You could indeed improve the set up I gave him. I just gave him something that would adequately manage a 6 ft prop with a common magnet size that works out nice and cheap.

It would perfectly well have gone on 8" discs and been a better compromise, but using 8" discs he would run out of space at the centrer of the stator and if he wants to use a standard trailer hub there would be trouble. I used 8" discs but I had to make a special hub.

Going up to 12 magnets makes the thing capable of managing an 8ft prop and it will need to be really tamed down with lots of series line resistance at 6ft. It seems pointless using 8 more magnets. Eight poles of the 2 x 1 x 1/2 will still be ok, I just happened to use the metric magnets as they are far easier to get here.

Rather than squeeze every bit out of a small design, this was generous enough not to have to worry about making it ideal. It was derived from Hugh's single rotor 8 pole 4 ft machine. I have scaled it up to 5ft with a second disc with no magnets and it would just probably take 6ft with care. Doubling up on the magnets gives it lots in hand for 6ft and there would be no real need to go to great lengths to get a wire size to make the coils touch. Unless it is 12v with normal line resistance it will be well towards stall and may need added resistance. At least the heat is better dissipated in the line rather than a higher resistance stator.