|Note: Recent advances by a putt putt enthusiest in South Africa promise to answer many of the lingering questions about why some engines work and some don't. Click here for more.|
A project like like the putt putt boat requires a community of enthusiests to work out the best engine design. There are dozens of variables affecting engine performance. I have experimented with some of them. But the problem is the variables interact synergistically with each other. Here are some variables and some notes. There is a lot of experimentation to be done!
Width of boiler: Most of my teenage students plot to make a bigger (and therefore more powerful) engines. If you succeed in scaling up the size significanly, I'd like to know about it. It has not worked for me. Not all complex systems can simply be scaled up. If the boiler is too wide, it seems to take in too much water, cooling too much. If the engine is too narrow, it becomes rigid so that the bottom doesn't flex in and out.
Length of boiler: It would be nice to shorten the boiler length, especially when putting it in the fancier boat body, where it tends to stick out. I have been able to shorten it about an inch, but sometimes the engine doesn't work as well. I also worry that the candle will burn the glue at the end and cause a leak because water (which moderates temperature in the boiler) doesn't get up at that end much.
Length of straws: The momentum of the column of water in the boiler is what allows the periodic cycle of the putt putt engine, so you need straws for it to work. However, shortening the straws (to a point) increases the speed of propulsion. The period of the cycle (number of putts per second) seems to be faster, too. I do not understand very well the interaction of straw length with straw diameter.
Straw diameter: In the construction directions I showed how to restrict the end of the straws to increase the speed of the water jet coming out, and therefore the speed of the boat. Should the whole straw be narrower (less turbulence?) or should only a very tiny bit of the end be restricted (less pipe friction)? I don't know. Could we learn something from rocket nozzle design or jet ski design to make it even more effective?
How far the straws stick into the boiler: How far they stick in determines where the water hits the boiler. A related point is, could the straw ends be shaped somehow maximize their effectiveness?
How much sealent goes inside the engine beside the straws: If the silicone only goes in a little bit, some water will pool there when you prime the engine. On the other hand, if the silicone fills the the entire gap between the straws, the water will just go from one straw into the other without pooling.
Overheated boiler: Even tiny leaks can cause a boiler to stop working, lose water and overheat. It emits a distinct smell of hot metal and glue. If the candle is allowed to burn under it for a long time, it could damage the glue sealent, making the leak problem even worse.
The frustrating thing is that even some engines with no leaks overheat. Such an engine will often work great, the slow down, start making a tinny sound, and finally stop.They will usually work again if you take away the heat for a minute or two. If the straws are clear you can actually see the water creep up again.
Most of the variables listed on this page could have some effect on to what extent it happens.
Angle of boiler to straws: Engines with a sharper angle seem to go a little faster. On the other hand, with a sharp angle it is hard to get the candle close to the bottom, which is where some engines heat most efficiently.
Candle placement: Where the candle goes under the long boiler can have dramatic effect on engine power. It might also relate to overheating. Closer to the front seems to generate more power sometimes, but have the opposite effect other times. Moving the candle back allows you to leave space between the candle flame and the bottom of the boiler, so carbon doesn't collect there.
Ease of Construction: When making innovations in engine design, try to keep it simple enough that ordinary people can make it. I'd like to simplify my instructions more.