ScaFi: length-scalable, compliant, parametric robotic fish design for operation in multiple environmental niches

Aquatic creatures provide inspiration for robotic design, with morphologies spanning a vast range of scales. These scaled forms are suited to the performance and environmental demands of their habitats, influencing characteristics such as swimming speed, efficiency, and tail beat frequency. However, in artificial systems, the design of a single robot that can operate effectively across a wide range of physical sizes remains a challenge, with existing platforms typically optimized for a single scale and lacking generalizability. To address this, we propose a compliant subcarangiform-inspired robotic fish that requires only a single motor and can scale from several tens of centimeters to a few meters in body length. The tail design consists of a rigid forebody and a compliant tail incorporating fiberglass rods and a cross-over tendon routing. By applying a scaling law derived from a simplified hydrodynamic-elastic model, we maintain similar tail deflection and kinematics across scales, enabling comparable wake formation across sizes. We demonstrate the scalability of the design using three physical prototypes spanning lengths from 50 cm to over 2.5 meters. Particle image velocimetry confirms the formation of bio-inspired vortex wakes, while swimming experiments and field deployments highlight the platform’s robustness and adaptability across diverse aquatic environments.