Bird-inspired tendon coupling improves paddling efficiency by shortening phase transition times

Drag-based swimming using rowing appendages, fins, and webbed feet is a widely adopted mode of locomotion in aquatic animals. To develop efficient underwater and swimming vehicles, various bioinspired drag-based paddle designs have been proposed, often facing a trade-off between propulsive efficiency and versatility. Webbed feet generate effective propulsive force during the power phase, while being lightweight, robust, and partially foldable during the recovery phase. However, the time-consuming process of mechanically folding and unfolding webbed feet extends the transition periods between the recovery and power phases, which in turn increased drag, and reduces overall paddling efficiency. In this study, we draw inspiration from the coupling tendons of aquatic birds. We implement tendon coupling mechanisms to minimize the transition time between the recovery and power phases. Hardware experiments demonstrate that our proposed mechanism improves propulsive efficiency by factors of $\mathbf{2. 0}$ and $\mathbf{2. 4}$ compared to designs without extensor tendons and based on passive paddles, respectively. Additionally, we find that distal leg joint clutching-previously shown to enhance efficiency in terrestrial walking-plays a negligible role in swimming locomotion. In sum, we present a novel principle for efficient drag-based leg and paddle design, with implications for understanding the swimming mechanics of aquatic birds and advancing bioinspired aquatic propulsion systems.