Why though are we using a pair of clockwise-rotating propellers, as here? Well from this photo it is clear that the overlapping areas of each disk will in fact rotate in the opposite direction.
This is important because overlapping propellers are not at all efficient, and altogether less so if the first swirls the efflux in the same direction as the next. An exception to this is the jet-engine, where the stages do all rotate the same way upon the same axis but are able to accommodate this with flow-straighteners in the interstitial stages.
Don't ask, basically.
Using the laser-guided set-squares I have here, however, it is clear to see that a ten-inch space (254mm) is a suitable separation distance for each pair of U7 motors.
