The answer is buried in the volume of text in this thread, but I'm happy to say this again because it's very important to what I've discovered.
Originally I used two different engine sizes because I happened to have a few aero engines not being used, and OS 15, an Irvine 20, a TT 25 and an MDS 40. The 20 and the 25 were the closest in power, so I used those believing it was a compromise. This turned out to be a happy accident.
In my head it made sense to put the bigger of the two at the back, because surely it would be pointless to put the small one there simply because the airflow over it from the front one would be faster and in greater volume than it could manage to meet, so it would add nothing. Expanding on that I figured if the small one ran the same size prop as the rear but with a pitch suited to its size, then the moving air would hit the rear engine in a pre-accelerated state, so I figured a heavier pitch prop than the engine should have on the rear could be run as it had fast incoming air to work with. In essence this worked out well. At the back of my mind was a real boat water prop system employed by Volvo Penta called 'Duoprop'. On that system a heavy pitch front prop feeds a contra rotating rear prop which has an extra blade. When I was on the shop floor I fitted loads and loads of Duoprop stern drives on motor cruisers, and latterly as a test engineer I'd regularly take them out on sea trials, and superb they are too!
So something at the back of my mind essentially came to fruition when I tested the principle, which was latterly found to be a form of 'Pressure Recovery', where the spiral or tangential airflow off the front prop which was largely lost force was re-directed to a straighter air flow, and where the front prop 'charges' the rear allowing it to reach peak RPM with a heavier pitch prop, thus more thrust. By looking at the exhaust smoke when I was running the engines very rich a clear narrow conical airflow is coming off the props, rather than the usual dispersion you see. This is actually visible on the second video of the test boat, where accelerated air is still visible some ten feet behind the boat in a comparatively narrow area
If you think about it, two engines the same, running two props the same, will have the rear one aiding the pressure recovery element, but it'll just be moving in air at a similar speed to what it would produce, effectively running a prop with zero pitch in respect to the differential between incoming and propelled air, i.e if it's hitting the prop at X miles an hour and is only capable of moving it on at X miles an hour then it's adding little or nothing.
So, when I decided to go for bigger engines for the project boats I had in mind a 40 and a 36, but in the affordable SC range the 36 is a different motor, and the 32 and 40 shared ABC type and an option for rear mounted needle valves, which makes it a little safer. I've had one clip off the test boat as a reminder that putting your hands between two aero engines so close together is dodgy ground, so I thought it OK. In reality the power difference is only slightly off line to the difference between a 20 and a 25, so not too shabby.
However, it's worth pointing out here that whilst true contra rotating props add power to a single engine, two engines lose any such advantage through increased weight, so I remain interested in doing this simply because I've wanted to for a long time, with the advantages I've proven of negligible, even immeasurable, torque issues, and that great sound. One other advantage I've found is that if you're less than careful with duration by configuring fuel tank sizes so that the rear runs out first the boat can be brought home without those tedious waits or recovery needs. Fact, I spend more time on the pond using this airboat than everyone else in the club, who seem to spend an inordinate amount of time waiting for dead boats to drift in, and the only time I've had to do that is when I flipped it in those heavy winds. The rear engine must stop first though, or at least bring it back on the rear very slowly, as the rear is totally dependent on the front engine for cooling at high RPM.
To me there is no reason why a 15 and a 40 can't be bedmates, you just won't be able to run the same diameter props. Something like an 8x4 on the front and a 10x8 on the rear will, I guess, still realise the two stage air movement, it's just that the 40 will not be fed quite so wholly.
I may find myself running different size props on the final boat too, but if I can safely get away with the same size I will. I guess you could run two engines the same cc with the different pitches and get the same effect, I've left that door open as my mount will allow the later fitment of another 40 easily, but then the weight thing would be less favourable. Dunno, there's still plenty to explore but I hope I'm in the right place for these particular boats. I must say I'm feeling encouraged by the new engines, the thrust off them is very good for the engine type and I'm not so nervous as I was when I first felt the weight of my extravagant mount. Also with the vibration issue looking good so far I'm heading back to pylon mounted throttle servos to make the installation a lot simpler. remember I have this alloy mount because I've found that other types don't last, even glass filled nylon radial mounts have let me down over time. But a well put together plywood mount would work well for some time, make it replaceable and there's a simpler solution to longevity right there. As I enjoy the act of building though I'll stick to my over the top resilient mount for these boats
