Build a boat that is powered by a candle

Fascinating: The motor of a Knatter-Boat consists of a tube and a tea light and does not require any electronics. Here are the instructions to build it yourself.

Hull made of styrofoam and burning spiral made of steel pipe


Knatter boats, also called putt-putt or puff-puff boats, have been around since at least the late 19th century. Century. According to the 2. After World War II they fell into oblivion in Europe, in India they are still a popular toy today. With the following instruction we can build our own Knatterboot.

Traditionally, Knatter boats were built of sheet metal – but no one should feel constrained by this in their imagination. Styrofoam, milk cartons or even Dutch clogs are just as suitable. Knatter boats use a water impulse drive for locomotion; the "motor" consists of a tube and a heat source (in our case: a tea light) and does not require any electronics. Unfortunately, the efficiency is rather weak: in engine construction, the principle was therefore quickly discarded. For self-made toy boats, however, it is ideal.

Production of the spiral evaporator

The first step is to form the firing spiral: To do this, wrap the tube tightly around the broom handle in its center: one and a half turns are enough. The result should be a flat circle with 2 protruding tube ends. By the way, steel pipe is relatively easy to deform; to avoid kinks in the pipe, it is enough to fill the pipe with quartz sand. Because once you have kinks in the tube, the drive no longer works. A fixed in a vise handle of course simplifies the work, but is not essential. If you work in a group, it is enough to have another person holding the handle.

Now each tube side must be bent twice slightly: Once in the first third behind the spiral and once in the second third behind the spiral (see photo). The length of the side pipes from the spiral to the end of the pipe should be between 12 and 15cm. It is advisable to mark the bending points on both tubes before bending, so that they are sure to be bent in the same way. In the next step, you bring the tubes to the same length with the hacksaw and deburr the ends with sandpaper. Now all that’s left is to cover it up: Wrap the spiral tightly with some aluminum foil.

Making the hull of the boat

Using a sharp knife or cutter, the styrofoam hull is cut first. With the jet nozzles ahead, the evaporator is now speared through the Styrofoam at an angle. You have to pay attention to several things: The spiral itself must be high enough above the hull that you can still mount a lit tea light under it. The jet nozzles must always remain below the water surface, ideally they are also bent a little upwards: This allows water to penetrate the tube more easily – essential for the effect of the "engine". In the last step, a rudder is mounted on the rattling boat. Just score the Styrofoam on the bow and wedge in a small piece of Tetrapak – that’s it.

If you make the hull of the boat from thinner materials, you may need a holder for the spiral; you can easily make this yourself from a rectangular piece of tetrapak. Using a pointed object, you drill two holes about 3mm wide in the fixture. Your distance will be determined by the spiral evaporator to be performed; the holes should be small enough for it to fit tightly in the bracket. The spiral evaporator is passed through the bracket and the hull of the boat, and the bracket is glued to the hull so that the evaporator remains firmly seated in the desired position.


Before launching, you fill the steel tube completely with water and put the boat in the bathtub, a large bowl, a pond… Now you ignite the tea light mounted under the spiral. After about 30 seconds, the boat should start with a jerky motion and after another 30 seconds, regular propulsion begins: the Knatter boat can now theoretically run for up to several hours – until the tea light goes out…


Knatter boats use a water impulse drive for propulsion. Before launching, the entire steel tube is filled with water: If one sets the boat now on a water surface, the liquid cannot escape. If one lights a tea light under the spiral, however, the water is heated up to the boiling point there. The resulting mixture of water vapor and air has a larger volume and squeezes water out of the pipe ends: the boat starts up in one go.

As the candle continues to heat the spiral, the water level in the tubes slowly rises again. In addition, parts of the water vapor condense in the unheated part of the tube and become liquid water again. Slowly, a negative pressure builds up in the spiral part – now new water is actively sucked into the pipes from the outside. When the water approaches the heated part of the brass pipe, it evaporates again: a slight overpressure is created, which forces water out of the pipes. The principle is the same as for the first jerk, but less powerful, because the system is now balanced. In a weak but regular oscillation, water is sucked into the tubes and pressed out again.

But why does the boat move from the spot, since the same amount is sucked in and pressed out?? Quite simple: When water is sucked in, it enters the tubes from all directions, it is impulse neutral. When squeezed out, however, it forms a jet; here the force goes in one direction and thus propels the boat.


Sometimes the Knatter-Boat doesn’t work the way you think it will..

– The jet nozzles do not work evenly, the boat has a twist to one side: In this case it is often sufficient to wrap the jet nozzles with aluminum foil. It acts as a cooling plate and ensures an even temperature in both nozzles.

– The Knatter boat is driven for a long time and then simply stops: Provided the candle is still burning, the reason for this behavior is often a layer of soot on the aluminum. Scratch this off and start again.

– The Knatter-Boot is driven for a long time, goes down a strong propulsion and stops: The engine is dry, the candle has warmed up the evaporator too much. In this case, refill the tubes and position the candle at a slightly greater distance from the spiral. It also helps if the jet nozzles are pointed slightly upwards, as this makes it easier for new water to penetrate.

– The Knatter-Boot does not drive at all: The candle does not heat the spiral evaporator sufficiently, reason is either a too large distance to the evaporator or a too short wick.


Knatter-boats can be made of all kinds of substrate materials; only it must be able to float. Styrofoam is very suitable for boat building, but unfortunately it dampens the rattling noise that is otherwise typical of boats. If you want to build a typical loud rattling boat, a metal hull is the best choice.

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