Gould,
Since you boat with a trawler I think you have forgotten what happens when a
planning hull is no longer planning but is "muscling" it's way through the
water. Most boats drop off of planning at 2200 rpm to 2800 rpm.
The numbers do look reasonable for a "planning hull".

Once a month I do test runs on a wide variety of different boats. A number of
them are planing hulls. I cannot remember a boat where an increase of 500 rpm
would have brought the boat from 9mph to 25mph. As I said, I'm not in a
position to comment on that specific boat, but the numbers seem strange to me.

Once a boat is over the bow wake and on plane, (which should be occuring
somewhere in the low to mid teens speed wise), in most cases fuel consumption
goes up exponentially to obtain additional speed. It would be interesting to
view a speed/fuel chart for the boat NOYB is referring to.

A boat with the operating characteristics NOYB described would be frustrating
as hell. A difference of 500 RPM increases the speed by 2 1/2 times? Unless you
wanted to run (and the conditions permitted) 25mph, you'd have a dickens of a
time setting the throttle for a speed somewhere between 9mph and 25mph and
keeping the boat there. Everytime a mosquito landed on the throttle lever you'd
pick up or lose a couple of miles an hour. :-)