Abstract: The invention relates to a propulsion and lift system for speed boats with a submerged foil. According to the invention, foil (4) includes main lifting surface (11) and hinged trailing edge aileron (12) as well as components (14, 25; 15, 16) to inject gas at high velocity from the lower surface of the main lifting surface and the upper surface of the trailing edge aileron. This provides lift at low speed, in particular lift augmenting at takeoff and defines a nozzle (13) with a two-phase flow which alone provides propulsion at high speed by expansion of a gas-liquid emulsion in said nozzle. Application is to the design of fast hydrofoils.