This paper describes the design, construction, and testing of a biomimetic pectoral (side) fin with actively controlled curvature for UUV propulsion. First, a 3D unsteady computational fluid dynamics (CFD) analysis tool has been adapted to computationally optimize any fin design, followed by a full parametric study based on our findings. Second, this said fin has been constructed, and our working optimized mechanical design is offered. Lastly, we make an experimental vs. computational result comparison for thrust, lift, and flapping moment data - showing that a UUV with this technology can have dramatic improvements in low-speed propulsion and control over traditional thruster methods.