This is conjecture and for discussion only. Assume you have a flat bottom boat with 700 lbs load and a 20 HP engine. At 3 mph you will get about 14 mpg. At 6 mph about 12 mpg - at 9 mph about 2.5 mpg - at 12 mph the mpg will increase to 10 mpg - at 15 mph it will drop to 8.5 mpg - and at 21 mph it will drop to 2.5 mpg again. I wish I could graph this as it would make a lot more sense, but the whole idea is your boat will be more efficient at hull speed and below. It will be least efficient as it climbs the bow wave or runs at max speed.