Improving Efficiency and Maneuverability through Design of a Robotic Fish with Independent Control of Tail and Caudal Fin Deflection

Bio-inspired soft robotic swimmers offer a promising route toward efficient and agile underwater locomotion. Yet, most existing designs rely on single degree of freedom tails that couple propulsion and steering, limiting controllability and efficiency. In this work, we present a compliant, tendondriven robotic fish featuring an independently actuated caudal fin that decouples tail bending and fin orientation. Actuated via fishing cables, all motors are housed within the central body, minimizing rear-end inertia and preserving hydrodynamic responsiveness. The design enables phase-controlled coordination between tail and fin motion, allowing transitions between efficient gliding and turning. Experiments show that at an optimal tail-leading phase of -60°, the robot achieves up to 50% higher cruising speed and $30-40 \%$ lower cost of transport relative to a coupled tail-fin baseline. The combination of tendon-driven actuation, low-mass tail, and decoupled control demonstrates that modest morphological complexity can enhance propulsion efficiency and maneuverability in soft robotic swimmers.