Gravity is Optional
Putt Putt Boat

A child's toy .. whose operation has confused many of the top physicist.

The putt putt boat has a simple boiler (partially filled with water) and a candle. When it gets hot enough, the water in the boiler flashes to steam and pushes liquid water out of the exhaust tube(s). So far, pretty simple, expanding steam pushes water out the back and the boat moves forward. Rotating sprinklers work on the same principle .. as do jet engines and rockets.

The problem comes with the next phase. The exhaust tubes are cool and cause the steam in them to condense. The lower pressure "sucks" water back toward the boiler.

By the laws of physics, for every action there is always an equal and opposite reaction. At first glance, logic says that when the boat "sucks" water back in, the boat "should" move backwards the same amount it just moved forward.

The reality is that the boat continues to move forward - in the direction opposite what most people predict.

The answer is that sucking is not a force. Therefore, it does not produce a reaction.

To be clear, there is a force that causes water to refill the boiler -- but it is not caused by suction.

Suction is not a force | A closer look | Center of mass analysis | Other configurations | Heat engine | References

A Putt Putt Boat - Made in India
The boat and 2 packages of extra candles Unwrapped with the cover off

I bought this boat at the 2013 Cabin Fever Expo - a vendor had a bunch of different types. He pointed out that the 2 candles that come with the boat are pretty worthless (just string in wax) and suggested that I use the ones he was selling separate (and using in the demo boat). In those, the wick is anchored to a metal plate so it won't simply float on the surface. He also said that regular cooking oil will work .. again, using a weighted wick.

The top of the boiler is a thin sheet of brass .. when this flexes it makes a good loud noise. The designs with rigid boilers are much quieter. (Guess which one sells best.)

Suction is not a force

If suction was a force, then it would be possible to remove water from mines using suction. However, it is well known that at atmospheric pressure, water can not be "sucked" to a height greater than 33.9 feet. The classic experiment is to fill a tube with water (or mercury) and invert it into a container of the same fluid. Assuming that the tube is long enough (longer than 33.9 feet for water, about 2.5 feet (29.92 inches) for mercury), the liquid will not go all the way to the top of the tube. The "empty" space is not really a vacuum because gas molecules from the liquid will be there .. and at a pressure dependent on only the temperature. The weight of the liquid in the tube is equal to the weight of the air in an imaginary column having the same diameter as the inside of the tube, and going from the bottom of the tube to space. (True only when the tube has a constant inside diameter and the diameter is large enough that capillary action can be ignored.) Because of these results, it is understood that "suction" does not pull the fluid into the tubes. Instead, the atmosphere "pushes" the fluids up the tubes until the weight of the fluid is the same as the weight of the displaced atmosphere.

The same thing happens when you suck on a straw - you create a pressure difference and the atmosphere pushes the liquid up the straw.

It is actually a good thing that a vacuum is not a force since, if it was, then the vacuum of space would suck all the water off the planet. (Bad news for us!)

Instead, a vacuum is simply the absence of anything .. including pressure.

Back to the putt putt boat - after the explosive expansion phase, water vapor begins to condense creating a partial vacuum. Since there is now a pressure differential between the atmosphere and the inside of the boiler, the atmosphere pushes water into the boat engine. The force is from the atmosphere, not from suction. As the water moves thru the exhaust tubes, it gains momentum. When the tubes are full, the water stops moving .. and transfers its momentum to the boat .. pushing it forward.

This is the weird part - the putt putt boat moves forward during both cycles - both when it is pushing water out thru the exhaust tubes AND when it "sucks" water back in.

Note: While suction is not a force, it requires a lot of energy to create a vacuum using a pump. It isn't that it takes a lot of effort to remove a gas from a container. (Breathing out takes almost no effort.) The effort is in keeping the atmosphere from getting back in.

On the other hand, the putt putt boat creates a partial vacuum by condensing water which, in fact, releases energy.

Note: At 1013.25 mbar, the atmosphere should support a water column of about 33.9 feet. However, the exact value is temperature dependent for 2 reasons
  • Water expands and contracts based on its temperature
  • The amount of water vapor in the near vacuum above the column is temperature dependent
At 60F, the saturation pressure of water vapor is about 17.6 mbar which will reduce the height of the supported column by about 7".
    33.9 feet (1013.25 - 17.6)/1013.25  = 33.3 ft
    (33.9 - 33.3)(12 in/ft) = 7.1 in

Note: It is frequently said that "Nature abhors a vacuum" - but the truth is that over 99% of the universe is a vacuum.

A closer look

In a more detailed analysis, there are more than just 2 states that need to be analyzed. Thus both the intake and exhaust should produce a forward force.

In reality, the boats continue to move forward during all phases because of the forward inertia. If you stop one of these with your finger, you can feel that there is a continuous forward pressure. It pulses, but the boat never pulls away from your finger.

There is no single "correct" explanation of these simple (NOT) toys. For instance, The secret workings of a transparent pop-pop engine says that during the condensation cycle, water does not flow out of the exhaust tubes back into the boiler. Instead, the water overshoots equilibrium and causes the pressure in the boiler to increase. Even though the temperature increases due to the increase in pressure, the boiling point of the remaining steam goes even higher, which causes the steam in the boiler to condense so that the cycle can repeat. To be clear, there must be liquid water in the boiler, they just claim that it condenses there and does not flow out of the tubes. (They have high speed video to support this theory.) In their description, the momentum of the water in the tubes causes the pressure to go significantly higher and lower than otherwise expected.

Regardless of what the video evidence says, because some of the steam is condensed in the exhaust tube, some of the water in the tube must enter the boiler. If this was not true, then eventually the boiler would run dry. The water lost via condensation outside the boiler must be replaced.

Non-linearities in the viscosity and drag of the fluid are important in scaling these toys. It is likely that there are maximum and minimum sizes that can be made to work, and that outside those bounds, it simply won't work.

Center of mass analysis

Looking only at a center of mass analysis - for a rocket or jet to move forward, it must eject matter. In this type of analysis, the center of mass does not move. For the jet to move forward at some mv, it must eject matter with the same mv (but in the opposite direction).

During the drive phase, the putt putt boat expels mass exactly the same as a jet (or rocket) .. and the same center of mass analysis applies. During the condensation/recharge phase, the same analysis applies. Therefore, a simple center of mass analysis states, quite clearly, that the putt putt boat should not work and that the fact that it does violates the first law of physics.

The flaw in this analysis is assuming that suction is a force. This system has 2 separate forces

  1. The candle produces steam which moves the boat forward
  2. The atmosphere recharges the system during the condensation phase
Since the boat never actually stops or moves backward during any of the phases, it is obvious that the net force is always forward.

When just the boat is considered, the atmosphere increases the mass of the system during the condensation phase. However, because this is an external force, the center of mass does not have to remain at a constant location. Thus, at the beginning of the next mass ejection phase, the center of mass is forward of where it was at the start of the previous ejection phase.

In 3-dimensions, when the boat takes on water, it sinks lower. When it expels water, it rises higher. As a result, the motion thru the water is a bobbing motion .. and the center of mass at the end of the expulsion phase is actually higher than the center of mass at the beginning of that phase.

There are basically 2 frames of reference - one for each force. As a result,

Other configurations

It may be possible to build one of these boats with some kind of floating piston in the exhaust tubes. In that way, the boiler and exit ports could be separated while the motion of the water columns is not affected.

A similar method of propulsion can be implemented using motor driven pistons instead of a boiler. Instead of using the same tubes for intake and exhaust, there would be 2 ports. When a piston pulls one way, it would "suck" water into a chamber. When it goes the other, valves would send it thru the exhaust tube. (Jet skis are similar to this, but use an impeller instead of pistons.) However, rather than have the input port on the bottom of the boat (like on a jet ski), it could be placed on the back. For slow motion (like a putt putt boat), the orientation of the input port should have no effect. However, jet skis move so fast that the Bernoulli principle would cause rear pointing inlets to have a lower pressure and significantly reduce performance.

Other configurations include a water top - it just floats and spins. alternate

It should also be possible to create

Heat engine

Because the flow of water in the exhaust tubes significantly over shoots at both extremes, there are both lower and higher pressures than expected using only static analysis. As a result, the steam engine is a type of single piston (beta) Stirling engine (known as a Fluidyne engine) where the water column acts as both the piston and the out-of-phase displacer. Granted, most Stirling engines don't use a phase change (they just heat and cool a gas), but the putt putt boat uses an external heat source and recycles the working fluid (water/steam) which fits the basic definition.

A similar engine I saw at Cabin Fever 2013.
Since this unit does not produce steam, the pipette (tube with a suction bulb) on the table is used to start it. Once the liquid starts moving, the over shoot at each extreme provides the extra dynamics for it to continue.


Author: Robert Clemenzi
URL: http:// / Putt_Putt_Boat.html