This paper describes the modeling, design, experimental testing and optimization of a flapping foil for use as an aquatic propulsive device on a robot capable of aerial and aquatic modes of locomotion. Motivation for the research stems from numerous avian species which use the same flapping mechanism as a means of propulsion in both mediums. The main aim of this research is to establish the optimal kinematic parameters during aquatic operations that maximise non-dimensionalised performance measures, such as propulsive efficiency. Optimization of said parameters enables the direct comparison between outstretched and retracted wing morphologies and permits scaling for future robotic vehicles. Static foils representing the wing in both an extended and retracted orientation have been manufactured and tested over a range of kinematics. The gathered results enable validation of previously developed numerical models as well as quantifying achievable performance measures. This research focuses on the mechanical propulsive efficiencies and thrust coefficients as key performance measures whilst simultaneously considering the required mechanical input torques and the associated thrust produced.