I forgot about the pop-pop boats till I found them again on the web as collectors items. There are actually quite a number of pages dealing with that phenomenon. Now recently I found this little boat in a shop in Stockholm and of course I couldn't resist! It's made in India and the price was SEK 50.00 ($ 6.00).

The following close up shows the hull and boiler. The boiler is basically a flat bowl covered by a membrane. The two pipes come out of the bottom and go to the back of the boat. Water is filled into one pipe end till it comes out of the other. Then the thing is put into water (without letting air go back into the pipes). Now the burner goes under the boiler and the whole thing starts. When the water turns to steam, it pushes some water out of the back ends of the pipes. The steam that expanded into the pipes cools down and condensates. The vacuum sucks water back into the pipes. One cycle is finished and it all starts over again.

The membrane is not really needed for the basic physical function but seems to improve the efficiency. Now the question is, WHY is the boat moving forward? One should think that pushing out water and sucking it back into the pipes should keep the whole thing in balance or just make it vibrate. But it really moves! So there is no balance of forces but instead the force (relative to the boat) created by the water pushed out is greater then the one created by the water sucked in. The reason is the focus and direction of both.

When the water is being pushed out, it is quite focused due to the straight pipe. When it's sucked in, it's much less so (if at all). That results in the forces pushing the boat forward being much greater than the ones pulling it back. That seems to be the whole principle. Click the button below to hear the noise those boats make and you'll understand where the name "Pop-Pop Boat" comes from.

I found this boat in a souvenir shop while being on holiday home in Germany. The saleslady said, they were only recently available again but the old shop owner would remember, to have sold such boats 35 years ago. Different from other boats this one has the pipes not soldered into the hull but glued. Probable a sign for cheap mass production. But the color scheme is really very nice.

These are the accessories the boat comes with. The little hose is used to fill the tube with water.

It's a Chinese replica of a 1950's Japan made model. I got it from a US mail order service for $ 11.00. It's sure the cutest of the three boats I have. Especially the "skipper" looks nice.

That's the accessories it comes with.

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Inspired by suggestions from other webpages I tried a simple test myself. A flat fish can, a tee-warmer candle and a simple 3 mm brass pipe bend to a coil. That's all!!!

I don't think there is more to explain. The picture shows it all. Note the waves! They are actually caused by the pulse of water coming out of the pipe.

Believe it or not, the thing is moving and continues to do so till the candle is put out!

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I think I'll go on with some more "fish can tests" to improve the performance. The first (single turn) boiler coil was just wrapped round a piece of wood. It worked but was not that easy to do. I almost ruined the pipe. Now I'd like some kind of simple gadget to make it more easy.

A small piece of wood for the base. Some thick steel rings from the hardware store and a screw to hold it together.

This shows the thing in action. The rings have a diameter of 14 mm. After bending is the inner diameter of the coil about 14.5 mm. The screw clamps are very important to fix the pipe but make the whole handling a little tricky.

Same thing but the screw clamps are replaced by four screws, two at either side at the pipe. Two screws with big washers are squeezing the pipe against the base. To get proper bends without folds, it is VERY important that the section A-B does not move while bending the pipe.

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Now this is a test with a boiler bend with the help of above tool. The difference to the first one is a smaller diameter of the coil and two turns now.

It seems to work better but I believe the flame is too small or the heat is not used efficiently. I think I will try some kind of fuel burner.

I used two of those small aluminum pots which normally contain the candles. I cut the height to about 2/3 and drill a hole in the middle of one. It is then pushed upside down into the other. A bit of mineral wool serves as wick. The finished burner is filled with lamp oil (olive oil is supposed to work too).

It works better than the candle but still the entire coil is not heated evenly.

A little cover made from tin can with holes in it serves as kind of a distributor which spreads the heat over the whole coil. This shows the best result so far.

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Richard is living in Texas, USA and was at the time we first got in touch (summer 2000) 77 years old. Richard told me that those boats were sold in the USA as kits with balsa wood hulls and prefabricated boilers during the 30^th of the last century. He had build himself a thing like that around 1939! If that's not amazing, I don't know!
I have got Richards permission to use all his information at my page and I'm really glad to be able to show it here.
Thanks Richard.

I quote:
"Thanks to the information in your WEB site regarding "pop-pop" boats, I became interested in recreating them myself. The first one was made very much like yours - a coil of small diameter copper tubing with a heat sink soldered to the top. It ran well but no "pop-pop" sound.
The problem was making a thin, flat boiler with a very thin top that would "oil can" with the pressure surges that propelled the boat.
First I made a simple die using 1/8" (3 mm) steel for backing plates and 3 mm aluminum die forms that produced a 3 mm deep recess in a small sheet of .015" (0.4 mm) brass. Out side dimensions of the recess is about 1" x 1 ½" (25 x 38 mm).

After drilling two holes for the water pipes, I soldered the top on. It was made of 0.003" ( 0.075 mm) brass. I found that there was no need to deform the brass top so that it would "oil can". The heat of soldering put several wrinkles in it.
I used 1/8" (3 mm) brass tubing to make the water pipes, beveling the top ends at a 45 degree angle to insure circulation would not be restricted if I soldered them in too close to the top of the boiler.
The boat's performance has been excellent - makes a lot of noise and goes fast.
I used brass shim stock, available in most machine shop supply stores, and brass tubing purchased in a model airplane hobby shop."

I asked Richard what kind of solder he would use? I was afraid, soft solder couldn't stand the heat of normal operation. Here is his answer.

I quote:
"I used soft solder. So long as there is water in the boiler, it will not melt. If, for some reason, the pulsing stops be sure to blow the flame out within about 10 seconds... ...The manufacturers back in the '30's used soft solder. We learned the hard way to never put a lighted candle under an empty boiler. The result was the boiler landing on the candle..."
End quote.

That's all from Richard so far.

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Mini-01 (homebrew)

Because the boat is small and it's my first, it's called Mini-01. But small can be trouble. The boat is fully made from metal and due to its small dimension and the comparatively high weight, its buoyancy is minimal. In fact the first tests showed that it needed some help. Therefore I fittet a 3.0mm plywood layer under the hull. The boats lenght is only about 8.0cm. I think with 10.0cm I could have avoided the trouble. But then again, its small dimension makes much of its charme. By clicking at the picture above, you can download instructions in PDF format. But please read these notes first. It may help to avoid failure.

The picture shows a kind of X-ray view with the parts and how they come together.

We start with the hull. Special thanks go to Mark Horovitz, who kindly allowed me to use his own hull-design and publish it here. Marc's own boat was described by himself in great detail at the excellent Pop-Pop Pages of Vance Bass. The only change I made to his hull was to cut off the three tabs. Instead I just solder the corners as they are. It saves some weight and for a small boat like this, it's still stable enough. I also added a bench and a bow-cover. For material I'm using tin-plate. It's cheap, light and very easy to solder. I use a pair of ordinary scissors to cut it. I first cut a bit generous. Then I do the final cut along the lines. That avoids distortion at the edges.
Now this is important: Do NOT cut the curve around the bow! This is being done AFTER soldering!

I first did a little test with cardboard to get a feeling for the dimension.

The walls are up but unfortunately I didn't have a big enough tin. So I had to cut bottom and walls seperately. The additional soldering regained me the weight, that I was trying to safe by cutting the tabs :o(

Soldering is done, now we can cut the leftover at the bow. Soldering is much easier this way.

The engine is a simple coil with four turns of 3.0mm brass tubes. Copper would work just as well. Inner diameter of the coil is 8.0mm.

I used this very simple tool to wind the coil. I used a thick board, drilled a hole with an 8.0mm drillbit and left it inside the board to serve as core. The screws hold the tube in place. That's very important to avoid kinks.

I drilled a second bit right beside the first and used it to form the bottom bends. Please note: the picture shows a coil with six turns. This boat uses a similar with only four turns.

This are the pieces for the bow-cover and the bench. First we must solder the engine into two 3.0mm holes in the stern. Then we solder these pieces into place.

This is the result so far. With water in the boiler and fire under it, it should work just fine.

Here comes the sailor for our litle boat. I call him Pop-Pop Eye ;o)

Cutting the sailor out of tin-plate was not as difficult as I thought. First I cut around the edges. Then i used my Dremel-tool with a small cutting disc, to form the contour. The little pin, soldered to his bottom, keeps him to his place at the bench. The little flag was easy enough to cut and solder into the hole at the bow.

It's the schematic for the burner. The dimensions are 1.5cm x 1.0cm x 0.5 cm but depend on the available space between the pipes!

Fold up the walls, solder the corners, then fold the lid down. My burner is stuffed with rockwool that is normally used for building insulation. I use alcohol for burning. I only apply as much alcohol as is needed to totally wet the rockwool. No alcohol should be able to leak out. The operating time is not that long but for such a tiny boat it's OK.

Burner and sailor are joined together by a little distance piece. Dimension and shape depend on the situation inside the boat. As the sailors position is fixed through the hole in the bench, so is the location of the burner underneath the boiler. Adjustments may be required here. The sailor has practically become the handle for the burner.

All done, ready for painting.

If that's not colorful, I don't know ;o)

Assembled and ready to launch.

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