Abstract
Pulse jet propulsion has been tested experimentally in a towing tank for a model-scale, rim-driven ducted thruster and compared to steady operation. Enabled by high-acceleration thruster technology, the experiment was conducted with the goal of reproducing reported propulsive efficiency benefits from pulse jet propulsion by dynamically varying the rotational velocity of a conventional ship propeller. The tests included operation in bollard pull and with forward velocity. Propeller revolution, thrust, and torque, and the duct thrust were measured to provide reliable data of the useful and total power generated by the thruster. Formation of vortex rings has been qualitatively verified with pitot and optical measurements, but with a limited duration before the vortex rings dissipated in the propeller wake. Comparing the generated thrust with the same average rotational velocity, pulse jet propulsion generates thrust up to 2–3 times that of steady operation, consistent with the quadratic relationship between total thrust and the propeller’s rotational speed. The thruster did not replicate the reported benefits of efficiency in tests with forward velocity. In tests at zero speed, the thrust-power relation was the same or deficient of the steady propeller operation for all cases tested. Acceleration-dependent dynamic effects from acceleration and retardation of the propeller have been identified. A static model is presented and used for evaluating the dynamic contributions to performance. The static model predicts the obtained results, explaining the reduced efficiency of unsteady operation primarily as a result of the higher-order relation between power and propeller revolutions compared to the total thrust.