Thinking at a model boat and its propulsion most people think at a propeller or a sail. To illustrate the force of blow back, which arises if mass is leaving a vessel with a reasonable speed, a model boat is also applicable, since it is easy to build and no moving parts have to be included. The model boat design presented just uses a styrofoam hull, two aluminium vessel usually taken to container cigars and two aluminium fastening brackets to fix the vessels to the hull.
To make it short, just consider the design sketch above. The vessels are half filled with water, which is then heated and partly evaporated with regular tea candles. The steam created rises the pressure inside of each vessel, so that mass is pushed through a small hole made into the covers of the cigar container. As a matter of inertia conservation (neglecting the fluid friction at the boat hull), the boat is pushed in opposite direction as the mass leaving the vessels, the boat is propelled.