I totally agree with having a conversation with someone knowledgeable about NL generators at the factory. But let me expand on the voltage implications.
I have got to believe that both US and Euro NL built generators use the same generator end, just wired differently. The generator has an output coil that produces 220/240 (the two are effectively the same so think of them as one value) and has a center tap that produces 110/120V from the tap to the other ends of the coil.
So to produce 220/240 in Europe or for a US boat that only needs 220/240 (not any that I can think of) you just wire to two ends of the coil to the 220/240V panel and the generator will produce 30 amps at 220/240. I am just using 30 amps to illustrate. The 5KW generator will produce less.
If you need just 110/120V then you parallel the two phases of 110/120V and you get 60 amps at 110/120V.
But some US boats need 110/120 as well as 220/240. So you wire each leg of the 220/240 to the panel as well as the center tap as a neutral, just like you home's electrical panel. Such panels have two busses, one for each leg of 110/120 that are supplied by each leg with the neutral (the one coming from the center tap of the generator coil) supplying the other side of the 110/120 circuit. In this case you cannot combine the two 110/120 circuits to produce more current. You are limited to 30 amps on each leg.
So talk to NL about what is going on. You probably need to do three things: increase the rpms to get more hz out of it, deal with the voltage regulation issues, and rewire the generator output to produce double the current for 110/120V. The latter may not seem important if one leg fills your needs, but doing it is easy and will be easier on the generator windings.
Sorry, this has been long, tedious and a combination of layman terminology and electrical terminology.
David