Objectives This paper investigates the influence of the span-wise stiffness of a flapping pectoral fin on its generated thrust.

Methods A scheme to fabricate a flexible pectoral fin with uniform span-wise stiffness is introduced. A series of experiments is then conducted to investigate the generated thrust and lateral force of the fabricated pectoral fin with different degrees of span-wise stiffness in the frequency range of 0.3−1.0 Hz. Combined with collected shots by a high speed camera, the impact of span-wise stiffness on generated force is analyzed.

Results  The experimental results show that the average propulsion force and the average lateral force generated by the flexible bionic pectoral fins are related to the size and distribution of the radial stiffness of the fin in the range of the tested flap frequency. By optimizing the spreadal stiffness of the flexible bionic pectoral fin of the robotic fish prototype (XJmanta), the maximum swimming speed can be increased by about 45%.

Conclusions The results of this study can be used to guide the design of flapping flexible bionic pectoral fins and optimize the hydrodynamic performance of ray-like underwater robots.