The Pop Pop Boat was my very first poor boy' s boat and cost me a fortune to buy-------S$0.80 60 years ago.I wrecked it while trying to make it run faster by adding more heat. The mini pop
boiler fell off and I spent days,weeks and months trying to gum it back. Potential Boat Repair/Builder and HMEM Engineer.
It was made in Japan. Japanese quality in 1952 was that bad.
Everywhere I have worked, the lunch room at some time has exploded in heated discussion about how the pop-pop boat works. This usually resulted in blokes making boats to prove a point and running them in the sink (or in one case, a kiddies wading pool).
The most satisfactory answer I have found has been given by the late Peter Payne who took out several US patents on variations of the pop-pop boat engine. A good description of the pop-pop boat and an excellent bibliography that includes Peter's work is given here: http://www.nmia.com/~vrbass/pop-pop/
Perhaps we will see an explosion of pop-pop boats in this forum.