How NOT to Build a Valved Pop-Pop Boat

About: I build stuff because it isn't available to buy, or is too expensive, or the ones you can buy don't do what I want them to. Sometimes I don't have a reason, I just want to build something. For me it's about...

The Problem
Pop-Pop Boats are notoriously inefficient for many reasons but the main problem is the action by which it moves forward.

The Explanation
When the engine pops it squirts water out of the exhaust. This water travels almost straight out with little divergence.

When the engine sucks in fresh water it sucks from all around the inlet. The water sucked in from the sides should be symmetrical so they sum to zero but approximately a third of the water in taken comes from the same vector as the exhaust squirted which cancels out say a third or more of the forward thrust when the net product is found.

The Solution
Put valves on the engine so that it sucks from the front and blows out the back potentially quadrupling the forward net thrust!

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Step 1: The Plan

With the concept set the principles of operation are simple. All that remains is to design an appropriate way of implementation.

These only have to be one way and should require no return so should take little force to open. I went straight for the shuttle valve. I found the smallest reasonably sized ball bearings I could and designed the valves around them so they would close from one side and free flow the other. Then, they would be mounted to allow free sucking from the front, no blow and free blow on the back, no suck. The ball bearings where 6mm steel.

Exhaust and Intake
I know through my research that the number of tubes is essentially irrelevant. One tube can work as effectively as two or more. The aspect to be considered is surface friction. As the output of the engine has an optimum cross sectional area the way to attain the least losses is to have one tube of the required diameter. Since I want to split the input and output I needed two tubes, one in the front for intake, one out the back for exhaust. I decided to have them both of the same diameter copper pipe of around 6mm internal diameter (1/4") To fit my valves. 

calculated from working engines boiler size to tube cross sectional area ratios.

Engine Type
Here I have two choices, the simple coil boat or the difficult to seal pop pop boat.

Coil Engine
There is a minimum radius to which you can bend a pipe without flattening or crimping it. My quarter inch copper pipe dictated that the coil would be rather large in diameter and hence hard to package in a boat hull of the size I was envisioning. I also know through research that pop-pop boats don't work well at larger volumes as the inertia of the water displaced with each cycle limits the frequency of operation to a point where there is little if any or the motion can be rather sporadic. This meant the internal volume of the coil that would form my boiler may be too large.
Classic Pop-pop Engine
These are simplest when constructed out of steel beverage cans. Instructions on how to do this are on the internet and I dare say here on Instructables. If you have trouble finding them, send me a message or e-mail and I'll either add a link here or send you the link. This would be good but the boiler needs to be at a fairly horizontal angle to catch the heat from the heat source. A vertical boiler would receive little to no heat as heat rises. This would mean that complex bends would be required to get the engine in the middle of these two pipes going on opposite directions. Fluid dynamics say that corners are bad as far as resistance and efficiency goes so this would not be optimal.

This left a completely fabricated boiler of my own design which was in the same size ratio as the smaller commercial pop-pop boats that you can buy but with an in line intake/exhaust orientation which made sense to me for the intake/exhaust placement I was envisioning.

Steel shuttle valves with a 1/4" copper pipe intake/exhaust and a conventional boiler with slight modifications.

Step 2: Making the Valves

I used a lathe to turn a piece of 1/2" brass bar down and a ball nosed drill bit to make the seat the ball bearing will seat into when sealed. I then made a cage to hold the ball bearing in the centre of the chamber whilest allowing it free movement and water around the sides.

Step 3: Making the Boiler

The boiler was made out of 1" copper pipe, almost flattened with the ends crimped and brased. I then drilled a hole out of the bottom for the tubes to be put in and a section out of the top for a steel diaphram.

This was mistake number two! Don't add a diaphram because it makes the engine sound cool, sealing it was almost impossible and it frequently blows a seal!

Step 4: The Intake/Exhause Manifold

I took a piece of 1/2" copper tube and two steel rods of the same OD as my 1/4" copper pipe and put a small taper on one end using a Lathe.

I then slightly flattened the 1/2" pipe in a vice and hammered in the two steel rods. I then took two smaller diameter steel rods and placed them between the two steel rods on the outside and squashed them in using a vice.

This was then cut to length and Brased to the boiler.

Step 5: Make the Hull

I used the hull patern from a pop pop boat website that uses the steel from a vegetable oil drum. The drum was opened with a can opener and a flat sheet was extracted using a pait of metal sheers. The patern was printed and stuck onto the metal with a low tack glue stick and cut out using shears. The patter was then removed and all the folds made before the hull was sealed with copious amounts of areldite.

Step 6: Put It All Together

The pipes had to be put in place after the hull was constructed as they would be on the underneath of the hull down the length of the boat. Holes are sealed with epoxy resin and it's ready!

Step 7: Test It!

The engine ran like a normal pop-pop boat but much less power until the inlet/outlet was high enough then the valves kicked in and it worked perfectly!... For one cycle...

After much thought I realised I has been an idiot and there where two fatal flaws to my design.

1. The weight of the ball bearings- The amount of energy it took to open or close a valve was roughly that which was normaly taken to move forwards so there was little energy left for propulsion.

2. Here is the real problem- It is a resonant engine. This means it works at a natural frequency which is dependant on the size of the boiler, the length and the diameter of the intake/exhaust pipes. As the engine is in operation pressure waves travel up and down the tubes at a relatively high frequency. By adding valves to the arrangment I had inadvertantly stopped the engine from consistantly running at its natural frequency as when it reached it, the valves would kick in, and it would cough and stop only to build up to another splutter in due course.

I have also realised that the lengths of the pipes are very different due to the placement of the engine and so this will make the engines harmonics rather unusual and probably not desirable.

Step 8: Conclusion

Back to the drawing board! Mechanical valves won't work! 

Since this boats design and manufacture I have designed and made a working valved Pop-Pop Boat .



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    9 Discussions


    7 years ago on Introduction

    what plans do you use for building the engien out of a beverage can


    8 years ago on Introduction

    This kind of valve are called Check Valve, is used in pneumatic systems like FESTO and water flows as well. They usualy are a conic shape tube openning for a better seal with the ball bearing and may use a spring to open or close at specific pressure. You may build easily with a standard 60 degrees drill tip according the ball bearing diameter.


    8 years ago on Introduction

    Good to see work on pop pop boats. I have used the valves from car windscreen washers in my boats, mounted inside the boat and connected with high-temp silicone tubing. You can insert concentric tubes into the outlets to increase velocity and encourage intake through the valves


    8 years ago on Introduction

    I have been chuntering round a type of water ram jet - using flap valves and a flash boiler (with a bigger burner than a candle) but two side by side to get alternate actions so avoid the issue of slow cycling.

    Alternatively there seems to be some mileage perhaps in a Stirling type system using a water column as the power piston.

    Again cycle time is an issue due to the water column returning under it's own timing, multiple units side by side could get round this - rather like a multi barrel machine gun.

    2 replies

    You can dictate the frequency at which the engine operates by the size of the boiler, the diameter of the tubes and the length of the tubes.

    The smaller the above, the faster the frequency.

    I'm curious as to how you where going to make the engines run alternately? I like your ideas, they sound interesting. It would be good to see how they work...


    8 years ago on Introduction

    You could of course add a simple flap valve to the system made from little more than a soda straw rather than the mechanically heavy ball valve. Not much alteration to do either.

    1 reply

    The problem isn't only the weight of the valves although that is a big problem. The bigger problem is that the engine works like a tuned exhaust on an IC engine and by adding automatically operated valves to the exhaust and intake system you will change the harmonics of the engine.

    As the engine operates a pressure wave travels down each pipe, bounces of the effective end of the pipe and back into the boiler.

    The first problem is I have pipes of different lengths, this means I've spread my power curve over an area rather that having a high peak power that the engine could sit at using it's own resonance.

    The second problem is that when the valves are closed they make the pipe effectually an inch shorter which means that the engine now has a different natural frequency and will try to change to run at that frequency, but, When it tries the length of the tubes changes again as the valves open and close. This means that an engine designed to run at a specific frequency is constantly building to a power band instead of achieving it.

    If you managed to valve both the inlet and the outlet with super light weight valves that don't change the length of the pipes then it has potential to work. I would theorise that there would still be pressure waves as the valves open and close that would mess up the harmonics and it would perform far below expectations.

    The solution here lies in a form of valve that doesn't vary the length of the pipes and doesn't physically close a pipe to stop the flow of water.



    8 years ago on Introduction

    Thanks for doing the work for me!!! I was thinking this approach for years. I maked a few pop-pop boats, they have many secrets.