Starting from 200v at 3000 rpm  with single phase and full flux you would be looking at a cut in of 3000/1.4 = 2140 rpm for 200v. At 50v you are looking at 2140/4  or about 535 rpm. As you may only have about half flux then your cut in may be up to 1000rpm.

With pvc blades I think you must have seen this 60W in a gale.

Single phase is bad, small high speed motors have little room for much magnet so everything is a against you from the start so you have done fairy well. If you want 48v then you need to rewind, it may do better at 24v or 12v with some reconnection of coils in parallel.

When I look at your excellent work I can't help feeling that if you had put half that effort into building Hugh Piggot's 4 ft machine you would have had an infinitely better result with some output from about 7mph upwards and about 150W at furling point.

I really see the motor conversion as a labour of love rather than an easy solution and anything smaller than about 3 hp 3 phase is a lot of hard work for little potential reward in the end.