Most underwater vehicles today have spinning propellers, but designers of these vehicles are turning to nature for inspiration on improving the technology. Tuna are one of the most efficient long-range swimmers in the ocean, and understanding their method of propulsion could potentially contribute towards the development of more efficient underwater vehicles. This study investigates the oscillatory motion of a tuna’s rear body – specifically, how the geometry of the tail fin affects the thrust, drag, power, and efficiency of the fish. CAD models of tail fins were designed with varying sweep angles while maintaining the same planform area and aspect ratio. These tail fin designs were combined with a forebody to create a tuna assembly then tested using a computational fluid dynamics (CFD) tool that simulates a water channel. The total force experienced by the fish is measured, and the velocity of water flow is fine-tuned to achieve cruise control, the phenomenon where the thrust generated by the tuna tail fin balances the force of the flowing water pushing it back. The thrust of each tail fin is calculated for a range of tail beat frequencies and for each tail fin design. Work is currently being continued on running the entire parameter space of simulations. Ultimately, the results of this study will lend more knowledge to what tail fin geometry is most efficient for cruising at high speeds, and this will hopefully lead to the integration of this technology in future underwater vehicles.