Pop-Pop Boats Are Weirder Than You Think

If you're wondering why the boat never cracked under thermal stress, it's because they made it out of quartz glass! How cool is that!
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Those glassmakers did an incredible job.

*I was introduced to the Pop-pop boat from Ponyo and always wondered how it worked. Amazing video, thanks for the explanation!*

for forward motion, the shape of the boat also acts as a latching/winching mechanism. It glides forward in the water easier than it glides backwards, due to the hull shape.

I am 69 years old, when I was a child my father brought home a metal Pop-Pop boat. I now have one to amaze my Grandchildren. Thank you for your video explaining how they work.

I've played with a dozen of these when I was little. They used to keep me entertained for hours.
I have forever been wondering exactly how these boats worked. At last your glass boat has put an end to that. High five !

I used to make bristle bots all the time as a kid and something that is really important is that the bristles are all angled in the same direction. That ensures that it is depressed forward when it vibrates before the bristles spring back to their original position with the brush slightly moved from where it was before

I saw a clip or two in this video with the engine running with asymmetric amounts of water in each tube (3:35). That's very strange. Other thermoacoustic engines require pretty precise resonance matching between the engine cavity and the load to get any work done. In this case you've got two mismatched loads with competing resonances somehow popping at the same frequency. I'm guessing the mismatch must be minor enough to force synchronization but would like to see further tests. Does the amount of water in the tubes effect the frequency of the popinating? If you make one tube a little longer than the other so it contains more water by default will it still run?

I remember when me and my dad would make these out of old soda cans. We would have to buy pre-made tubes and tanks but it was still fun. He said that these were a very common toy in India and other places in Asia

This is very interesting, reminds me of a pulse jet, both initially seem like the forces should be balanced and no thrust should be produced, and both essentially consist of just a cleverly engineered tube.

My dad and I built one of these for a science fair in 2nd grade. Won the fair and blew the teachers and judge away. We used coiled copper wire instead of a tank.

The drawings you refer to with the metal tank show a tank that is very flat with a height that is very similar to the tubing (~1D in height and only a few tubing diameters in width), and is slanted down towards the tubing. Your glass tank is several times the tubing diameter in both height and width, and doesn't have the same fluid dynamics as the metal tank. The tubing in the glass system are perpendicular and not tangental to the tank, again causing different dynamics. These changes might acount for the tank not cycing like the metal system.
However, the end result is "the same" type of oscilation but different efficiencies occuring in different locations.

The original boat tank is very thin, about the same thickness as the pipe, compared to your glass one. This may make it easier to push out more air, especially if one tube extended all the way to the back of the tank.

"The batteries are just there for weight" - exactly what someone would say to hide their perpetual motion machine. You can't fool us!

A mechanic you failed to mention about the metal pop pop boat is the top piece of metal in the chamber pops in and out just a little bit. This assists the positive and negative flows of the water. That's one of the reasons the metal one moved faster. Weight is likely a factor as well I'm sure.

I think the boat has a shape that helps forward movement and not bacwards, so it will tend to go forward even if you apply the same force in both directions.

I think the motion could be as well explained with the change in the center of mass. If you have 2 masses on a boat and you change their position, the center of mass will move, leading to the movement of the boat. If you do that repeatly, the shape of the boat will lead to a forward motion. It will be interesting to do this experiment with a pendulum on a boat.

I think it's the same effect as blowing a small flame out with a phone speaker. Even though the speaker both pushes and pulls, the net effect if stronger in one direction

I used to collect put-put boats as a kid: the role played by the thin metal diaphragm is crucial

For the forward motion i have another theorie that could apply. If the water is pushed out the tube, it will take the surrounding particles with it. like if you blow in an empty bag from a distance, the bag fills quicker as if you press it on your mouth and blow it like a ballon. When the water gets sucked in, a smaller volume of water gets accelerated to get sucked back in the tube.

I always like to put my guess for the big question on these types of videos… I’m at 7:30 and we’re about to figure it out. Basically, even though water is going in and out, theoretically putting the same force on the boat each time in opposite directions, we should stay still or perhaps shift back and forth slightly.
However, because of the shape of the hull, there is very little resistance when moving forwards, and a large drag resistance when moving backwards. So even though the same force is imparted on the boat, the boat slides forwards easily, but doesn’t ever want to slide backwards.

I would argue the bristle bot is not truly oscillating motion because of the rotary nature of the weight. The weight has a tendency to hop the bristle up and forward then down and backward which has a “hopping forward” effect. You can change the direction it hops by changing the direction of the motor. Great vid. Beautiful glass boat. That visual took so much speculation out of the equation. Two thumbs up

I had one of these as a child, but had completely forgotten about it. Thanks for the memory bump, and the explanation of how it works.

I'd assume that the shape of the boat also plays a factor in its forward trajectory as the bow of the boat is more streamlined for less resistance while the stern of the boat is not.

The glass one is cool, but I also like the little flutter of the metal one on the top.

I'm thinking this is equivalent in operation to the thermoacoustic engines I've seen -- the extreme form of which is known as a "howler tube" -- a hot mesh inside a tube open on one end, which will oscillate in audio frequencies (depending on the size of the tube). These operate like a Stirling engine without any mechanical displacer mechanism -- which appears to be a similar effect to what's happening in your see-through pop-pop engine.

Would an adjustable torch effect the frequency of the pops? If so it could possibly act like a speed control.

Also something as simple as the shape of the boat where the front allows for less water resistance vs the back of the boat would contribute to the impulse (change in momentum) to forward motion, even in the hypothetical condition that the forces cancelled each other out

I built one when I was a kid out of thin galanized steel and a coil of thin copper tube I got at a hobby store. Was really heavy but worked suprisingly well with a sterno can heating the coil.

Excellent. If I ever find myself on a deserted island with an oil drum, a couple of pipes, and enough wood to make a raft and bonfire, I know how to escape.

Great explanation! Now for our next question: would be possible to make a life size, working pop-pop boat?

Played with this boat since my childhood. Always wondered how it actually works. Thanks a lot for this beautiful explanation!!

Never heard of pop-pop boats..but the propulsion part could ALSO be explained because the back of the boat is squared off resulting in more friction. I would love to see one with a more canoe shaped design.

That's a very interesting mechanism. I wonder how energy efficient it is- that is, for however much heat you're putting in, how much force is getting put onto that boat? Would an air-based mechanism work for a wheeled vehicle if everything was scaled up? I know jet engines suck in air and push it out the other way, and I know traditional combustion engines turn pistons using explosions that expand and recede (basically just a giga-fast version of this sort of phenomenon). But if you tried to do something like that directly with the fluid mass like the pop pop boat does just on a larger scale, does that actually work?

What I wanted to know is who came with the idea of the pop pop boat in first place and added a metal plate? The guy was a genius

I remember seeing one of these in ponyo when I was little, I thought it was so cool. Still do actually, would love to have a little boat like that big enough for me to actually fit in

I made these from plastic ketchup bottles and thin metal piping from the back of a old refrigerator when I was in grade 5 for a science fair project... The pop pop boat I made didn't actually have a "tank", I just made a coil in the center from the mental piping and used that as the "tank", I found 3 loops worked best. And because it was just basically one long metal straw, I just sucked one end of the pipe with the other end in the water to easily fill it...

I was wondering why the pop-pop boat was so familiar, and then I realized it's the same one from Ponyo, and I've always wondered with mechanics behind it were.

The shape of the boat will influence it's motion as well. For a typical V shaped haul, it's much easier to push the boat forward, as apposed to reverse.

I think the net forward direction has a much simpler answer: The boat's shape makes it easier to move forward than backwards. If the boat was, say, a circle, you'd see the boat scoot around everywhere but in one single direction.

Now imagine doing this with a full scale boat

I had one of these as a kid and I was absolutely certain that it worked like a pulse get, sucking water in one pipe and pushing steam out the other. My boat wasn't terribly good so it went in circles, that may have contributed to my theory

cuando yo tenia aproximadamente 5 años de edad , este era uno de los juguetes que más me fascinaban , incluso llegó a comprarme uno mi Mamá , y con el tiempo se descompuso , uno crece y no lo recuerda hasta que videos como estos nos traen de regreso estos recuerdos.......tal vez no sean originarios de México y hayan sido una copia de algun otro país , pero yo ya tengo 68 años de edad ......así que hagan la cuenta.......muy bonitos.

I like how he even used a see through tooth brush in his bristle bot demonstration.

Not sure if it was asked or if I missed it in the video but couldn't the issue between the pop pop & your glass made (which was made flawlessly!), Wouldn't the heating element size be a factor?
Would you need roughly double the heat for the glass boat due to the size of the tank?
Does that matter in this case?
Like you said, clearly the glass tank doesn't move & pop like the Pop Pop's does but I'd imagine the amount of heat it's receiving is considered

I would be incredibly interested to see someone make a full-size version of this propulsion method

Hello Steve. I was delighted to find this podcast as I remember my dad buying my brother and I, each a put put boat with exactly the same little engine in it. This was back in 1954. The only difference was that instead of a little Tea Candle, we'd use Methylated spirits in the container. The little engine was filled exactly the same as you did, with water being injected into one of the pipes at the rear, and we knew the tank was filled when water squirted out of the pipe not being used to fill the little tank. As demonstrated in your video, the little tank was slightly sloped toward the outlets, and the little boat would scoot off as soon as the water got hot; with one difference. When it ran out of water in the tank; instead of sucking the water back in; the tank would empty and the solder that held the tank to the two outlet pipes simply fell off. There was no sucking in of water as the little thing motored along. Poor old dad made a rod for his back for after each "Maiden Voyage" if we did not grab hold of the boat and remove the small pan containing the metho, the "boiler" plate would fall off. I have been looking at making a little boat for all my grandsons who would love them.

Beautiful little glass boat those guys made! Though I do have an idea for a true recreation of the pop-pop boat that can have more of a way to deform. Using clear plastics similar to straws would help for the tubes/enclosed space, and making the boat itself out of the same or similar materials would help. Getting a heat-resistant plastic would also help with this as well, to make sure the flame doesn’t burn through the plastic. I think I know why you didn’t do this, since the plastics burning is uh t o x I c but it would be interesting to see one with a high melting point that wouldn’t produce too much toxic gas and smoke.

The shape of the the boat also helps it move forward. The aquadynamics are favorable for forward motion, as the front is in a more curved and arrowhead shape and the back is more squared. The more squared back can't move through water as easily as the curved front, so you can get more net motion forward than back.

This was my dream boat as a child.
Now, I've almost understood how it worked.

I always thought that they worked because of the pop and that's sort of accurate. Water gets pushed out horizontally but the return pressure pushes against the popper vertically, allowing it to move forward. It's cool to see that it's just the air pocket though. Love the channel and this video made me a subscriber 😁👍

I kinda knew how this worked back when I was a kid and watched "Ponyo"
In the film, the kids had a pop-pop boat but it was huge
One big enough to find two kids
And they kinda showed how the thing worked
They even had to get a new candle at one point to "refuel" the engine
They had a little rudder to steer, and the temperature difference powered the engine
It was a cool idea that makes mw want a man-sized Pop-pop boat to drive myself on the lake

No-one could be disappointed with any good visual explanation - even if it makes your channel a 'transparent version' channel!

Now I am curious to see how far this can be scaled. could it be made into a full size boat like in the movie Ponyo? or does this only work because the tubes are such a small diameter the water surface tension helps to keep the air inside?

Thank you for explaining that. I had never seen one of these boats and came across it a couple of days ago. I could not figure out how it works and another video basically said that it was magic, without actually using the word magic, which was highly frustrating. Now I have some understanding.

Another factor in the oscillating momentum leading to forward motion could be the design of the boat itself. The angled front has to put less effort into displacing water, whereas the flat back essentially provides a wall preventing backwards momentum, as the angle to displace is much greater than the front.

Fascinating, I was scratching my head pondering the dilemma of how it moved forward vs just back and forth. Very cool!

Thank you! I was always unsatisfied with the standard explanation! I was particularly delighted by the explanation of how the forward motion is caused--a crucial aspect that is totally ignored in the standard explanation.

As you've probably already been told, the "water flowing into the pipes from all directions" explanation that you rejected is the same reason Feynman gave as to why his s-shaped water sprinkler wouldn't work backwards. So it might be interesting to run that experiment in a clear box, like Hero's engine in reverse.

I'd be really interested to see a scaled up version of this. Maybe even normal boat sized with manual controls for direction, maybe even solar-powered electronics. That leads to the question: Could this be a sustainable method of normal boat travel? and What would the effects of expelling this heated water be on sea life and the health of the waterway in general if many many boats like this were being used?

They operate very similarly to a stirling engine. The column of water in the tubing is the piston that responds to the vapor expanding and contracting.

Most honest sponsorship/review out there. Love your videos and appreciate your approach to this.
Cheers!

It is such a pleasure to see science video on Clip-Share by someone who is actually very well educated for a change. I have had so many cannels I like end up going full sensationalism. Too many times channels like infographics (and the plethora of other ones from the same guy) just make stuff up and embellish. I am so sick of thinking I am learning something only to later find out it was all completely wrong. I feel like I can actually trust this channel because Steve is genuinely very intelligent and has a masters degree in physics from oxford which is about as good as it gets for education.
Thanks Steve, keep up the fantastic work!

Cool video, played with one when I was a kid. Always wondered how it worked. Now, have you tried putting the boat in a bigger boat, to see that they don't move ?

Actually more than resonance I think this is a nice example of limit-cycle oscillation, where a steady supply of energy (from the candle) produces an oscilatory motion.

You know, we have to give credit to this man. What a great explanation for just wanting to hide the fact that the boat uses batteries to run

Something else that would help with the forward morion of the boat (vs reciprocation) is the shape of the body in the water. Aerodynamic in one direction and a bluff body in the other, reducing drag in the forward direction, and increasing it in the reverse.

I would like to see a version of this where the front and back of the boat are the same shape.
I think forward goes because it's pointed and it's easier for the boat to move, but backwards it's up against a flat back and resists movement slightly more.

I feel like I wasted my life. I had lived bit over 30 years and had never ever heard or seen of pop-pop boats. This has saved me. Now I know what is pop-pop, how it works and I even know that pop-up boats made from glass don't make pop sounds. Thank you

The moka pot has a very similar working principle to what's explained at the beginning of the video. I once thought that the coffee in it is extracted by steam, which then cools down and condenses in the upper chamber, but in fact it does not even have to reach the boiling point to work.

There is a very critical part you're missing.
The resonance is controlled by the bulk modulus of the fluid.
In a glass chamber, you purely have the bulk modulus of the gas in the chamber.
The metal pop boat has a variable stiffness based on the size of the diaphram, which tunes the resonance to that of the heat input of the system.

Also consider that the boat is, well, boat-shaped. The pointy bow is more hydrodynamic than the flat stern. So when water is expelled from the tubes, it can move forward more easily than it can move backward when water is sucked back in.

Fascinating. One thing that I didn't hear you mention was that water doesn't compress, air does. You can put pressure on water but not compress it like air can. So, the air on the suction cycle acts like a pillow.

It seems like one could construct something like this with closed tubes (they’d have to be quite long, I think) because radiant cooling. As long as there is more force in one direction than the other. A thing will move. For example, sitting in a boat with a shotgun braced against the forward wall, and an impenetrable target placed against the back of the boat, it should move the boat forward-there will always be more energy released from the barrel of a gun than goes into the kinetic energy of the bullet. If the boat is long enough, and the boat has an open surface, the energy of the bullet striking the back of the boat is negligible.

The oscillation always begins by pushing water. So within each push-suck cycle, the boat first pushes the water, gaining some momentum, then sucks the water back, losing that momentum. During this cycle the boat moves forward a little bit, like the brush head. At a higher frequency then it looks like moving forward continuously.

Thanks a lot for this scientific explanation of such a simple toy boat. In my opinion there is physics all around us every day and everywhere, and much more than we realize.
I'm sure, later in life kids might regret some day, when they didn't pay attention during physics lessons.
By the way, this Anker power station is great. I got one too. It's very useful for a camping weekend to power some lights or other stuff.

You glossed over the flexing metal making a popping sound, but that compliance is actually another spring in the system. I believe this added spring sort of amplifies the steam effect which is what makes the metal boat faster than the glass one.

I love how steve makes simple things complicated.
Luv u steve 💖

Great video. I used to play with them in my childhood and I was pretty intrigued how it worked. Thanks to you now I know. Might get a couple of pop pop boats to play with too 🤪 I also watched your entire advertisement of Anker.

Another interesting video. Thank you. With regards to converting reciprocating motion into forward motion, have you had a look at children's flicker scooter (e.g. clip-share.net/video/eUKlmfbJyIQ/video.html)? There's no obvious equivalent to the impact that you identify with the pop-pop boat, but side-to-side motion still seems to make the scooter move forwards rather than backwards. Would be interested in your thoughts.

In my teaching days we use to make these with the kids but the 'engine' was just a length of copper brake tube formed into a coil for the boiler with the ends soldered into the back of the tin boat. Without the flexible tin on top of the boiler they didn't pop so audibly but still worked. Also we didn't fill the tubes with water, just tilted the hull slightly so one exhaust was under water and the other not so a few drops of water entered one end, that was enough to set them off.

Shape of the boat might also contribute to the forward motion as well. The back of the boat is flat and has higher surface area and resistance when moving backwards, while the front is curved and pointed and lowers the resistance when moving forward.

I was thinking the forward motion may have had something to do with the front of the boat being streamlined, where the back of the boat is a blunt, flat surface that would be resistant to motion in that direction.

Another possible reason for the oscillating motion moving the boat forward: the shape of the boat generates much more drag being pulled backwards than it does being pushed forward, so the net effect over a full cycle is in the positive direction.

Awesome glass job. Surface area must count for a lot in the tank as well.

The one thing you missed is the aerodynamic design of the boat when considering why the boat moves forward and not stay in its position. While you give a much more complex answer to that question, I think the answer is very simple. Considering that both push and pull forces are equal, it is alot easier for the boat to move forward due to less water resistance while moving forward, which is not the case while being pulled backwards. And to experiment whether this holds true, invert the boat design with the pipes popping in the front.

I think because the 'tank' in the metal boat has a lower volume than your glass boat, it's not an issue that the tank is only partially full.

After watching one of your recent videos, I was wondering on how fluid viscosity has an influence on say a greedy cup, how high or low does the viscosity of the fluid have to be before the reaction is broken

I guess the other reason for the boat to move forward is if you see the shape of the boat, the forward part of the boat does not have resistance due to water while moving forward because of its bow shape compared to its stern which has more resistance due to water.

Reminds me a bit of a pulsejet engine, the way it oscillates to produce a net force. Obviously it's very different in a lot of (very big) ways but I can see some similarities in how they work

I assumed the boat was propelled because the water inside first is expelled, and when it's sucked back in, that only stops the new forward momentum, so it's oscillating between forward motion and stopping rather than forward and backward motion that fully cancels out.

The pop pop works because of the 2 different metals the boiler is made of.
I think it''s steel on the top & brass on the botom.
The water keeps the brass cool whilst it's in the boiler thus keeping it "small".
When the water boils out the brass heats up from the flame & expands thus pulling water back in again
The water cools the brass, it "shrinks" agian over & over

I've seen explanations about the last topic that go like this - if you were trapped on a frozen lake that is perfectly slippery, such that there is zero friction between your shoes and the ice, if you tried to walk, you'd stay in place (or, well, fall, but stay in place). However, you could escape the situation simply by... breathing.
Now, I'd have to see some actual calculations to estimate how quickly an average person would be able to move in such a precarious situation. I guess it would be hard to reach any appreciable speed that way, because air resistance would still slow you down, and I don't think you'd be able to accelerate very quickly. But, at least in theory you should be able to reach some non-zero average velocity.

I'm wondering if this effect only works at small scale or could it be somewhat larger

I think the momentum cancellation is a bit dodgy. What I think is happening, is that that the water being pushed out propels the boat forward, and the water being sucked back in brings the boat to rest. So in one cycle, the exact same amount of water is being expelled and sucked in, but the boat has moved forward by some distance. And the cycle repeats.

I bet the shape of the boat helps too. Since there’s more drag when the boat is accelerated backwards, it will have a slight preference to move forwards with the periodic motion from the tubes. Could be worth building a setup where the tubes are mounted in reverse to see if the drag being different is dominant over the argument you made for the collision between water and air.
Great Job on the video!
Edit: I said it was the first video I’ve seen on this channel. Turns out that was a lie. You got a new sub today

As a child, this tinplate toy was a must have every year during my annual vacation to Varanasi, India. I still have a few left, preserved somewhere within my chattel.
Thank you for posting this video.

Until now, I used to think that due to unevenness (asymmetry) in piping geometry, the hydraulic system creates a draught. That is, water flows in through one of the pipes and steam+water mixture leaves through other. Due to the exit momentum being more than the opposing entry momentum, there is a resultant forward force. BUT, Alas, it is much more complicated than that!

I can't help but wonder how the boat would act if it weren't designed to cut through the water like a normal boat. Maybe just reverse the motor in the same boat and see what happens?

Could the forward motion also come from the hydrodynamic shape of the boat? Like a hooked arrow head that goes forward easier than it goes backwards, the backwards motion would simply be resisted stronger than the forward motion.