- 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 end of one of my videos,
they are paying me.
It's important that you know that,
but let's just get into the details.
It's the Anker 521 and it's got 256 watt-hours of power.
So, it's an absolute beast.
It'll give you 200 watts of power from the mains adapter.
It's UK for me.
But if you bought it in America, obviously it'd be American.
If you bought it in the EU, it would be EU.
It also has the car charger.
That'll do 200 watts as well.
It's got fast charging USB ports
and it's got a fancy light as well.
My laptop maxes out just over a hundred watts
if I'm using it and charging it at the same time.
So, it can easily handle that.
It can also handle small appliances like a mini fridge
if you're gonna take that on holiday
or you can pump up your 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 a full charge from 16%.
That's pretty much in line with what I've experienced,
but it will get you up to 80% much quicker than that
in about an hour and a half from no charge.
It uses lithium-ion phosphate technology,
which is the longest lasting lithium technology
on the market.
You can charge cycle this thing 3000 times
and you'll still have 80% of charge left,
which is six times better,
than standard lithium iron technology.
It comes with its own charger obviously,
but because it's got a USB-C port for charging your devices,
you can actually do it in reverse.
You can charge the power station
from USB-C as a backup option.
I'm often in the market for this kind of power pack.
And as you can see Anker is the name that I look out for.
And that's not just me.
Anker is recognized,
as the world's number one mobile charging brand.
They've been in the business for 10 years.
They've sold over 200 million products in 146 countries.
Next, time you go on a camping trip or a weekend away,
think about taking one of these things
and when you're not camping,
keep it in your home 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.
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it's the one the algorithm thinks you'll enjoy the most.
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