What do fishes and fighter jets have in common?

Multi-fin systems, like fish or fish-inspired vehicles, are~governed by~unsteady three-dimensional interactions between their multiple~fins. In~particular, dorsal/anal fins have received much attention because they~are~just upstream of the main thrust-producing fin: the caudal (tail) fin. We~used a tuna-inspired fish model with variable fin sharpness to study the~interaction between elongated dorsal/anal fins and caudal fins. We found~that~the performance enhancement is stronger than previously thought~(15{\%} increase~in swimming speed and 50{\%} increase in swimming~economy) and is governed by a~three-dimensional Dorsal Fin-induced~Crossflow that lowers the angle of attack~on the caudal fin and promotes~spanwise flow. Both simulations and multi-layer~Particle Image~Velocimetry reveal that the crossflow stabilizes the Leading~Edge Vortex~on the caudal fin, similar to how wing strakes prevent stall during~fixed-wing aircraft maneuvers. Unlike other fin-fin interactions, this~mechanism~is phase-insensitive and offers a simple, passive solution for flow~control~over oscillating propulsors. Our results offer new insights into~dorsal/anal~fin shape and placement in fish and fish-inspired vehicles.