Small-diameter ducted contrarotating propulsors for marine robots

Many marine robots, including industry and institution-grade systems, rely on substandard propellers for thrust. This is especially true in small systems. Many of these systems are moving toward onboard power, which is often restricted. Inefficient propulsion becomes a major design problem in these vehicles. Using computational tools, we design, test, and evaluate a small-diameter ducted contrarotating propulsor with a high thrust coefficient. We perform a parametric study to determine the optimum operating point and match a motor and gearbox to the propulsor. A three-dimensional vortex lattice code is then used in conjunction with a Reynolds-Averaged Navier-Stokes (RANS) flow solver to adjust the mean camber surface of the propeller blades and to determine the desired duct offsets. The propellers and duct are manufactured using computer-numerical-controlled machines, as are the components for the motor housing and the miter gearbox used to achieve contrarotation. Propulsors are assembled for use on the MIT ROV and for testing in the recirculating water tunnel at the MIT Marine Hydrodynamics Laboratory. Propulsion tests are conducted to determine KT, KQ, and eta at different rotation speeds. Results show good agreement with design and modeling, with some extra losses due to manufacturing roughness and other unmodeled factors. Propulsors were installed on the MIT ROV Team's entry into the Annual MATE ROV Competition, and performed well in real-world conditions. Lessons learned in testing and missions at the competition identified some points for future improvement in the design