Totally agree on fuel efficiency at optimum RPM and would suggest IMHO it is somewhat mitigated by longer cruising times at lower kts. As an example using Glenns #'s if I am going on a 20 NM trip at 20 kts it will take 1 hour and I will burn 25 gallons and at $4/gallon it will cost $100.

If instead I go 14 kts then the trip of 20 NM will take me 1.43 hours and at 15 gph I will burn 21.45 gallons and at a cost of $4 will cost me $85.80 for the trip.

I think a lot of the incremental fuel cost comes down to the generator time and load. I don't know why one might run the generator while moving and Moorings specifically indicates not to. A lot of my incremental costs are running the generator in the evening to keep the boat cool.

My latest trips have been "show the BVIs" trips to people who will most likely never see them again so we tend to go faster although not flat out