  Natural selection has tuned many flying and swimming animals across different
species to share the same narrow design space for optimal high-efficient and
energy-saving locomotion, e.g., their dimensionless Strouhal numbers St that
relate flapping frequency and amplitude and forward speed fall within the range
of 0.2 < St < 0.4 for peak propulsive efficiency. It is rather challenging to
achieve both fast and high-efficient soft-bodied swimming robots with high
performances that are comparable to marine animals, due to the observed narrow
optimal design space in nature and the compliance of soft body. Here,
bioinspired by the wing or fin flapping motion in flying and swimming animals,
we report leveraging the generic principle of snapping instabilities in the
bistable and multistable flexible pre-curved wings for high-performance,
butterfly swimming-like, soft-bodied flapping-wing robots. The soft swimming
robot is lightweight (2.8 grams) and demonstrates a record-high speed of 3.74
body length/s (4.8 times faster than the reported fastest soft swimmer),
high-efficient (0.2 < St = 0.25 < 0.4), low energy consumption cost, and high
maneuverability (a high turning speed of 157o /s). Its high performances
largely outperform the state-of-the-art soft swimming robots and are even
comparable to its biological counterparts.
