Ours puts out 219V with no load, 217V with max load, and 218V with half load. It's been that way for at least 3 years, since I started looking at this measurement.
I asked the only Onan-recommended, Onan certified dealer, in this part of the world why I am not getting 230V on a 230V boat, and whether he should adjust it.
He bent over, took a pic of the genset spec plate and showed me his phone: the genset spec reads "220V". He said we should leave it alone.
Now with this thread I a am not so sure.
When he comes by tomorrow, I'll ask him to check the RPMs. Should be 1800, uh?
SM2K No. 350
Marina d'Arechi, Salerno