Abstract: A marine propulsion unit including a conduit having opposed, inlet and outlet ends. A first pivot pin is pivotally secured within the conduit adjacent its inlet end. A second pivot pin is pivotally secured within the conduit adjacent its outlet end. The forward end of a flexible sheet is secured to the first pivot pin, and the rearward end of the flexible sheet extends freely from the outlet end of the conduit. The flexible sheet is also secured to the second pivot pin between the forward and rearward ends thereof in a longitudinally compressed manner so as to impart a waveform to the flexible sheet. At least one crank arm is secured to the first pivot pin for oscillating the first pivot pin to cause the rearward migration of the waveform in the flexible sheet from the first pivot pin to the second pivot pin.