Abstract: The Keelung Sill is an underwater ridge, located between the Keelung Harbor and the Keelung Island on the northern coast of Taiwan. Due to its shallow depth, the sill constricts the waterway and over the sill the tidal currents accelerate. Our measurements showed that the highest tidal current speed was 2.7 m/s. We have also developed an underwater power generating unit, which included a 80 cm horizontal-axis turbine, an underwater generator, a gear box, and a supporting under-structure. The turbine was designed to generate 3 kW at the current speed of 3.2 m/s. Tests in our cavitation tunnel have been conducted. It was found that the device generated 1800 W at a current speed of 3.2 m/s. This showed that there is a 40% power loss in total, including the loss of mechanic, generator, and power converter. We also designed and tested a 120 cm turbine in the cavitation tunnel. The results showed that the start-up speed for the 120 cm turbine is 1.38 m/s, lower than the 1.7 m/s for the 80 cm turbine. The power output for the 120 cm turbine was also about 1700W at 2.55 m/s when the gear is changed from a ratio of 1:7 to 1:10.