Mobot mobot: an ocean sunfish ( Mola mola ) robot

Abstract The Ocean Sunfish ( Mola mola ) has one of the most unusual body geometries and swimming strategies of all fish species. Effectively lacking a caudal fin, these fish propel themselves by synchronized flapping of their extremely long dorsal and anal fins—a form of locomotion known as median/paired fin (MPF). Long misunderstood to be poor swimmers, Mola mola are increasingly being recognized for their surprising swimming efficiency and agility. MPF propulsion can be modeled as a combination of pitching and heaving in a hydrofoil, a well-studied phenomenon, and the mechanical simplicity of these motions lend themselves well to the creation of compact and robust propulsion systems. Here, we present a novel bio-inspired marine robotic test platform based on the body geometry and swimming strategy of the Mola mola . We analyze the forces generated by various flapping frequencies and patterns (synchronous and asynchronous), and the flow behavior for both single flap events and continuous flapping. We observe that there is a linear trend between flapping frequency and thrust force for both synchronous and asynchronous flapping up to the maximum frequencies obtainable with the current design. We then test the flapping parameters resulting in the highest thrust forces for both flapping patterns in a free-swimming arrangement and show that the synchronous flapping results in larger steady-state swimming speed.