Pop pop boat

The Knatter, putt-putt or Puff- Puff - boat is a toy boat with water pulse drive ( Dampfjetantrieb, Pulsarmotor, pulsed waterjet ). The simple motor comes from any moving parts such as pistons, valves or wheels. As an energy source is a candle or other heat source. At the stern of the boat protrude below the waterline two pipes side by side and parallel out. Through the pipes water is sucked and discharged again. Although just sucked as much water as is ejected, there is a positive balance of momentum that propels the boat forward. Cause of the positive pulse relative to the boat as a balance envelope is the fact that the water is indeed drawn from all directions, but is preferably discharged again in one direction. When suction is thus produced a spherically symmetric and thus pulse neutral Valley, in ejecting a pulse -prone free jet.

Principle of operation of the engine

Flat evaporator

View from below

A common design of the engine is shown in the top images: A flat evaporator (yellow) of copper sheet is positioned horizontally over the flame. From two sides of the evaporator, the recoil tubes ( dark gray) run to the stern of the boat where they open below the water line. The second tube is not necessary for the function. But it facilitates the bubble-free filling with water before use. If the water reaches the boiling point, it comes in the evaporator to a sudden evaporation and the water in the tubes is expelled. Because of the inertia of the water in the tubes and cooling of the evaporator by the expansion of the steam produced in the evaporator, a vacuum which means that fresh water is drawn into the evaporator through the tubes. Thus, a new cycle which keeps the system in a continuous oscillation begins.

Noise generation

The characteristic eponymous crackle of the boat is not created directly through the evaporation process, but by a special design of the evaporator: The top is formed of spring steel sheet as the clicker or brass sheet. With the rapid pressure changes these top jumps back and forth, producing each a cracking noise.

Evaporator coil

A simpler but equally effective construction of Knatterboots based on a single thin copper, brass or aluminum tube of approximately three to four mm inner diameter. The tube is wound in the middle part to a screw that the two ends come off parallel and equal in length. The screw serves as an evaporator. A screw evaporator operates silently and can be a boat with appropriate energy supply - for example, by a spirit burner - accelerate to speeds of 10 to 20 centimeters per second.

In the evaporator, the tube is heated at least to the boiling point of water. On the jets, the tube has water temperature. At the meniscus, the steam -to- water boundary, the following processes take place: By the water vapor pressure, the water is forced in the direction of jets. The previously filled with water pipe sections are so cool, the water vapor condenses. After the condensation is due to the volume difference of steam to water, a negative pressure to the air pressure, and the meniscus moves back towards the evaporator.

Other names

The Knatterboot is known by many other, mostly onomatopoeic names:

• Puff- Puff Boot ( before the Second World War)
• Pütt - Pütt boot
• Dampfjetboot
• Pop-pop boat (English )