This paper presents a new mechanism with unique and prominent features for a helical swimmer robot. "Double Helices Propulsion Mechanism" consists of two parallel helices with a single axis rotating in the same direction. The outer helix acts as the main propulsion component, and the inner helix, which is made of a Shape Memory Alloy (SMA), controls the forward velocity during swimming. This mechanism, by varying the geometrical parameters of its helical tail, can change the forward velocity of the helical swimmer robot that is required by its predefined missions. In order to study the effects of geometric parameters on the forward velocity in the single helical swimmer, a hydrodynamic model based on Slender Body Theory (SBT) is implemented. Moreover, in order to validate the predicted results, a scaled-up macro-dimension prototype with a single helical tail and Reynolds number of less than one is built. Finally, the performance of the double helices system is estimated by modelling the dynamics of the motion in different tail lengths. This comparison indicates that this mechanism increases the forward velocity and efficiency of swimmer robot; it can produce variable forward velocities at each frequency. © 2018 Sharif University of Technology. All rights reserved