  Sinusoidal undulation has long been considered the most successful swimming
pattern for fish and bionic aquatic robots [1]. However, a swimming pattern
generated by the hair clip mechanism (HCM, part iii, Figure 1A) [2]~[5] may
challenge this knowledge. HCM is an in-plane prestressed bi-stable mechanism
that stores elastic energy and releases the stored energy quickly via its
snap-through buckling. When used for fish robots, the HCM functions as the fish
body and creates unique swimming patterns that we term HCM undulation. With the
same energy consumption [3], HCM fish outperforms the traditionally designed
soft fish with a two-fold increase in cruising speed. We reproduce this
phenomenon in a single-link simulation with Aquarium [6]. HCM undulation
generates an average propulsion of 16.7 N/m, 2-3 times larger than the
reference undulation (6.78 N/m), sine pattern (5.34 N/m/s), and cambering sine
pattern (6.36 N/m), and achieves an efficiency close to the sine pattern. These
results can aid in developing fish robots and faster swimming patterns.
