Hydrokinetic energy conversion systems are emerging as a viable solution for harnessing kinetic energy. However, the typical deployment location is highly space-constrained due to both the nature and the other uses of the river. Therefore, a modified conversion device to overcome these constraints is necessary. The research objective of this work was to evaluate and enhance the performance of multiple coaxial horizontal axis hydrokinetic turbines (HAHkTs) mounted on a single shaft. The hydrodynamic performance of different configurations of single- and multi- HAHkTs with blades made of carbon fiber polymer composites was evaluated in a water tunnel. Increasing the number of rotors of the turbine system from one to two enhanced efficiency by approximately 75% and lowered the operational tip speed ratio. The third rotor also enhanced the efficiency, but the improvement was less (about 32%) due to the slower flow passing this rotor. A duct reducer was also incorporated, and its effect was studied. Finally, the wake behavior and its effect on the multi-turbine system operation were examined by using a particle image velocimetry system. From the structure aspect, composite materials have the appropriate properties that suit the water turbine blades. The composite blades were manufactured using the out-of-autoclave process.