The key is to have a very small passageway heated to boiling temperature, then introducing water into it. A more efficient boiler is the diaphragm type. This variant replaces the coils with a small metal pan whose top is a slightly concave piece of very thin, springy metal such as brass shim. The result is a boat which runs well and makes a vaguely motor-like noise. Matt Welland has made a full-size drawing of a commercial boat made in India, showing the arrangement of the diaphragm boiler as well as the general construction of the boat.
M aking a pop-pop boat is utterly simple.
According to the archivist at Boy Scout headquarters , the Cub Scout book contained this project from 1954 through 1977. Put a flame under the coil, and you have a pop-pop boat. To run the boat, fill the tube with water and light the flame. From the top piece, I cut out the inside of the hull with a scroll saw. I glued the two pieces together with waterproof wood glue, cut the point of the bow on the saw, and then drilled the tube holes at an angle in the back transom of the boat.