In this work, we present a bio-inspired fin prototype as a propulsion system for a submarine robot. We explain the rajiform locomotion, which is intended to be imitated with the movements of the fin. It is also explained how the undulations on the fin allow it to have different behaviors, thus generating thrust in different directions, which in turn generates several types of motion for the fin. Our design describes a sub-actuated fin composed by a number of rays. This means that with a single actuator, more than one ray is able to move. We explained how the physical construction process of the first prototype was carried on, as well as some of the tests on which its control parameters were varied to test the performance of the sub-mechanism.