How To Build a Valved Pop-Pop Boat

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.

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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.
sd_fd4 months ago
My idea was to make a trial with a water jet vacuum pump, that is not the original augmentor theory.
I made it as a vacuum pump.
LetsBuildOne (author)  boat-sketch1 year ago
I'm not sure what you mean by that...
boat-sketch2 years ago
Made an experiment with a simple pop-pop boat. The speed of the boat was ca. 1/2 of the original.
LetsBuildOne (author)  boat-sketch2 years ago
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.
Aron3132 years ago
Any video????
Forgive my ignorance, but aren't you just getting more power from nowhere? Is this possible?
LetsBuildOne (author)  RocketManDave3 years ago
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 backward units 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?
LetsBuildOne (author)  dcanada3 years ago
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
LetsBuildOne (author)  dcanada3 years ago
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.
LetsBuildOne (author)  dcanada3 years ago
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
dcanada dcanada2 years ago
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.
LetsBuildOne (author)  dcanada3 years ago
Wow, Thanks for the link, there is a lot of very useful information on there. I've visited it before but I don't think it had all this extra information on it.

To which section are you refering? I found the section on 'Proof of propulsive dissymmetry' and the one on 'Working principle
of a pulsed waterjet' but neither say anything about the suck not being useful. In fact in the latter document there is a diagram that describes a little as to why the thrust augmenter works. I hope the original content owner doesn't mind but I'll upload that pic for illustrative purposes.

Are you aware of how a pulsejet works? If so are you aware of the thrust augmenters they use? This is real proven technology just given a different, very similar application. Here's a little pdf on the subject:

Maybe re-read the section in my instructable on how a thrust augmenter works or do a google search on them for clarification. I'm not suggesting that the sucking gives significantly more power. I'm suggesting that the established retarding forces the intake entails can be minimised and re-directed to have less effect on the forward motion and, if done correctly, may even add to the propulsive forces. This is all explained by Newtons second and third laws of motion; force is equal to mass multiplied by its acceleration and every action has an equal and opposite reaction respectively. The majority of the extra thrust comes from the vortex piston. Not the redirection of the sucking.

Glad that my comments are of interest to you. Going back to the eclecticspace site I could not find that particular paper either. I will ask Jean-Yves if it is there in it's complete form. It is however on an invitation only private forum that I belong to and if it works I will copy and paste it below. It sounds elitist but it is not and in fact there is little new happening there now. The photos and graphs did not copy but you have the text.

I would not use any of Jean-Yves material without permission but have no doubt he will grant it. We are continually trying to make contact with people who are interested in putt putt engines and boats and creating a library of information that is available to all. The Unofficial Mamod and Other Steam Forum have created a sticky thread that we hope will serve that purpose. The link to that thread is below and I encourage you to join in. It will help others if this conversation we are having goes on record at the Mamod site.

Another site is on Yahoo but it is mostly a talk forum.

You may PM Jean-Yves via either of these forums.

I'm not a scientist or engineer so my creations are the result of much trial and error. The increases in force and speed that I have achieved have come from listening to Jean-Yves and others, applying their knowledge and maximizing the design of the basic solid boiler pop pop engine.

The best proof of your theory is to build two identical boats and engines and test them with and without the inclusion of your ideas. I sincerely hope you can discover a way to make gains.

Jean-Yves Full Member Posts: 215
pulling or pushing a boat?
« Reply #1 on: August 04, 2007, 12:38:02 PM »

Hi all,
I just finished a new test to demonstrate that sucking water has no effect to propell a boat. There is no pop-pop engine in this demo, but it means that during the sucking phase of the reciprocating cycle there is no negative effect. It also means that it would be useless to complicate pop-pop engines with check valves in order to suck ahead and deliver backward. (I must confess that I had this stupid idea 3 years ago).
See attached document.

Proof of propulsive dissymmetry.doc (110.5 KB - downloaded 4 times.)

Experimental proof (one more !) of the dissymmetry of a propulsion by reciprocating waterjet.

Purpose of the experiment: to show that sucking water on the bow of a ship has a negligible impact; though a jet backward propels.Principle of the foresee assembly:

A symmetrical hull (according to transverse and longitudinal axis) to avoid endless discussions.

Electric pump located in the middle of a pipe of ID « d ».

Internal diameter of the cross-over equal to more than 2xd.

Thus, on one hand the pressure drops upstream and downstream will be nearly unchanged. And on the other hand, the speeds inside the cross-over will be very low and the resulting forces being opposed they will cancel each other.

Longitudinal sucking Longitudinal delivery

Measurement of the boat velocity in both cases.Practical results:

Test conditions are described in appendix.

Measurement of the boat velocity when propelled by jet. Result: 72mm/s.

Measurement of the boat velocity during sucking. Result not measurable. Velocity lower than 1mm/min.

Knowing that the thrust evolves as the square of the velocity, one can calculate the ratio between pushing and pulling forces.  ; i.e. the pulling force (when

sucking) is roughly 20 millions times weaker than the pushing one.

Conclusion : this experiment confirms the theory. Sucking ahead has a negligible effect, and the related assumptions used in other documents are justified.

Photo of the (so-called) ship.

Measurement of the pump flow. Result: one little in 64 seconds.

Wetted surface of the hull: 591cm2

Internal diameter of the pipe: 4.4mm

Internal diameter of the cross-over: 10mm

The measurement of the propulsion velocity took place outside on a bench that is 4m long. The first meter was used for initial acceleration, and the last one for stopping. The real measurement was done on the central 2 meters. To eliminate some eventual wind influence, tests were run in both ways.

The first sucking measurement took place on the same bench. The velocity was seen low, but influenced by the wind. The boat was going ahead or astern by some centimeters. New test were run indoor with a shorter and wider bench. Transversally the hull was kept at the centre of the tank by means of two slides (with a play of some millimeters). When the pump was energized, the vibrations got rid of the light permanent friction forces.

Vortex at the suction side.

Test conditions and lighting allowed us to observe the water surface and the movement of some particles just above the suction side.
LetsBuildOne (author)  dcanada3 years ago
Very interesting! It may be important to take into account the engine dimensions I'm using as the boat without the thrust augmenter has a noticeable 'two steps forwards, one step back' style of motion. The boat engine is relatively low frequency and the boat itself is very light weight, essentially the engine plus a few grams of foam. I am tempted to hypothesis that the higher the frequency the less difference it makes in sucking the boat back because of it's momentum. With this low frequency, low inertia boat it's effects are far more prominent, it looked like it was vibrating on the spot in one video.

I'm currently building a single seat racing car that is due to race at Silverstone in just over a week so I'm rather busy. As soon as I'm finished I'll rebuild two identical engines and hulls and get some experimental results for you. You sound enthusiastic and knowledgeable in the field of pop-pop boats so I'd like to bounce a few ideas off you and see if you've heard of any previous experiments. One thought I just had while reading that paper is the asymmetry of most pop pop engines and if one could be designed asymmetrical purposefully to suck from the front and blow out the back. I agree with the paper you quoted but I believe that noticeable gains can be made in performance from solving this fundamental issue with this engine. It's physics, we're wasting energy...
The two steps forward and one step back motion we refer to as shuttle. Your conclusion that the weight of the boat is a factor is correct. The added weight of the augmenter may be the only difference in your boats now. The shuttle is caused by the boat moving back and forth rather than the water in the pipes moving hence low thrust. The lighter the boat, and, the larger the pipes, and, the lower the hertz are all contributing factors. The fix is not so simple as every thing done to fix it carries some kind of penalty. Reasonably good compromises are available as you can see from the performances in my videos. Jean-Yves and I have also come up with a way to (mostly) cancel it out and with larger tube engines, 1/4" ID and up significant gains can be made. With smaller ID's gains are questionable at this point. I am testing one now.

If you are going to build one or two engines to test your theories, (and I hope you do) I suggest starting with a proven design that should provide consistent performance with few idiosyncrasies so that test results will have reasonable validity that others could attempt to confirm by duplication. Two of my designs are available online.

The first is on a thread at the Mamod site. It requires to be assembled with silver brazing. Silver solder or glue will not do. The link is....
2 tube engine build.....

A better choice is described in one of my youtube videos. Easy build. No high temperature brazing required and can be built to be noisy or silent. Silent is better for testing because it is more powerful, more consistent and the noise won't drive you crazy if testing is prolonged. To make it silent use a rigid material for the diaphragm. Brass, aluminum, copper or steel (rusts) in .015 to .020" thickness will do. The link is......
Part 13-Easy engine build.

So far symmetry has proved to be important to getting good results so I will be interested to here your thoughts on how asymmetry might be an advantage and what you mean by the engines being asymmetrical now.

I have passed on your comments re a thrust augmenter to Jean-Yves and another clever fellow who's interest is mainly in rocketry. We will see if they can reach agreement on the potential merits. I would like to try it if there seems to be a potential for gains in force.

I'm happy to discuss any subject with you though I would rather do it on the Mamod site where you gain the advantage of other valuable input.

Good luck at Silverstone. My racing days are long gone.
Wow, that is really impressive! I've only ever made coke can-diaphragm pop pops but am hoping to use brass tubing on future models. Will take another look at your instructable then ;)
Araldite degrades above about 100 C, in the presence of water, so solder should make a better joint. If your solder joints are failing, I should look at the technique you joined it with.
LetsBuildOne (author)  steveastrouk3 years ago
Water boils at 100 C, the engine shouldn't get much above this point. Dependant on the specific type Epoxy resins such as Araldite start to fail just over 100 C, Specifically between 120 and 180 C. 50/50 solder starts to melt around 180C.

With Epoxy resin it is easier to apply, seal, requires little skill and no extra tools such as a blow torch or soldering iron . When the flame laps over the edge and hits the join it just scorches the adhesive and the unit remains sealed as the rest of the glue maintains it's integrity.

Solder is the proper way of doing it but when the flame laps around the edge and catches the solder it has a tendency to melt a corner then for the molten part to melt all the solder it's attached to. So when the solder fails it blows the whole seal. It also takes more skill to seal. It's easy if your proficient at soldering but if your not a technical person it can be quite the challenge and this instructable is aimed at people of any skill level.

Experiment for yourself. Both methods work, I prefer soldering but Epoxy does the job and is much easier. I'd suggest you invest in some high grade Epoxy, if your buying it from Walmart or home depo the chances are it isn't going to work. No rapid or clear, you want the grey opaque stuff that comes in two half kilogram paint tins that takes 48 hours to set. Surface preparation is more important than when soldering; The surface should be keyed with some emery paper and cleaned with a solvent that doesn't leave a residue such as electrical contact cleaner. Your Epoxy should be weighed when you measure it out to ensure you get equal amounts of each and should be mixed until it is of a consistent colour. While setting it should not me disturbed.

The slow setting J-B Weld epoxy has a higher max temperature rating (600 deg F per the package) than the quick set epoxys. One can braze with a mapp gas torch, but a large area will be heated. For possible lower brazing temps than bronze rods, two pack nickel silver brazing rods are available at the Adavance Auto parts store for ~$5. The best high content silver brazing rods are very $$$ now days.
LetsBuildOne (author)  zoomkat3 years ago
Wow! The website suggests 500F but that's still 260C which is mighty impressive! Thank you very much for the tip, We don't get that stuff in the UK so I wasn't aware of it. I may have to get some imported off the net. 260C will be plenty for pop pop boats I'd imagine but as you suggest I think brazing would be a much more appropriate solution. It'll be a while until I get round to my next engine though as I am currently rather busy but when I do I think brazing is the way forward. There's no way I'll blow a brazed seal with a candle...
zoomkat3 years ago
Did you test the boat both with and without the augmentor to compare total thrust generated and boat speed in water?
henrib7363 years ago
could your perhaps upload a video if its posible as it would be great to see it work.
LetsBuildOne (author)  henrib7363 years ago
I intend to, though the boat needs a new hull and the engine re-sealing and I have my finals in a month. As soon as that's over it's top of my list though!
casman3003 years ago
could your perhaps upload a video if its posible as it would be great to see it work.
LetsBuildOne (author)  casman3003 years ago
I will! I need to rebuild the engine and hull but that shouldn't take long...
rimar20003 years ago
Congratulations, you've managed to carry the cat to the water! Pardon, the pop-pop...
LetsBuildOne (author)  rimar20003 years ago
Cat?... Because of the long tail?...
"Carry the cat to water " is a saying that means "achieve a feat"
LetsBuildOne (author)  rimar20003 years ago
No wait!... Because it's hard to get a cat near water you're saying I've done something very difficult!... I apologise, I have never heard that turn of phrase before, I assumed you where comparing the boat to a cat which made no sense to me for fairly obvious reasons.
rickharris3 years ago
How did you shape the inlet end of the thrust augmentor? Interesting solution.
LetsBuildOne (author)  rickharris3 years ago
I put it in the chuck holder (like a three jaw on a Lathe) on the milling machine and inclined it at an angle. Then used a slot milling bit to drill in to the centre and turned the chuck by about 140 degrees ish... raised the mill out and turned it round a bit more to leave a web. Then back in and round another 140 degrees to make the other slot. A load of small holes would do or you could turn both on the Lathe and then attach them to the tube using three bits of thin rod bent and glued down the length of the augmenter at equal intervals with an angle down to the 1/2" copper pipe and then glued around that pipe at equal intervals also.

On Pulse Jets they are rolled out of sheet metal and welded. Due to the size of the device I decided against that method as my engineering department doesn't have a roller small enough to my knowledge and welding would have been tricky with the thin metal and dodgy angles.