## 1 Introduction  
Unmanned underwater vehicles, intelligent enough to follow specific objectives and tasks, have been largely developed in recent years. These vehicles have found a large variety of applications in scientific, military and industrial fields [1]. Their development tendencies as well as new design concepts vary from large systems to biomimetic mechanisms [2]. The use of biomimetic autonomous underwater vehicles (BAUVs) represents a great strategy for exploration tasks inside rivers, lakes, and oceans; for underwater inspection; and for ecosystem monitoring. Moreover, what biomimetic robots try to attain is to deliberately replicate a natural system's performance into man-made systems to correctly deliver tasks without disrupting the environment's natural cohesion. Building and designing fish-like systems for underwater vehicles ensures highly efficient motion, hovering and agility of response under water [3]. Just as fish propel their bodies by only using their fins, BAUVs can swim when they are properly equipped by mechanisms that correctly imitate natural propellants. Control over BAUVs' maneuverability will then directly depend on its designed propulsion system. Correct motion inside water assures and increases their chances of doing a job efficiently. BAUVs travel in water using undulatory or oscillatory propulsion. In this technique, a pressure difference is created in water by moving some part of their body in a wave-like pattern. To generate oscillation in the second-last part of the vehicle's body and/or its caudal fin, different types of mechanisms may be employed. Traditional multi-joint robotic propellers have been designed to properly imitate fish-like swimming by allowing motion on robot's tail in an undulatory fashion. These mechanisms may use simple configurations based on two serially linked motors to generate sideways flapping [4] or dorsoventral flapping [5]. Furthermore, larger configurations can be designed to improve cruising speeds and turning, where three [6] or four motors [7] are linked for such purpose. Some of these multi-joint designs are supplemented with the addition of smart materials such as memory alloy wires to enhance the shaped induced propulsion [8]. By generating rhythmic movements on joints from a serial link of actuators, oscillatory amplitude, frequency, and phase are controlled to manage the flapping from the vehicle's propulsion system [9]. However, these designs present several deficiencies, such as generation of noise, higher energy consumption, and a unidirectional thruster vector [10]. Furthermore, many BAUV swimming capabilities are way less efficient than those found in real fish. In contrast to the novel BAUV-designs, fish can swim at speeds ranging from 3 BLs\({}^{-1}\) (body length per second) and up to 10 BLs\({}^{-1}\), while the oscillation frequency of their bodies and/or caudal fins might get up to 5 Hz depending on the species [11]. Novel solutions to improve control and efficiency in swimming locomotion inside BAUVs are being developed to excel the performance of such traditional propulsion systems [12]. Parallel robotic mechanisms have become a novel approach to generate oscillation inside BAUV's propulsion systems. These systems are made up of one fixed platform and one moving platform linked by several limbs, allowing an end effector to reach many poses by controlling multiple combinations from limbs' dispositions [13]. Hence, end effectors attached to the moving platform can attain wider ranges of workspace. According to [14], limbs used on parallel robots often consist of an actuated prismatic joint connected to the platforms through passive spherical and/or universal joints. The use of linear actuators allows the system to only deal with compression or traction and not bending, which increases position accuracy. Since multiple links move and support the same platform, the use of parallel mechanisms brings higher structural stiffness, a quality desired in most of the cases in propulsion systems.