If you want to measure the usuage at 1800 rpm you'll get a very differnt number at (say) 2800 rpm.

I'm sure (within normal ranges), that the NMPG efficiency will go go down, as the rpm's go up. The use of a large frame alternator with a real recharging battery load can have an impact (expecially on a smaller engine and load).

Motoring into a short, rough seaway and into a head wind obviously changes the equation too.

Our cruising range goes up about 60-70% at about 2000 vs. 2800 rpm. That's going 7.3is vs. 8.7ish knots (for example purposes).

You need to look not only at the HP curve, Larry -- you need to look at the whole system of pushing the boat through the water. The loading chart (remember "hull speed") is more telling.

On you multi hull, that has less impact because of the wetted surface differences to a mono hull ; but it's still there in principal (just a bit more muted).