Hi Rene
I read your post from months ago. So you may have found your rpm problem. water need more speed, larger supply pipe . You may have done this. try to run piping with out 90 deg fitting, will slow the speed of the water.
One note i see on your slip ring the brush has not been seated to the ring. This will create hot spot for all the current to pass through. I recommend to shape the brush to the surface of the slip ring. I tape sand paper to the ring and work the brush back and forth to sand it into shape. use a fine sand paper 150 and polish 220. I hope this is a bit help full. and flashing the winding can help to induce a magnetic field into the gen. have you found how to flash the winding ?