The big one is 28 volts, the little guy in the image is a 14 volt model.
For whatever reason Niehoff decided on charging voltage designations.
With large bearings and being brushless, this is a start it fully loaded and leave it there, forever, alternator. The automotive ND alternators cannot take that kind of full load punishment.
This unit requires a 6-rib serpentine belt, Forget moving the alternator to adjust belt tension, that is what idler pulleys are for.
It uses an internal voltage regulator but it is easily defeated so a yacht programmable voltage regulator can be used.