The present invention relates to a propulsion module (1) for generating wave-like motion in liquid. The propulsion module (1) comprises: a motor (7) for generating rotational motion; a set of conversion devices comprising a first conversion device (91) and a second conversion device (92) each comprising a movable arm (27) for converting the rotational motion from the motor (7) into oscillatory translational motion of the respective arm (27) along a movement axis, which is angled with respect to a length axis of the propulsion module; a drive mechanism for coupling the motor (7) to the first and second conversion devices (91, 92) for transferring the rotational motion to the first and second conversion devices; and a flexible structure (19) extending along at least a portion of the length axis of the propulsion module (1), the flexible structure (19) being configured to be guided by the movable arms (27) to generate the wave-like motion of the flexible structure (19). A distance between the first and second conversion devices along the length axis, and/or a maximum range of motion of the movable arms (27) along the movement axis is/are adjustable.