Abstract: A marine propeller system comprising three individual elements, two of which are propellers mounted on a common shaft driven by a prime mover and the third being a cylindrical tube surrounding and movable relative to the two propellers, the tube having its interior surface in the form of a pair of equal and opposing truncated cones meeting midway of the ends of the tube. The rearward propeller is conventionally keyed to the driven shaft and fast thereon and the forward propeller is axially movable on the shaft and is arranged to be driven by the rearward propeller whereby, depending upon hydraulic pressures within the cylinder, the forward or driven propeller is shiftable relative to the rearward or driving propeller to maintain a spiral discharge from the cylinder with annular wake influence lines and substantially free of cavitation regardless of vessel load or speed.