A novel marine propulsor concept is presented for high hydrodynamic efficiency. Optimum performance is sought by thrust-producing counter-flapping foils cyclically oscillating along a closed trajectory. The thrusting device exhibits low propulsion loading and low viscous losses. In particular, the concept incorporates- lifting surfaces located at the ship stern with propulsive swept area larger than that of conventional propellers; - optimum spatial distribution of axial velocities induced by the propulsor in the wake; - elimination of transverse velocities induced by the propulsor in directions perpendicular to the ship motion; - minimization of unsteady viscous losses compared to conventional oscillating foil concepts for thrust production.The arrangement of lifting surfaces allows for optimum generation of a steady and uniform overall thrust over the propulsor swept area. In addition, no rudders are needed for directional motion control. Gains in efficiency relative to other propulsion concepts are estimated by potential flow theory. Significant efficiency improvements relative to other advanced propulsors like CRP, cycloidal propellers, etc. are foreseen. URANS methods are used to validate and quantify energy savings in open water conditions for a particular design case.