When the Control Valve is flipped, the Steam Cylinder is connected to the Condenser, which condenses the steam, creating a vacuum. Because of the vacuum in the Steam Cylinder, air pressure now pushes the Steam Piston and Water Piston down, forcing water out of the Water Cylinder, through the Truck Line and out the stern. By reaction, this mass of water thrown backward propels the boat forward, in the same way as a jet or rocket. At the same time, more water is pumped into the Water Tube Boiler by the Injector Pump, and condensed water and trapped air are pumped out of the Condenser by the Air Pump. Notice the water passing through the Trunk Line Flapper Valves during the Steam Stroke? These allow water from the Thrust Stroke to continue moving out the Trunk Line during the intake of new water through the River Valves.
Illustration by Sam Moore