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As ocean exploration deepens, new demands on propulsion methods are proposed due to the complex underwater environments. Marine animals exhibit excellent locomotion properties, which provide a promising direction for the development of high-performance underwater robots. The unique hydrofoil-paddle propulsion mode of the sea lions foreflippers is a key contributor to their efficient locomotion. Inspired by this, the present work simulates the hydrofoil motion of California sea lion foreflippers with a transient computational fluid dynamic model using dynamic mesh technology to investigate its hydrodynamic characteristics and performance. The result shows that thrust generation during the recovery and power phases is dominated by lift-based propulsion, while during the paddle phase it is dominated by drag-based propulsion. Maximum thrust in a stroke cycle occurs in the power phase, while maximum efficiency at the same motion speed of the hydrofoil is achieved in the paddle phase. The effects of the motion parameters, including Strouhal number (St) and the dimensionless flapping amplitude (h), on the hydrodynamic performance are also investigated. Analysis results show that the optimal thrust and efficiency are achieved in the St range of [0.220, 0.293] and h range of [1.0, 1.864], with the highest efficiency of 25.27% occurring at St = 0.293  and h = 1.846. The present work provides valuable theoretical guidance for designing the bionic robotic foreflippers.

Unmanned underwater vehicles (UUVs) capable of agile, high-speed maneuvering in complex environments require propulsion systems that can dynamically modulate three-dimensional forces. The California sea lion ( ) provides an exceptional biological model, using its foreflippers to achieve rapid turns and powerful propulsion. However, the specific kinematic mechanisms that govern instantaneous force generation from its powerful foreflippers remain poorly quantified. This study experimentally characterizes the time-varying thrust and lift produced by a bio-robotic sea lion foreflipper to determine how flipper twist, sweep, and phase overlap modulate propulsive forces. A three-degree-of-freedom bio-robotic flipper with a simplified, low-aspect-ratio planform and single compliant hinge was tested in a circulating flow tank, executing parameterized power and paddle strokes in both isolated and combined-phase trials. The time-resolved force data reveal that the propulsive stroke functions as a tunable hybrid system. The power phase acts as a force-vectoring mechanism, where the flipper's twist angle reorients the resultant vector: thrust is maximized in a broad, robust range peaking near 45°, while lift increases monotonically to 90°. The paddle phase operates as a flow-insensitive, geometrically driven thruster, where twist angle (0° optimal) regulates thrust by altering the presented surface area. In the full stroke, a temporal-phase overlap governs thrust augmentation, while the power-phase twist provides robust steering control. Within the tested inertial flow regime (Re ≈ 10 -10 ), this control map is highly consistent with propulsion dominated by geometric momentum redirection and impulse timing, rather than circulation-based lift. These findings establish a practical, experimentally derived control map linking kinematic inputs to propulsive force vectors, providing a foundation for the design and control of agile, bio-inspired underwater vehicles.