also: ぽんぽんじょうき
pop-pop boat; toy steam boat
A small toy boat propelled by a simple steam engine, often made from a tin can or similar material. The name is onomatopoeic from the popping sound of the engine.
I have memories of playing with a pop-pop boat when I was a child.
A pop-pop boat is a toy boat that moves using the heat of a candle.
steam passenger launch
Original meaning; refers to a small steam-powered passenger boat, now largely historical. The toy boat sense derives from this.
In the Meiji era, steam passenger launches were a common sight in harbors.