I'm guessing this is the right place to put this, from and r & d point of view it seems to be unique. It's also a bit pointless but the 'why do we climb mountains' thing kicks in here.
The idea is simple enough. I like airboats, and many years ago I resigned to one day making a boat where contra rotating propellers would remove the troublesome torque issues they have. Using model aero engines these airboats don't like turning right, the torque effect on such skimmy bouyant designs is pronounced. Now its been years since I thought of this, and a long time since I started to make it happen, but I get to it when time allows between more regular projects.
I like trialling things before comitting, especially if there are unknowns in there, so the first thing to do was put together a basic boat to check out the principle. The result of that is here:
https://youtu.be/Q6BiGlR36UcThose are two tiny engines, a .20 and a .25 I happened to have spare. After many trials a couple of things came to light, not least that I was getting more cumulative thrust from engines arranged like this than from the engines capabilities running alone added together. A happy accident I looked into and developed. In essence the front engine, the smaller one, is pre accelerating the air into the second, which is able to run a much heavier pitch prop than usual. After much debate and alternate engine trials I've settled on a .32 at the front, and a .40 at the rear. And this design of craft for them:
My tests showed two main issues. Vibration from two engines on a common mount was quite destructive, so I made a setup with isolated mounts. No existing rubber mounts suited space restrictions so I made my own from polyurethane. After this is was getting a bit silly on the weight front on the mount, which is where the fuel tank on rc airboats is usually kept. On my trial boat though I had the tanks on the deck as can see in the video, but inevitably there were issues, so as the tanks could ceratinly not be up on the mount, I turned to Perry fuel pumps. I tried a few variations on plumbing on these and settled on a system that works really well. The pumps deliver a surplus of fuel to a point just by the carb inlets, the carbs draw off what they need, and any excess is teed off back to the tank. That rig is shown here where the fuel tank distance is representative of the final boat:
https://youtu.be/4a-mfEhPY3kHere's where I am, top image it's next to a 10cc hydro
I'll get back to it now