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Undulatory fin motions in fish-like robots are typically created using intricate arrays of servo motors. Motor arrays offer impressive versatility in terms of kinematics, but their complexity leads to constraints on size, hydrodynamic force production, and power consumption, particularly when studying propulsive performance at high-frequencies. Here we present an alternative design that uses a single motor and a tunable rotary cam-train system to achieve a spectrum of fin motions running from oscillation (wavenumber < 1) to undulation (wavenumber > 1). Our platform enables thrust, lift, power, and wake measurements at prescribed pitch amplitudes, frequencies, and wavenumbers. We demonstrated the platform’s oscillating and undulating capabilities via force and wake measurements in a water tank. Studies of fin wavenumber offer design insights for fish-like underwater robots, particularly those with stingray-inspired designs.