To swim fast or to go far: answers from 1-guilla, a bio-inspired undulatory robot

Natural undulatory swimmers are observed to adapt their waveform kinematics when migrating or when swimming against strong currents. To characterise the effects of waveform kinematics on the swimming performance of undulatory swimmers, we designed a bio-inspired anguilliform robot. We measured the robot’s swimming speed, efficiency, in terms of the cost of transport, and body kinematics in free swimming experiments, for a broad range of kinematic parameters, including joint amplitude, body wavelength, and frequency. We find that speed, in terms of stride length, increases for increasing maximum tail angle, described by the newly proposed specific tail amplitude. Maximum stride length is reached for specific tail amplitudes around unity. Minimum cost of transport requires a lower specific tail amplitude and body undulations close to pure travelling waves. Live anguilliform swimmers display a range of specific tail amplitudes that match our robot’s efficient regime, suggesting similar mechanisms of efficient locomotion. The results improve our understanding of anguilliform swimming and provide guidelines for improved design of undulatory swimming robots.