If you have gone as far as converting and rewinding these car alternators I don't see why you are trying to play with a voltage doubler that is not going anywhere.
I suspect your basic problem is no wind and there is no cure for that.
I also have doubts about your interpretation of measuring power into resistive loads.
If you have an alternator that will produce 100W into a battery at 700 rpm you should be close to managing something. If you had rewound it with more turns then it would have started producing at a lower speed. Not sure of blade sizes you are using or what cut in speed you actually have into a battery.
If your trouble is lack of wind then boosting your 6v to 12v will bring you to cut in but you may still not have sufficient power to do any useful charging. If the only trouble is low volts then rewinding with more turns of thinner wire is the way to go.
Try charging a 6 v battery and if you can get several amps then you will do some good if you rewind. If the thing just brings the speed down and the current is tiny then you have no wind or unsuitable blades.
You can easily get up to about 500rpm with fast well built 5ft blades in less than 10 mph. If your alternator genuinely produces 100W into a battery ( not a resistive load) at 700 rpm. with it's inherent low efficiency it ought to be cutting in at 12v at 500 rpm or below. I suppose there comes a time when small fast blades will not start a pm converted car alternator against eddy drag in a sensible wind but I would have expected a 5ft prop to start in speeds where you are producing enough to supply the iron loss.
For a very low wind area rewinding for a lower cut in of about 350 rpm and using a fast 6ft prop you should be able to get something useful in winds over 10 mph.
If you want to try boosting the present alternator then I think you need to go to electronics rather than voltage doublers. They need monster capacitors and with 3 phase you are struggling to do anything better than half wave.
A mixed bridge boost converter would be infinitely better but is not likely to be an option without some electronic skills.
Flux