC motor, a custom propeller, and a magnetic gearing system. A duct was incorporated as a means to improve hydrodynamic performance and to protect the propeller. All subsystems are purpose-built to maximize their efficiency at the desired operational point. A detailed computer model was developed in MATLAB Simulink in order to predict thruster performance. OpenProp was used to design a number of efficient propellers and were tested using the 400W thruster in an open water flume tank. The propellers did not perform as well as expected so additional experiments were performed in an effort to identify the source of discrepancy. It was determined that propeller surface finish and blade thickness have a significant impact on performance. More importantly it was concluded that the quality of flow in the flume tank was not suitable for accurate testing. Bollard tests may provide more accurate results in the future. The larger 3kW thruster was designed with a greater focus on efficiency. A parametric study was performed in order to identify an ideal propeller geometry that would suit the needs of the Sea Dragon. A custom DC motor was designed such that its efficiency is maximized in the operating regime of the vessel. Simulated results indicate that the overall efficiency of the unit to be between 0.5 and 0.6 which is very attractive for unmanned, untethered vehicles. The thruster will be tested in a tow tank and then onboard the Sea Dragon.