wind turbine voltage multiplier
You probably need to connect your field winding up to get more output but running at this dismal speed is going to be a poor generating solution. No, a cockcroft walton multiplier won't help much here because they need several volts p-p to make any headway due to the internal diode volt drops. Drop the idea of the CWM and stick with understanding how to set the field winding current. I believe a lot of automobiles use the field winding current to control output voltage hence the final alternator output is regulated.

If you are not using the field winding then you are relying on residual magnetism in the rotor for generating your output voltage so it will be low.