The annals of human locomotion are crowded with designs that work gracefully as models but prove to scale poorly; the Putt-Putt Boat is just such a vehicle. Sometimes called pop-pop boats, hot-air boats, or (most appro priately) flash steamers, the Putt-Putt Boat maneuvers around a bathtub or swimming pool under its own power with no moving parts, driven by a simple, valveless flash boiler made from a coil of copper tubing and a rudimentary alcohol burner.
Flash boilers are one of those technologies that seem ideal but never really work out. They are light, small, easily constructed, and don’t need to be warmed up to produce a head of steam—all factors making them popular in early aircraft designs and nineteenth-century steam cars. Unfortunately, they also instantly overheat if they lose water pressure—not a big deal for a bathtub steamboat but a bit of a critical failure when you’re flying above the Himalayas or steaming across the Mojave Desert.
a utility knife
a small hacksaw
an electric drill with a 1/2″ bit
a Sharpie fine-point marker
a ruler or tape measure
a sturdy curved surface, such as a broom handle, table leg, or beer bottle
(optional) an awl (You can get by with a nail and a pencil.)
(optional) a tapered half-round file or similar rat-tail file
(optional) a large rubber band or masking tape