Rattle boat

The Knatter- , Putt-Putt- or Puff-Paff-Boot is a toy boat with a water pulse drive (steam jet drive, pulsed waterjet ). The simple motor works without moving parts such as pistons , valves or wheels. A candle or other heat source serves as the energy source. At the stern of the boat, two pipes protrude side by side and parallel below the waterline. Water is sucked in through the pipes and expelled again. Although as much water is sucked in as it is discharged, there is a positive balance of momentum that propels the boat forward. The cause of the positive impulse in relation to the boat as a balance sheet is the fact that the water is sucked in from all directions, but is preferably expelled again in one direction. When sucking in, a spherically symmetrical and thus pulse-neutral sink is created , and when ejecting a free jet with a pulse .

How the engine works

Flat evaporator

Rear view, with both flow cases
View from below, drive and candle

A common construction of the motor is shown in the pictures above: A flat-lens-shaped evaporator (yellow) made of soldered copper sheet is positioned horizontally above the flame. The recoil tubes (dark gray) run from two points on the evaporator to the stern of the boat, where they reach a little way into the water below the waterline . The second tube is not necessary for the function, but it facilitates the air bubble-free filling with water for commissioning.

When the water in the small evaporator reaches the boiling point, it will evaporate very quickly under the constant warming of the candle. The overpressure of the (hot) steam accelerates the water in the pipes, which is ejected backwards into the open with a swing.

At the end of the ejection phase, the superheated steam, which was previously under overpressure, has relaxed under the momentum of the flow in the pipes to a little below ambient pressure and has therefore cooled significantly.

The overpressure of the environment is now at its maximum and cold water begins to flow into the pipes from the outside with a reversal of direction. A relevant amount of steam now condenses on the water front advancing in the pipe, so that negative pressure is maintained here and the water flow is pressed further in the direction of the evaporator. Finally, under the momentum of the liquid flow in the tubes, water is injected into the small evaporator.

The temperature of the side of the evaporator - empty of liquid water - which in the meantime continues to face the flame, has increased significantly in the meantime due to a lack of heat dissipation. The heat capacity of the evaporator helps to evaporate the injected water, which has been preheated by steam condensation, very quickly. A new cycle begins.

These boat models operate with a fairly stable pulsation frequency of the order of 1–10 Hz .

Physically, such a drive can be seen as an oscillator that is energetically fed by the heat flow of the flame. The speed of the water flow in the pipe, the amount of steam and pressure in the vapor phase , temperatures on the walls of the evaporator and pipe fluctuate rhythmically and out of phase , because the various energy reservoirs interact, coupled via the inertia of mass, heat capacity and the phase transitions boiling and condensing. The high energy of evaporation and its viscosity also go into the water medium.

Noise generation

The characteristic, name-giving rattling noise of the boat is not caused directly by the evaporation process, but by a special design of the evaporator: The top is made of spring steel sheet like the cracking frog or sheet brass. During the rapid pressure changes, this upper side jumps back and forth, producing a cracking noise in each case.

Screw evaporator

A simpler, but just as efficient construction of the Knatterboot is based on a single thin copper, brass or aluminum tube with an inner diameter of around three to four millimeters. The tube is wound into a screw in the middle part so that the two ends come off parallel and of equal length. The screw acts as an evaporator. A screw evaporator works noiselessly and can accelerate a boat to speeds of 10 to 20 centimeters per second with the appropriate energy input - for example from an alcohol burner.

At the evaporator, the pipe is heated to at least the boiling temperature of the water. The pipe has water temperature at the jet nozzles. The following processes take place at the meniscus , the water vapor-to-water boundary: The water vapor pressure forces the water towards the jet nozzles. The pipe sections previously filled with water are so cool that the water vapor condenses. After the condensation, there is a negative pressure compared to the air pressure due to the volume difference between water vapor and water , and the meniscus moves back towards the evaporator.

Other names

The rattle boat is known by many other, mostly onomatopoeic names:

Puff-Paff-Boat (before World War II )
Pütt-Pütt-Boot
Steam jet boat
pop-pop boat ( Engl. )
Candle boat

Movie

In the anime Ponyo - The Great Adventure by the Sea , a girl with magical powers enlarges a rattle toy boat, which from then on serves as a real boat for two children. The drive was the candle, enlarged with the magic.

literature

Jürgen Becker: Putt-Putt-Boot . In: Walter Czech (Hrsg.): Physik, teaching materials for teachers Sek II , supplement 18, 1994, Stark Verlag, Freising
I. Finnie, RL Curl: Physics in a Toy Boat , American Journal of Physics, Number 31 (1963), pages 289-293
Jens Johannsen: Study on the Optimization of Chug Boats . In: Journal Steam and Hot Air 4-2008, p. 16 ff., Neckar Verlag

Web links