- This is a pop-pop boat
or a putt putt boat. And I've seen various
different explanations online for how it works. So, I thought the only
way to know for sure is to build a transparent version. This channel is rapidly becoming the Transparent Version of Things Channel, but anyway, let's have a look at the
regular pop-pop boat first. You've got this little
tank here with two pipes, coming out the back, and if you follow those pipes they come out at the back of the boat, under the level of the water. The first thing you have to do is fill the tank and the pipes with water. You then need to light
a fire under the tank. In this case we are using
a mini tealight candle. Very quickly the pop-pop boat starts to make this popping sound and
it races around in the water. Now, according to the common explanation the that tank remains full of water, until it reaches boiling point. And then you get this
sudden release of steam, which forces the water down the tubes. As that steam continues to expand, that follows the water down the tube. The tubes are much cooler than the tank. So, the steam condenses back into water, that reduces the pressure inside the tank. And so the water is
sucked back into the tank. All this steam condenses, and you're once again left
with a full tank of water. Once back inside the hot tank, boiling happens again and
the whole cycle continues. But with the glass version
of the pop-pop boat, we should be able to see if
that's actually what's going on. The first thing you notice actually is that it's quite hard to
fully fill the tank with water with no air trapped inside there. Look, if I don't make any
effort to tilt the boat in funny ways to remove air bubbles, this is how much water
you get in the tank. Most of it's actually air. It is possible to get rid
of almost all the air, by tipping the boat upside down, whilst just squeezing water in, until you essentially flush
the whole system with water, but you don't do that with
the metal pop-pop boat. So, my assumption is, actually there's probably
a big bubble of air inside the metal pop-pop boat during operation. The first thing you notice
as the tank starts to heat up is that this little pocket of
air gets bigger and bigger. Thanks by the way to Limo Holland and the Scientific Glassblowing Team at the University of Southampton
for making this boat. So, I imagine two things
are happening here. That little bubble of air is expanding, because air expands as
temperature increases, but also some water
vapor is being created, even at these temperatures below
the boiling point of water. The gas cavity continues to expand, until it reaches these two tubes. And actually it continues
to expand beyond that, down into the tubes, but you'll notice there's
a little reservoir of water still in the tank. And then at some point, the water in the tube
starts to oscillate back and forth inside the tube. By the way, the batteries are only there, 'cause the boat was
too heavy at the front, though I do appreciate, it looks like a really dodgy
perpetual motion machine with not so well hidden batteries, but I promise they're
only there for the weight. Perhaps the oscillations would occur in different places in the tube, depending on how full the
tank was to begin with. Here's a test where I
don't make any attempt to fill up the tank. I just make sure the two tubes are full. Look, there's only a
bit of water in there. In this setup once the
gas starts to expand, it fills the two tubes almost entirely. There is still a tiny bit
of water left in the tank, which is slowly going down as
more and more of it vaporizes. But eventually you do seem
to get some oscillation, right at the very exit
of the exhaust pipes. And that does seem to be
enough to get the thing moving. But to go back to the full tank test, well the oscillations still
happen inside the tubes, not in and out of the tank. So, at least with this
version of the pop-pop boat that's where the action is. On the surface, this seems quite different
to the description, you tend to see online, where the tank fills up with water, then fills up with steam,
then fills up with water. But although the reality
looks quite different, I believe the method of
operation is very similar. The best way to think about it, and actually it's not
often described in this way is in terms of resonance. This packet of gas inside the
tank and the tubes is elastic. If you squeeze it, it will push back. And so if you do it in the right way, it will oscillate. It will resonate at its natural frequency, a bit like the air inside
this glass syringe, boyoyong. But you'll notice that the air resonating, inside this glass syringe is very damped. The oscillations die away very quickly. So, we need some kind of
external energy source to keep that oscillation going. And of course that energy
is supplied by the candle, but how exactly? Well, when you have something oscillating with simple harmonic motion and you want to increase the amplitude or at least stop the amplitude going down, because of damping then obviously
the force that you apply, needs to be applied
with the same frequency. Like you need to push and
pull at the same frequency, but you also need to
worry about the timing. Like if you push, when the gas is pulling and you pull when the gas is pushing, that's not going to work. Whenever the gas is expanding, the engine needs to be
applying an expanding force and whenever the gas is contracting, the engine needs to be
applying a contracting force. In technical language that means that the force applied is a quarter of a wavelength outta phase with the displacement of the oscillator. But where does this force come from? Well, it comes from a
temperature difference. And that makes sense, like if you just uniformly heated up, the gas and water inside the engine, well, some things will expand and the temperature will change, but it won't oscillate. For it to oscillate, you need some exchange
between hot and cold regions. That's how any heat engine works. For example, the Stirling engine that I use in my video explaining entropy, link in the comment in the description, in this case as the gas
expands into the tubes, well the tubes are much colder and when a gas gets colder, the pressure goes down, especially if it's water vapor, because some of that
steam will condense back, into liquid water, drastically
reducing the pressure. But of course as the gas contracts, it ends up back inside the tank, where it's hotter. As the gas heats up the pressure goes up. And so it expands back into the pipes and that cycle continues. At this point, you may have worked out
that a pop-pop boat, doesn't need two pipes. It will work just fine with a single pipe, but having two makes it
a lot easier to fill. You squeeze water into one pipe and it expels air out of the other. With the glass pop-pop boat, you don't get the popping sound. That's because with the metal boat, you have this plate of metal on top. And as the pressure goes up and down that metal plate flexes up and down, causing a popping sound.
(metal popping) But there's nothing really
flexible about the glass boat. So, you don't get the popping sound. The final thing we need
to explain is like, how is it that water going in and out of the exhaust tubes at the
back leads to propulsion? 'Cause it's not obvious, like intuitively it's
a reciprocating motion. Everything that happens in one direction, then immediately happens in reverse. So, surely the boat should
just go back and forth on the spots and not
actually travel anywhere. Well, it's actually reasonably common for a reciprocating mechanism to lead to non reciprocating motion. Take for example the bristlebot. The bristlebot works by vibrating the head of a toothbrush up and down. And that's obviously reciprocating motion, but it leads to forward
progress of the bristlebot. If I slowly press down on
the head of this toothbrush, it leans forwards on the bristles. And if I slowly let go, it leans back up again. And so that reciprocal motion, leads to no change in position, but if you do it quickly, well, first of all I press down and that causes the
bristlebot to lean forwards. And if I quickly release, well the bristlebot springs up and the bristles slide forward. And so with each cycle, the bristlebot moves forward. So, how does the reciprocating mechanism in the pop-pop boat
lead to forward motion? Well, one suggestion is that as the jet of
water leaves the exhaust, it's collimated, it's all moving in the same direction and because every action has
an equal and opposite reaction, that causes the boat to move forward. Whereas when the water is
drawn back into the pipes, it's drawn in from all directions. So, the reverse motion
of the boat is weaker, but actually that argument doesn't stand, because all we really care about is the momentum of the water
that interacts with the boat, that's the water inside the tube. And it's the same mass
of water going forwards, as going backwards. The real answer is that
after the expulsion phase, the boat can continue to glide forward with the new momentum that it has. Whereas when the water
is sucked into the boat, yes, there is an opposite reaction force that pulls the boat back. But at the end of the suction phase, there's essentially a collision. The water slows down inside the tubes, because it's colliding with
the air inside the tubes and the air is enclosed
within the entire boat system. So in other words, the reason the water that's
being sucked in slows down is because it's colliding with the boat and it's imparting momentum on the boat, which exactly cancels
out the reverse momentum, as it was being sucked in. In summary during the expulsion phase, there's no cancellation of the momentum, being imparted by the expelled water. But during the suction phase
there is a cancellation. Altogether that leads to a net
forward motion of the boat. Anker sent me this portable power station to do a quick review at the
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pool with an electric pump. In terms of capacity, you can charge your iPhone 12 20 times, my beefy laptop, maybe three times, it's saying 3.7 hours to
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in case of a blackout. The link to buy the Anker
521 Portable Power Station is in the description. I hope you enjoyed this video. If you did think about
watching this video next, it's the one the algorithm
thinks you'll enjoy the most. (upbeat music)
if we had two 1 way valves on the exhaust which meant the boat threw out the water column as normal out the back but then sucked water in from a forward facing column
Double propultion pop pop boat?
Make a round boat and spiral the pipes out opposite sides both pointing clockwise or counterclockwise. I want to see if it'll spin fast enough to create a wet black hole.
The momentum of the water being sucked in and the water being pushed out is not the same.
Mass is the same, yes, but velocity most certainly isn't. Momentum = mass * velocity.
You should do a video on the Feynman sprinkler.
Very interesting & well made as usual! I don't quite accept the "collision" on the intake being that important of a factor for moving forward (and having no "anti-collision" on the output.)
Just like the oscillating syringe example, the pressure and vacuum forces both slow and stop the motion, then reverse it, with seemingly equal force.
Am I missing something?
I think you didnt get the explanation right. The old explanation was definitely right, but you just left a point of it..The point is, that as the water inside the boat goes out of it, it will push some water, that is outside the boat with it, and this water is the water, that gains the thrust. And when moving inwards, of course it will pull some water with it, but mostly not from behind, but from all other sides, and thus theres a netto thrust. So to summarize: the water moving in and out will move the water outside the boat in a way, that the netto movement of water outside the boat is to the back..Thus it gains thrust. Another example is a normal boat. It doesnt even push anything from the inside to the outside,but still it moves forward. How is it? Well same explanation: the mechsnism of any normal boat does a movement,that forces water on the outside of the boat to move, and thus it gains thrust. The only difference to this one is, that in this case the "mechanical parts" moving the water on the outside are itselve out of water (you could also put a stick in the end of the tubes, with a droplet shape at the back of it, and it would surely still work) Edit2:One can see a cone shaped wave coming from the boat, this does imply, that there is a net backwards movement of the water, which enforces my statement, i think
Off topic but the useful looking sponsored item is unfortunately "Currently unavailable. We don't know when or if this item will be back in stock."
Steve, Thank you. You had me at hello... I mean "resonance" (at 4:20), and then again at "When you have something oscillating with simple harmonic motion" (at 4:57). :) Thank you for sharing another enjoyable journey of discovery inspired by curiosity.
Hi Steve, Here's two questions for you:
Does the concentration of red water dye change in the evaporation/condensation 3:10 and water propulsion/withdrawal 9:20 stages?
Does the plate of metal that oscillates to create the "popping" sound contribute towards the movement of water? 7:03
I've theorised that the force pushing the plate outwards causes an opposite force pushing into the tank because of the elasticity of the metal, forcing the water back out the boat. Kind of like a diaphragm? Like 4:15
Thanks for another great video!