by Marc Horovitz
Photos and drawings by the Author
Pop-pop engines, or water-impulse engines, as Basil Harley calls them in his book, Toyshop Steam,
date back a hundred years or so. Their origins are obscure, which adds
to their charm and mystery. The pop-pop engine is simply a tube of
coiled copper that has both its ends in the water. The coil is heated
and the boat in which it is installed mysteriously moves forward.
How this works, in scientific terms, is still open to conjecture and it
has been the source of some controversy. What appears to happen is that
a little water in the tube flashes into steam, forcing the rest of the
water out the back, making the boat go forward (see one of Newton's
various laws). The vacuum thus created within the tube then draws in
more water and the cycle repeats.
In practice there are many variables, as follows: the length and
diameter of the tube, the heat source, the weight of the boat, and
whether or not the flame is shielded. Any and all of these will affect
performance. I’ve not seen a great many pop-pops at work, but of
the ones I have seen, none put on a performance that I would call
great, or even good. However, if one considerably lowers one's
standards, a well-designed pop-pop boat will perform in a manner that
could be described as acceptable.
Pop-pops have always been the stepchildren of the steamboat family.
They can truly be called steamboats, but they bear little resemblance
to a proper Steamboat. They have no moving parts, after all. Not even
one! In the early days of pop-poppery, the German manufacturers often
built a thin, flexible metal diaphragm into the water/steam circuit.
This diaphragm was dished slightly and, when the water flashed into
steam, the thing would spring out, making a noise like a metal cricket
that you snap by pressing it with your thumb. That's how the boat got
its common name. (The Germans call them toc-toc boats for the same
reason.) Similar inexpensive boats are available today from India.
Building the boat
Building a pop-pop boat is a good exercise in simple sheet-metal work.
You can build it out of brass, but I prefer tinplate from an old
paint-thinner or olive-oil can. If you use brass, don't build the boat
from anything more than .010" thick.
The hull I show here can be made from a single piece of metal. I have
deliberately not included a description of any type of superstructure.
You can be as creative as you like here. I've included little more than
the bare minimum required to get a boat working in the water.
The pictures pretty much tell the story, so just follow along. Click here to download the full-size hull drawing.
Photo 1. Print out the two-page PDF of the hull.
Photo 2. Cut each half along the double-dotted match line and tape the pieces together to form the whole pattern.
Photo 3. You’ll need some raw material. I used a flattened out
gallon olive-oil can (check with your local Greek restaurant).
Photo 4. I use Krylon #7010 Spray Adhesive to stick the pattern to the
metal. All it takes is a light coat on the back of the pattern –
you want it to easily peel off later.
Photo 5. Glue the pattern to the metal. If you’re using a printed
can like I did, you’ll need to decide if you want the printing
inside or outside the boat. Either one is fine, but glue the pattern to
the outside. Also, if you’re planning not to paint the boat,
place the pattern to take best advantage of the printing on the can.
Photo 6. First, drill the 1/8” holes for the water tubes. Then,
with a stout pair of scissors, start cutting out the hull. The solid
lines are “cut” lines and the dotted lines are
“fold” lines. Cut the sides and the transom (back end), but
do not cut around the bow (front end). Leave the bow a flat chunk for
Photo 7. This is what the cut-out hull should look like at this stage.
Photo 8. I like to do my bending in a vise. Here, I’ve cut some
wooden blocks the length of the transom fold. I’ve carefully
placed the blocks and the work piece in the vise, making sure that when
I make the fold, the side tabs aren’t in the way.
Photo 9. With your fingers, carefully make the fold by simply pressing the metal over the block.
Photo 10. Use a soft mallet to make the folded corner sharp. If you
don’t have a soft mallet, put a piece of wood over the fold and
smack that with a hammer. Now follow the same procedure to fold the
side tabs. They can go either inside or outside the transom -- I think
they look better inside. Also, fold the front tab on the one side.
Photo 11. Folding the sides up is the same as folding the tabs, but
without the wooden blocks. However, if the fold line is longer than
your vise is wide, make some wooden blocks of the proper length and
Photo 12. This is what the completely folded hull should look like now.
Photo 13. Now carefully peel off the paper pattern and throw it away.
Photo 14. Before you can solder you must prepare the surfaces. If there
is any printing on the surfaces to be soldered, it must be got off. On
mine, the tabs had printing on them. I ground it off down to bare metal
with the edge of a cut-off wheel in my Dremel. Do this wherever
Photo 15. The boat will be soft-soldered together. I like to use a paste flux (right) and 1/32” tin/lead solder.
Photo 16. My tool of choice for soldering tinplate is a good old
35-watt soldering iron, available at any electronics-supply store. I
prefer a V-shaped chisel point, which can be easily filed if yours is
Photo 17. Each surface to be soldered should first be tinned –
that is, coated with a thin layer of solder. First, butter a very thin
layer of flux over the entire area to be soldered. Then charge your
iron with solder. This just means to get some extra solder on the tip.
In general, solder will not be applied directly to the work, but
transferred to the work from the iron. Then slowly wipe the tip of the
iron across the fluxed area. The solder should easily transfer, leaving
the area shiny silver. Try to get as thin a layer as possible. In the
photo, the area below the iron has already been tinned.
Photo 18. On my boat, I found it advantageous to hold the tinned sections together with a clip while soldering.
Photo 19. To make the joint, charge the iron with solder, then run it
along the edge of the joint. The tinned areas should heat up, drawing
the solder into the joint. You can run the iron over the outside joint
as well. The photo shows a soldered inside joint.
Photo 20. This is the outside of the joint. If it is not neat enough
for you, it can be cleaned up with a small file. Solder the bow-ends of
the sides together in the same way.
Photo 21. Here’s the boat with all three tabbed joints soldered.
Photo 22. The bow of the boat can now be soldered to the bottom. Flux
both surfaces, press the bow down onto the bottom for a good fit,
charge your iron, and solder it together in small steps. Be generous
with the solder. The photo shows the finished soldering for the bow.
Photo 23. Now that the hull has been fully soldered together, the
excess can be cut away with scissors. Clean up any rough edges with a
file or sandpaper.
Photos 24 & 25. The finished basic hull.
Making the motor
To make the pop-pop “motor,” you’ll need some pieces
of K&S 1/8” OD copper tubing. This comes in 12”
lengths. You’ll need one 12” length and two 3-1/2”
lengths. You’ll also need a couple short pieces (1/4”) of
the next telescoping size up.
Photo 26. Holding one end of the long copper tube in a vise, heat the rest of it until it glows red to anneal (soften) it.
Photo 27. When it’s cool, carefully start bending it into loops
until it begins to harden up again. You should be able to feel it as it
gets harder. This is called work hardening. The photo shows how far I
was able to bend the tube after the first heat.
Photo 28. Now repeat the process, reheating the hardened tube. This photo shows how far I was able to go on the second heat.
Photo 29. After the third heat, I put a piece of 1” bar in the
vise and finished wrapping the tubing around it, which gave it a nice,
even appearance. Counting across the top, you should have five loops.
Photo 30. The two smaller pieces should be heated and bent as shown here.
Photo 31. This picture shows all the parts necessary to make the motor. They are ready to be soldered together.
Photo 32. To solder the pieces together, clean the ends with a piece of
emery or steel wool. Wipe a little flux on them, as well as inside the
collar. Put the pieces together and apply the iron. In this instance,
you may want to apply solder to the joint while it’s heating. A
small torch will do this job quickly. Don’t worry if the legs are
not pointing the right way when you solder them on. They can be easily
twisted into alignment later. The picture shows two finished motors.
Installing the motor, and sundries
Spread the legs of the motor so that they easily slide into the holes
in the bottom of the boat. The design is based on a votive candle,
which is around 1-1/2” in diameter. The kind I use each come in a
little aluminum tray, which keeps them from getting messy.
Photo 33. The legs of the motor should fit easily through the holes,
pointing to the rear of the boat. If the holes are too small, you might
have to open them out a little with a round needle file or a very
slightly larger drill bit.
Photo 34. This is what the legs should look like under the boat. It’s OK if the ends of the legs touch the hull.
Photo 35. Solder the motor to the floor of the boat using the soldering iron.
Photo 36. Carefully bend the legs out so that they are parallel with the bottom of the boat.
This finishes the basic boat. However, on mine, I wanted a flame
shield, so decided to make a “boiler” out of some more
tin-can bits. I found a nice mango-juice can that is about 2” in
diameter. A tomato-paste can might work, too. Worst case, you can roll
one up from flat metal. I punched some 1/4” holes around the
bottom to let air into the fire. I made a top for the boiler out of
another piece of tinplate, into which I punched some more holes so the
hot gases could escape. Then I made a decorative smokestack out of a
tinplate tube I rolled up. All of this is soft-soldered together in the
same manner as the boat. I soldered some tabs to the floor of the boat
to keep the boiler from wandering when under sail. I also soldered in a
tinplate ring to hold the candle in position.
Photo 37. The tinplate “boiler” in position over the motor.
Photo 38. The boiler just slips off. Here, you can see the candle in
place under the motor as well as the tabs that hold the boiler in place.
Photo 39. I thought a rudder might be handy, so I made a simple one.
The tiller is just a bent piece of 3/32” brass rod. The rudder
itself was cut from tinplate and soldered on. The assembly is held onto
the transom by a simple tinplate bracket, soldered on. Friction retains
the rudder setting.
Photo 40. The boat looked too open, so I covered the front half with
another piece of tinplate. I made the pattern for this by putting a
blank piece of paper on the table and turning the boat over on it,
tracing around the bow. I added a little to the width so that I could
give it some curvature. I cut the piece out and tried it on for size. I
carefully trimmed it until I had a reasonable fit, then I soldered it
in place. Voila! The boat is finished.
Copyright © 2008 Marc
Horovitz. May not be reproduced without permission.
Prepared for the Web and hosted by Vance
Bass on the Small-Scale
Live Steam Resources site.
Go back to the