Spin is all you need for fast and efficient swimming*

The biomimetic engineering of fish-like robots has sought to recreate the specific characteristics of body or tail undulutaions with limited success in emulating the efficiency, speed or agility of the fish. In this talk an alternative design of a fish-like robot is presented, in which the spin of an unbalanced internal rotor is used generate parametric oscillations of a flexible tail and small amplitude oscillations of the main body. The oscillations of the tail generate thrust by the added mass effect according to the slender body theory and the reverse Karman wake it generates. The tail oscillations can switch from the first to the second mode depending on the spin frequency of the rotor. This novel locomotion generated by spin parametic oscillations is highly efficient at about a third of the efficiency of a tuna at speeds of 2.5 body lengths per second. Such efficiency in a free-swimming robot (untethered and unsupported by any cables in a water tunnel) is the highest amongst swimming robots. Moreover the the spin motion of the internal rotor confer gyroscopic roll stability to the robot and at the same time allow controlled roll and turning ability.