How does a steamboat move?

All steam engines work like so: A fire or heat source heats water to cause steam (heated water vapor) to expand. That steam is then released through a valve into a chamber where the force of the expansion can be used to move machinery... Usually, to push a piston to an "out" position. The steam then escapes through another valve, and the piston is returned to the "in" position to start over.

One way of pushing the piston back into the "in" position is if there is another piston that operates on the opposite "stroke" - moving out when the other comes in - so that one piston is always pushing out, which turns the wheel and pushes the other piston back in.

If there's a second inlet valve to let steam in in front of the piston to push it back into the "in" position, that's called a 'reciprocating' steam engine.

So... In the case of a steam boat, the piston is usually attached at it's other end to an offset attachment on a paddle wheel, although it could connect through a series of gears to a propeller, as well.

The paddle wheel is covered in a series of paddles that hit the water like a series of simple oars, rowing the boat forward.