Ducted Propellers for High-Speed Underwater Propulsion

Cavitation limits subcavitating marine propellers to modest forward speeds. At higher speeds the propeller can be designed to operate under cavitating conditions; however, for efficient operation a supercavitating propeller requires a fully developed cavity. The transition zone between subcavitating and supercavitating propellers represents a sizeable region of speed and advance ratio where performance is poor, as neither type of propeller can operate efficiently with partial cavitation. This situation manifests itself as a propulsor performance gap. The analysis presented herein suggests that a high-speed diffusing duct can extend the cavitation-limited speed of the subcavitating propeller to much higher speeds, and that it can thereby operate efficiently within the propulsor performance gap as well as in the region heretofore proprietary to the supercavitating propeller. Both base-ventilated and superventilated ducts are considered in this analysis. As examples, the performance of the two types of high-speed ducted propellers is analyzed for two bodies of revolution at a depth of 10 ft and at velocities of 50 and 70 knots, respectively, and compared with supercavitating propellers. The performance proves to be very attractive.