How to Build a Valved Pop-Pop Boat


Introduction: How 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...

Learning from the mistakes I made in my first attempts I researched Valves with no moving parts and found two.

1. The Tesla Valvular Conduite, uses the coanda effect to restrict the flow of liquid down a conduit in one direction. This is perfect but too complex to manufacture.

2. The Thrust Augmenter. Technology usually found in Pulse Jets, this can also increase the thrust of the engine.

Step 1: How a Thrust Augmentor Works

A Thrust augmentor is placed a short distance after the exhaust directly inline with the opening. It has a converging intake followed by a diverging section.

As the fluid is forced out of the exhaust it enters the opening to the thrust augmentor draging in surrounding fluid from the local environment as it does so. This increased mass of fluid then accelerates out as the augmentor diverges.

At this point the pulse jet sucks in and this creates toroidal vortices in the augmentors diverging section which expand to the walls of the augmentor and travel along as fluid pistons, pushing fresh fluid infront and pulling in fresh fluid behind. Such a ‘piston’ is a more effective than a simple flat pressure front because it stays together over a much longer distance before degrading. In addition, its swirl also appears to act as a rotary pump, drawing even more fluid with it. All this air adds to the reaction mass and greatly increases thrust.

This means the fluid traveling down the augmentor has significant inertia capable of resisting the sucking action of the engine thus the intake fluid must come from else where. The shape and possition of the thrust augmentor mean that the intake comes from the front of the boat and in effect, sucks it forwards.

This means that all of the work done by the engine should be used to move the boat forwards ultimately increasing overall thrust and hence the boats velocity.

Step 2: The Engine

The engine was built out of 1/4" copper pipe with a 45 degree bend in it to allow for heating. The boiler was contructed from a coke can in the same fashion as the drinks can and drinking straw pop pop boats that can be found via a quick google search. I believe the website is 'science toy maker'.

The top of the can was opened using a can opener and the wall metal cut into a sheet using scizzors. This sheet was then folded in half and the two open side edges folded over. Next the pipe was slotted into the open edge and all gaps sealed with copious amounts of solder.

NOTE: I would suggest araldite instead of solder as the solder has a tendancy to melt during operation.

Step 3: The Hull

I built this boat to test the thrust augmentor and was very short on time so the hull is rather crude but it does the job.

It is a piece of foam with a 1/4" hole drilled at 45 degrees to the surface towards the back of the boat in the centre. The front is cut with a hot wire to an aproximate boat shaped point.

Step 4: The Thrust Augmentor

The Thrust augmentor was much heavier than required manufactured out of 1" plastic bar on a Lathe.

The whole piece was drilled through to 14" to accept the engines tube then a diverging nozel was turned out of one end using a boring bar and the front end tapered to lower water resistance.
This was then taken out of the Lathe and put in the chuck on the milling machine to mill the slots in the front that will act as the intake.

Step 5: Putting It Together

The engine was pushed throught the hole in the foam and secured on the underneath with duct tape as I was in a rush.

The thrust augmentor was a push fit onto the end and hung off the back.

Step 6: Testing

It worked! Disturbingly well in some respects. A larger test area was required and a more stable hull but the concept was a success and that was all that mattered to me :)

Changes I'd make:
1.Make the thrust augmentor lighter, maybe out of rolled aluminium or plastic sheet or at least turned down in the thick sections.

2. Extend the hull of the boat to go up the sides of the thrust augmentor like side pods. This would stop the centre of mass issue I had where the boat would float with it's nose out of the water tail down.

3. Epoxy for sealing the engine. Probably with some form of protection so the flames cannot directly reach the seals as they lap around the edges of the boiler.



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    I made a simple engine, using quick set jb weld and it burnt of. Probably because I tried to run it off a higher heat source than a candle.

    1 reply

    Yeah, I've made that mistake. More heat does not necessarily mean more power in this case.

    JB weld should be good to ~260°C. The chamber only needs to reach a little over 100°C to operate. A candle is usually sufficient. A candle flame temperature is ~1400°C.

    Making sure the flame never directly contacts the adhesive is important. The JB weld should only increase in temperature only via conduction through the boiler skin. You can achieve this by using larger tabs all folded on top. This way, the underside and edges are all JB weld free.

    My idea was to make a trial with a water jet vacuum pump, that is not the original augmentor theory.

    I'm not sure what you mean by that...

    Made an experiment with a simple pop-pop boat. The speed of the boat was ca. 1/2 of the original.

    1 reply

    Interesting. Please can you describe the boat, engine and augmentor used? The geometry and relative sizes potentially make the difference between this doubling or halfing the speed.

    Any video????

    Forgive my ignorance, but aren't you just getting more power from nowhere? Is this possible?

    10 replies

    Hi Dave, 

    As described by the first law of thermodynamics energy cannot be created or destroyed, only transformed. The device utilises energy that would otherwise slow the boat down to make it go faster:

    Numerically, one cycle of the engine is split into two motions; Suck and blow. Lets say the total energy per cycle is ten units.

    The blow power represents around 50% of the total cycle energy. It is likely to be slightly higher than this due to the energy put in by the heat source. When the engine blows most of the energy goes into pushing the vessel forwards so lets round that to 50% and so five units of energy pushes the boat forwards.

    When the boat sucks in new water this represents the remaining 50% of the energy per cycle. This is done from 180 degrees though so only a proportion of it sucks the boat backwards. lets say 50% of the suck power which gives 2.5 units of backward energy per cycle.

    Summing these we find that the 5 forward units minus the 2.5 backwardunits gives a total forward energy of 2.5 units.

    This device redirects the energy that usually slows the boat down into pushing/pulling it forwards, so the vast majority of the energy that comes out of the engine is used to move it forwards. So when the engine blows, 5 units push it forwards. When the boat sucks 5 units suck it forwards, though less efficiently than the blow action as it sucks at an angle so not all the energy goes into sucking forwards, some is still wasted in turbulence and lateral water movement. Thus the vast majority of the energy goes into forward motion giving just under 10 units forward motion. Four times more than before, Though in practice this would be unlikely to reach double the original due to losses in the system.

    NB- these numbers are for illustrative purposes and have no experimental measurement or mathematically modelled backing though the general principle still applies.


    How did your race go? Are you back to putt putt's yet?

    I got back last night, we came 22nd out of 124. Bit disappointing as the car is faster than that but we entered every event so we did better than most years. It is difficult to compete with the top teams that have tour buses, lorries, carbon fibre monocoques, four years development, professional drivers, 45 people on their teams and budgets rumoured to be over £250,000 or even into seven figures. We have nine team members, £10,000 and nine months of time to design and build the car. We do almost all the work ourselves, with a little welding and CNC from the technicians. We made a beast of a car and put up one hell of a fight though! Few had more torque or power than us and we had one of the best power to weight ratios without using expensive composites and manufacturing techniques.

    Ah yes; The Putt Putts! I got a Job at Jaguar on Thursday so the next month is going to be crazy until I move to my new house that I haven't started looking for yet in a new city that hasn't been decided yet. I will make time to do these tests for you though as I said I would and I am a man of my word.

    Right. I have some 3/8" copper pipe to make my engine from though I'm at home now so I have no TIG or brazing facilities. On your recommendation I will make a new engine with no diaphragm. I only made one thrust augmenter so It will have to be a single intake/exhaust engine so I propose a coil boat with one intake/exhaust cut off at the coil and crimped closed. I also propose a simple foam hull to keep weight down and make build time shorter. Do you happen to know any tried and tested dimensions for a single tube coil boat engine or any good geometries as this will be new territory for me and I'm short on development time?

    Considering the well financed competition sounds like a commendable performance.

    Having only one augmenter is a limiting factor. The problems with the engine you propose to build are that it will be very low power and a type that is difficult to run at consistent power. This will make testing results hard to measure with accuracy. Single tube engines are also very hard to fill though injecting the water through a small tube that can reach the closed end of the engine or using a bleeder valve will work.

    I don't know what would be easier for you to build, another augmenter and something like my Easy Engine Build that requires no brazing or your plan when a reasonable test result is the main objective.

    Regarding coil engine design these two links could help. I have done little work on coil engines.

    Guss Flogels page at science toymaker

    Jean-Yves pages at eclecticspace - Coil Engine Performance

    Some reading is required to get to the parts you want. It's usually at the end isn't it? When the pipe leaves the coil it seems advantages to have it go up a bit first before down to the water. Length of the pipe from it's highest point to the outlet is usually best at about 62 x pipe ID. and the angle of rise from the outlet to the high point between 10 and 15 dgrees.

    If I can make it work I will add a photo of a typical engine side view. This has a boiler rather than a coil but that does not affect the layout.

    When testing remember to have them both the same weight.

    Good luck.

    Mamod engine 004-detail..JPG

    I used a lathe and a milling machine to make the first thrust augmenter. If I where to make another I would not have those facilities at my disposal so I would have to develop a new fabrication method and make two so that they are identical. For now I will keep it simple with one pipe.

    That is a very good point about the weight of the boat. I hadn't considered that. Though the boat without the augmenter is lighter so you would expect it to be faster though the shuttle more prominent which explains the reduced shuttling with the augmenter and the increased mass.

    If I where to use the 62xID relation you suggest my boat would be half a meter long. This seems a little excessive. my precious engine was around 15-20cm long and operated effectively. I am also considering whether the standard horizontal coil would be best or a vertical coil which could be made more longitudinally symmetrical Though apart from weight distribution I can't see how an asymmetrical boiler would affect performance.

    Any thoughts?

    Your comments in quotes. My reply was in maroon bold italics but it did not paste that way. Expect it is clear enough.

    "I would have to develop a new fabrication method and make two so that they are identical. For now I will keep it simple with one pipe." --------A quick and easy fabrication method is what is stopping me from doing a test.

    "That is a very good point about the weight of the boat. I hadn't considered that. Though the boat without the augmenter is lighter so you would expect it to be faster though the shuttle more prominent which explains the reduced shuttling with the augmenter and the increased mass" -------Correct. Just put the augmenter on board when it is not attached to get the same weight.

    "If I where to use the 62xID relation you suggest my boat would be half a meter long. This seems a little excessive. my precious engine was around 15-20cm long and operated effectively."-------- For example 1/4 ID pipe length would be .25 x 62=15.5".for the unheated (see photo) part of the pipe. Some of this can extend past the transom. Shorter can work on some engines but if you get steam bursts or burnout go longer. " I am also considering whether the standard horizontal coil would be best or a vertical coil which could be made more longitudinally symmetrical." ------Should be no difference. In this case I would use vertical as it better accommodates the rise of the pipe before tuning down to the water.-----"Though apart from weight distribution I can't see how an asymmetrical boiler would affect performance."-------It should not in this case. When I mentioned symmetry before I was referring to multi pipe engines where the pipe bends, lengths and ID's etc. need to be the same.

    An engine need not have either a boiler or a coil to operate. A pipe closed at one end will work. However the larger the volume of the "heated part" be it coil or boiler makes the engine more friendly.

    Right oh! That sounds like a plan. Vertical boiler, 3 coils? 40-50cm long is a bit big for my testing facilities which are yet to be assigned so I am thinking a maximum of 30cm. Add the augmenter as cargo for the benchmark. Run three runs in each direction to get an average and account for wind and other factors.

    I will commence fabrication shortly and upload the results soon there after.

    Afterthoughts. Never heard of anyone building a single tube coil before. Maybe you are breaking new ground though not likely.
    I think Jean-Yves found horizontal better than vertical but that was with two tubes and I don't think a large difference. A bent a piece of wire showing a compromise. I have built many engines where the tube doubles back and it did not hurt performance. The advantage is a shorter OA engine.
    3 to 4 coils supposed to be best.
    Stopping a moving boat, backing it up or generally messing with it can affect it's power for several seconds. It takes a few feet to build up it's speed and power because forward motion affects the balance between the power and intake strokes. A long straight run is best for speed testing but requires a guide wire to keep it on track. I test in a 8', diameter circular pond tethered to a center point. An average of ten laps or roughly 100 meters seems to give honest results though I guess the turning scrubs of a bit of speed.
    Good luck.

    single tube coil 002.jpgsingle tube coil 001.jpg

    I'm wondering how the project is going and if I missed a message or you got bogged down on your move etc.

    You might want to research further re your findings. Suck does not create a force usable in this manner. A simple test is to blow out a candle by blowing through a straw. Then try blowing it out by sucking through the straw. Much information is available at Go to pop pop and then to Jean-Yves pages for a lot of good information that I have applied extensively in building my boats and engines. See Putt Putt 4 video's channel on you tube or Google Canadian Putt Putt Boats. Many have tried to build putt putt engines with valves. It is not difficult to make them work but not with an increase in power. Other information is available at the Mamod steam forum. Look under steamboats and launches. Hope you continue working on these engines as new innovative thinking might some day lead to real advances.