Making your own steam boat sounds cool. Especially a small mini one that you can use in the bathtub, (with or without you in it also). The design seems simple, with the hardest part being the soldering of the copper tubes, because it is much harder to solder copper tubes then copper wires, but these are small, simple ones, so it can’t be too much trouble, and it probably doesn’t have to be perfect.
The concept itself of it is simple, put a small candle or a small container of fuel under the tiny boiler and you have the basis of a steam boat. I like it, I think I want to make one of these for my pond.