I can not follow your description of your Kaplan, so I can not give suggestions and "whys" the turbine is just producing a low % of what you expect.
If you have photographs, it may help a lot.
This low power could be caused by early heavy loading that stalls the turbine, common with people without much experience with Hydros.
It seems that the generator is not allowed to develop its normal and expected RPM.
Can you describe in detail what you have, generator voltage ( unloaded) what type of load and how you are connecting it.
Do know how to weld ( Tig or Mig) and cut 1/16 or 1/8 inch thick stainless for a possible fabrication of a propeller.
Nando.