Dynamic characteristics and Propulsion efficiency analysis of water boatmen based on high-speed experiments and CFD simulation.

Investigating the propulsive performance of drag-based underwater organisms is crucial for designing highly maneuverable bioinspired underwater robots. Water boatmen are typical drag-based small aquatic insects with agile locomotion. Due to their small size, measuring their dynamic characteristics and propulsive efficiency has long been challenging. This study integrates high-speed videography with computational fluid dynamics (CFD) to quantify the propulsive efficiency and dynamic forces acting on the body and propulsive hind legs of the water boatman. By driving CFD simulations with experimentally resolved kinematic data, we simultaneously determined the instantaneous total resistance, hind leg thrust, water drag, added mass force, and efficiency. The result shows that the water boatmen achieve an average propulsive efficiency of 49%, ranking them among the most efficient drag-based swimmers. This high efficiency, relative to other aquatic organisms, is primarily driven by three mechanisms: substantially reduced hind-leg drag during the recovery stroke, energy recovery via added mass forces during the deceleration phase, and a rapid decline in body drag late in the power stroke. Collectively, these mechanisms improved the propulsive efficiency, offering valuable bio-inspiration for the development of highly maneuverable and efficient underwater robots.