Underwater propulsion using a flexible elastic fin that mimics fish locomotion is investigated. Because the degree of elasticity is important for thrust efficiency, a fin with a dynamic variable-effective-length spring as a variable-stiffness mechanism is experimentally analyzed. The yawing of the elastic fin in still water is simulated using fluid-structure interaction analysis. There is a clear relationship between the effective length of the spring and Young's modulus for the dynamic stiffness of the fin. Additionally, using the kriging surface response approach, thrust efficiency is demonstrated to be better under dynamic conditions.
The use of underwater robots and vehicles for marine observation, undersea work, and the development of seabed resources has increased in recent years. One such robot is the fish-type robot, which swims with fins instead of thrusters. The fish-type robot developed by Kyushu University uses artificial muscles instead of motors. However, the power of the artificial muscle used in the fish-type robot is lower than that of a motor, meaning the driving force of the fish-type robot is lower than that of a robot using motors. To solve this problem, a fish-type robot that moves its body using servo motors and moves its fins using artificial muscles was developed. Several kinds of swimming performances of the fish-type robot were examined by experiments and calculations. Additionally, the fish-type robot does not produce suction flows in contrast to typical underwater vehicles. It has been inferred that animals living in the water are not harmed in any way by the swimming mechanism. The proposed fish-type robot and a common thruster-type vehicle were driven in a water tank containing a large amount of imitation seaweed. The two devices were driven under the same conditions, and the effects of obstacles in water were studied.