I kept a very careful check on my consumption last year. Boat and engine as described on the left.
Cruising speed 3600 rpm, which gives 23 knots and seems to be the most economical speed.
Average Consumption over the season 1.75 miles per litre - yes mpl. Litres per hour works out at 8.9, but that's because I spend a fair amount of time pottering about finding places to anchor, so average speed over the year is around 15 knots. (Taken from GPS av spd at the end of each trip)
I think the consumption increases exponentially to enormous amounts between 4000 rpm and max 5600 rpm, but by then I'm doing 39 knots and struggling to hang on.
Take it steady and I think you'll be pleasantly surprised. Incidentally, my heavy 38ft. ketch used about the same amount of fuel per trip (under power) as my little RIB. (They both work out at around 8 nautical miles per gallon )
Even more useless info. 5 miles motoring with my Johnson 3.5 auxiliary used the same amount of fuel as the main engine at 3500rpm over the same distance. Took ten times as long though.