It was too rough on the Bay Saturday for me to want to extend the
"testing" to the higher RPM ranges, but I did note that the apparent
sweet spot for my little Parker and its Yamaha F150 is 4000 RPM. At that
engine speed, with three guys aboard, plus 75 gallons of fuel (3/4
tank), fishing gear, three batteries, cooler full of ice, et cetera, the
GPS and the boat speedo agreed we were doing 27 mph and the fuel
management meter said we were getting precisely 4.5 mpg.

I don't know whether that is good, bad, or indifferent; it just is.

On a different day but with a similar load, we saw 41+ mph at 6000 rpm.
I don't recall either the the *exact* gph or mpg at that speed, but it
was typical for a 150 hp engine...using the one gph per 10 hp "formula."
There's really no need to run at that speed, so I rarely do.

My Parker is relatively heavy for its length (2850 pounds, sans engine,
fuel and gear), and has 21 degrees of deadrise at the transom, so
comparisons with lighter boats with flatter bottoms aren't too relevant.