This paper describes a novel lightweight propulsion scheme for use in high-speed ships in which large amounts of water are injected into the fan discharge duct of an aircraft- type turbofan engine. Theoretical design and off-design performance for this water-augmented turbofan engine show that dry thrust theoretically can be augmented 380% at 25 knots and 90% at 100 knots. The reduction in theoretical performance caused by nonoptimum rates of water injection and by two- phase flow losses is not sufficient to detract from significant thrust augmentation. The results show that in many marine vehicle applications the slightly lower propulsive efficiency of the water-augmented turbofan engine, relative to more conventional propulsion systems, is more than compensated for by its extremely low system weight.