Intro: 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.
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.