Although the water is still jetting out the ends of the straws because of momentum, the situation inside the boiler is very different from phase 1. The boiler is dry, no longer generating steam pressure because the water droplets have boiled off into steam. The column of water in the straws is no longer being pushed. But the momentum of the exiting water in the pipes keeps it going outward. Behind it --inside the boiler-- it expands the amount of volume available to the fixed amount of air inside. This creates a lower-than-atmospheric pressure inside the boiler. Colloquially, people call this a "vacuum", which isn't really right because there is still air in there, just less of it. "Rarified atmosphere" is a better term, I think. Whatever you call it, it's a huge change from phase 1, when there was high pressure in the boiler!
The bottom of the boiler bulges inward (concave), again making a sound (I called it "gnip" ('ping" spelled backward) in the illustration).
We could say that the suction inside the boiler is now trying to pull the water column in the straws back into the boiler. More accurately, we should say say that normal atmospheric pressure is pushing the water back in. In any case, the water's momentum has been pushing against the air pressure. When the water's momentum is finally gone, the air pressure springs it back toward the engine. Some of the water squirts back into the aluminum boiler. Remember, that boiler was dry in phase 2. The candle has still been heating the metal, though, so the droplets of water explode into steam. There is high steam pressure in the boiler, again pushing out the water, and so on. The cycle repeats several times per second until the candle burns out.
Sir Isaac Newton's Third Law of Motion is the familiar, "For every action there is an equal and opposite reaction." This is what propels jets and rockets--even in the deep vacuum of space. So, when the water jets backward out of the straws (action) the boat jumps forward (reaction). OK so far. But then the water rushes back into the straws, which would suggest the boat should actually kick backward in reaction. Why doesn't the putt putt boat just shake forward and backward in the water and not really go anywhere?
For years I assumed the whole answer lay in the shape of the boat. The pointed front cut forward through the water, while the blunt square back of the boat resisted going backward, sort of a hydraulic ratchet. But when I actually put it to the test--mounting engines on square boats, or even backward on pointed boats-- the engine still chugged forward. Clearly, some other principle is at work here.
I am speculating that the answer lies in the different way the water exits and enters the ends of the straws. Consider this: If you blow through a straw, you can blow out a candle from more than a foot away. But if you suck in the air, you have to be within an inch or two of the flame to blow it out (I am not suggesting you breath fire to try this out). This leads me to think that in the first case, much of the air keeps going straight when blown out. But in the second case, air enters the straw from all angles --not just in front. If we then apply Newton's Third Law, most of the reactions cancel each other out in the second case.
However, Dr. Jeff Bindon explains it another way, and I am unable to find a flaw in his reasoning. He uses this analogy to illustrate why sucking water into the engine does not cancel out the expulsion phase:
Imagine a man standing at the front of the flat deck of a small boat. He starts to run across the deck towards the back and as he accelerates, he exerts a force on the deck and the boat is propelled forwards. He continues to accelerate and exert a force until he comes to the end of the deck and leaps into the water. The boat is now moving and as it does so, it passes another man in the water and he grabs hold of the ladder at the back of the boat. The boat jerks him into motion and the boat is slowed slightly. He climbs onto the deck and proceeds to accelerate and run towards the front of the boat. He exerts a force on the deck and the boat is retarded. But he does not want to leap off the front of the boat so he decelerates and exerts a propulsive force on the deck. By the time he has stopped, the propulsive force he has exerted is exactly equal to the retarding force he initially exerted. Or you can argue that he walks slowly to the front of the boat and exerts no force when he starts or stops. So, a series of men floating in the water, climbing on board, running and jumping back into the water can propel a boat !!
Although the putt putt engine is the most challenging project on this site, it would be even harder if you had to solder it together, as is the traditional way to make them. Furthermore, most solder does not stick to aluminum, so you would have to find special brass sheets instead.
It is not a good idea to run the engine dry, but when water is in, it stays below the temperature that would damage the epoxy and silicone. Consider that a water balloon can be held above a lit candle without bursting. The water keeps the rubber cool.
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