For the unmanned underwater vehicles (UUVs) with power between 20kW and 60kW, the electric motor is too heavy, while the power of variable displacement motor is too high. In this paper, the high speed on/off hydraulic control propulsion system for a UUV is designed. The three main parts of the system, the hydraulic subsystem, the propeller, and the amplifier are described in detail. An integrated valve-manifold is designed, which can be installed in the rear of motor to simplify the structure and to improve the response performance of system. A novel quasi PFM amplifier which can make sure the enough switching time of on/off valve and good propeller control performance in the entire working conditions is developed. Since the propeller is driven in switching mode, the speed fluctuation cannot be avoided. Therefore, the relations between the system parameters and speed fluctuation are analyzed by mathematical method, from which the approximate analytical solutions of speed fluctuation and average speed are obtained. Based on these, a flywheel is added on the propeller shaft for the purpose of filtering the speed fluctuation. To choose the inertia of the flywheel, a trade-off between the fluctuation degree and the response speed has been done. Experimental results prove the system has good dynamic performance that the respond time of on/off valve is only about 3.5ms and the fluctuation degree is 12.55% while the response speed of propeller is quick. Simulation results show that the speed closed-loop control system has satisfactory transient and steady-state performance and good tracking ability.

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