Making a pop-pop steam boat: a good experiment for children

Maurice Jégado

A pop-pop boat

1. What is a pop-pop boat?

Pop-pop boats are powered with a simple boiler to which two thin copper tubes are connected. The tube ends extend out the back of the boat into the water. The boiler is heated by a small flame which heats the water inside. When the water vaporises, the steam jets out the back, propelling the boat forward in a way similar to rocket propulsion.

The pop-pop boat originated in a British patent in 1891 for water pulse engines by Thomas Piot, although a similar boat was published in a French journal that dates back to 1880. Since then many experiments both commercial and domestic have been conducted all over the world for the delight of many.

2. Plans for making a toy boat

Constructing a boat within a single-session workshop with children requires making simple choices. After such a simple experiment, children may pursue with more sophisticated individual improvements if they wish. Our choices are explained below.

Making the hull

For making the hull, we rely on [Harrison] where the author gives a detailed plan for getting the body out of a milk carton which is water-proof and easy to cut. This is cheap and furthermore has the advantage of illustrating the virtue of recycling materials. This is of educational value to children. The steps are listed below:

• Cut off the bottom and top of a milk carton, open up the carton so that it is flat.
• Print the hull pattern (at a resolution of 300dpi) which is a slightly modified version with respect to the reference above. Dashed lines correspond to folding lines while solid lines correspond to cuts.
• Line up the pattern on the carton and hold it with clips.
• Cut along the solid lines and fold along the dashed lines so that he print side of the carton is inside.

Once this is done, we are ready to assemble using staples and hot glue. First, assemble the front of the boat and then the rear. Be sure to scuff up the surfaces that are to be stuck together with sand paper before applying the glue. Once the body is glued, you can add a few staples to improve the robustness of the structure.

To cover the inner printed side of the hull, we use an aluminium foil which happens to be similar colour to the outer side.

The boiler

For the boiler, we are faced with two types of design: either using a coil principle or relying on a more sophisticated version based on a little shallow chamber with a flexible diaphragm.

Making a water tight chamber may prove a bit tricky and require soldering or careful epoxy gluing. Therefore, we opt for the coil principle for children as a basic experiment although the chamber principle leads to higher performance boats making a stronger and clearer pop-pop noise due to the vertical movement of the upper membrane of the chamber.

Making the coil raises a number of questions.

• What material can be used? Copper, stainless steel, or brass can be used. We use brass as small diameter tubes can be easily found in DIY shops.
• What should the diameter be? We prefer the 3mm diameter as it is quite light and performs correctly. It is quite easy to bend without kinking. The 4mm might perform better but tends to capsize the boat. The 2mm would be ideal in that it is cheaper but experience shows that the tubes tend to get dry and the boat stops functioning quickly.
• How to bend the tubes? If you have a professional tube former kit as used for car brake pipes, the task will be easier for you. If, like us, you don't have access to these professional tools, you can take a round bar of about 12mm diameter (a pipe will do) and clamp it into a bench vice. Press the tube firmly against the cylinder and bend the tube around the cylinder. The procedure can be made easier and safer by prior annealing. To anneal, first heat the zone of the tube with a propane torch or a camping gas and plunge the hot tube into cold water. You can repeat this procedure several times for making a full turn of the tube.
• How many turns should the coil have? Two turns are enough for a basic experiment although in [Flogel], the author reports that three turns lead to better performance. Now, in that reference, the coil axis is horizontal while in our version the coil axis is perpendicular to the boat surface.
• What about the shape of the coil? In our experience, giving a slight down curve to the tube before reaching the coil, and a down slope towards the water surface ending with horizontal tubes inside the water works better. As discussed in the next section, ideally the liquid piston should be located in the down slope towards the water surface although this is difficult to check with non-transparent tubes.
• How long should the tubes be? This is a question which we do not have an exact answer to. In practice, a length of about 10cm worked fine in our experiments. You can proceed with your own experiments to tune this value. For cutting the tube, a tubing cutter or a hacksaw can be used. Clean the debris out of the cut.
  As a variant, we might use a shorter metal part which is cheaper, and lengthen the tubes with plastic, flexible drinking straws. But, of course, the straws should be of the appropriate diameter and be sealed with silicon so as to avoid any leaks at the junction.
• Setting the coil. To set the coil in the boat, we take a small wooden bar previously pierced with two holes, two holes are also pierced in the top rear of the boat. The coil tubes are then inserted through these holes and the bar is held by pressure between the hull sides.

Putting the boat to test

We are now ready to see the results of our efforts. Before putting the boat to test, it has to be primed filling the coil with water. You can suck water from one tube, tap one of the tube ends with a finger, and put the boat on the water surface. Of course, tube ends have to be under the water level. Alternatively, you may use a syringe to fill the tubes.

Light a candle ('tea light') such as those used to keep food warm on the table and slip it under the coil. After a while the boat should begin to pulse and move.

With the help of a torch you may see the reflection of the ripples formed at the surface of the water. You may also add a few drops of black ink so as to see the water movement more distinctively. If you hold the boat in your hand pressing it gently, you'll feel the oscillation pulse.

Don't be too much demanding on your first experiment. Building a pop-pop boat that works is relatively easy but building a boat that starts reliably and pulses regularly and "indefinitely" is not an easy task, it requires perseverance in a trial and error process.

To guide the boat to run endlessly in circles in a small tray, you may stick a drawing pin or something the like onto the bottom surface at the centre of the tray, as suggested in [Bindon]. Glue a tether strip at the bottom of the boat, make a hole in the strip and anchor it to the pin so that the boat clears the edge of the tray.

If you have access to a local pond, why not not organize a collective experiment and a competitive race with all children? Now, adding a rubber or a ballast might prove necessary to give the boat more stability. For that purpose, you may add a small piece of water-proof material on both sides at the rear of the boat.

3. Understanding the physics of the pop-pop

For the sake of this discussion we will rely mainly on [Bindon], [Crane], [Renaud], and [Simon].

The basic pop-pop process is an oscillatory cycle [Crane]. The lower part of the tubes is filled with water at atmospheric pressure which acts as a piston rod while the upper part is filled with a mixture of water steam and air. The oscillatory momentum of the water pistons causes pressure fluctuations in the upper part of the system while steam condensation on the walls of the cold tubes causes a drop in pressure necessary for pulsation to be maintained.

To piece together the basic steps of the oscillation cycle, let's start when there is reduced pressure in the boiler and the water is being sucked up in the pipes. The liquid piston compresses the mixture of air and steam. As steams enters the boiler, micro drops of liquid in suspension flash vaporize immediately resulting in super heated steam being produced at a pressure which may rise well beyond the atmospheric pressure. The compressed gas pushes back the piston forcing the water down the pipes and giving the boat the propelling pulse. The steam quickly condenses on the cooler surface of the tubes thus reducing the pressure in the upper part below the atmospheric pressure which causes the next cycle to start.

Notice that in the standard cycle, liquid water normally does not reach the boiler. In fact, the interface between steam and liquid water may be quite distant from the boiler. In [Renaud], it is reported that the smaller the diameter of the tubes, the more distant the interface zone from the boiler.

In [Bindon], the author claims that air plays an important role in the good functioning of the motor. Air bubbles mixed with cold water are sucked at each cycle. But this air has to be discharged periodically although over a longer period than the period of the basic cycle. Failure to discharge the air excess might be one of the reasons why the boiler surface gets dry causing the boat to stop functioning for a long time.

This observation is also confirmed by [Renaud] who speaks of low-frequency sucking that alternates with sets of high-frequency sucking. In a low frequency sucking, the liquid water would move as far as the boiler and the pulse would stop for a short while.

In [Renaud], the author also models the motor with various equations. Quantitative values based on measurements made on a toy boat as well as computed from the models are reported. We reproduce here part of these results to give a flavour of magnitude.

• Frequency of the pulse. The pulse frequency is quite high and would range from 3 to 10 Hertz/second for toy boats. The thinner the tubes and the smaller the boiler, the higher the frequency.
• Yield. The yield of a pop-pop boat is very bad. More than 99% of the energy is lost in heat.
• Pressure variations. The pressure within the boiler of the toy boat used for the experience ranges from -2mbar to +10mbar. According to the theoretical model developed, the maximum variation between the low and high pressure might reach up to 60mbar.

As for the explanation of the forward movement of the boat, it just derives from the application of the momentum laws in physics. When water is being expelled out, the boat communicates a momentum quantity to the outside gaining the opposite itself in a way similar to rocket propulsion.

4. Annotated bibliography

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