In this paper, a vertical axis water turbine (VAWT) was developed to supply energy for underwater vehicles. 2-D transient numerical studies were conducted to determine the operating performance and power output of the turbine. A RNG k-ε turbulence model was used to perform the transient simulations. For both the four-bladed turbine and three-bladed turbine, the influence of blade pitch angle on the turbine performance was investigated over a range of rotor rotation velocity ω. It was found that the three-bladed turbine showed a larger power output than the four-bladed turbine. For a three-bladed turbine, a maximum averaged power of 29.33 W was generated when v=1 m/s, θ=120° and ω=0.3 rad/s. Contours of the velocity field of a three-bladed turbine under an inflow velocity of 1 m/s were also analyzed, providing helpful reference for the future design of turbines.