Abstract: An apparatus is described for converting river, tidal or ocean current energy into another form of energy, such as electrical, mechanical, or chemical energy. The apparatus can include a first underwater sail(s) coupled to a first cable and second underwater sail(s) coupled to a second cable. The sails are configured to catch water flows and move their respective cables along pulleys. The first cable and the second cable can form a main cable loop that is coupled to a bull wheel. A second cable loop configures the sails to best catch the water flow. The pulleys can guide the main cable loop along a rotational direction. The rotational movement of the bullwheel can be transferred to a generator to convert the water flow into another form of energy.