Almost 12 gallons in maybe 3 hours - doesn't sound terrible if you spent even half the time running into the wind.
Fuel useage can not be tracked on a linear scale (although that may seem logical).
As an example, I have some "on-water" test numbers for a 2 stroke, 50 horse Merc.
At idle, in gear (700 rpms) it burns about .3 gallons per hours giving it the eqivalent of 8 mpg.
At 1500 rpms it burns .4 gph but because the boat speed has effectively doubled (from 2.4 to 4.9 mph on the test hull), now it's getting about 13.5 mpg.
However, when you hit 2500 rpms, the gph jumps to 1.7 and mpg drops to 4.2.
But then when it's running at 3500 rpms, the fuel usage just barely tic's up to 2.0 gph giving it 9.9 mpg (this motors "sweet spot").
From there on the fuel usage goes up and the mpg goes down until at near wide open throttle it's burn about 4 1/2 gallons per hour (not bad for a 50 horse) and getting the eqivalent of 7.2 mpg.
So as you can see the ratio of fuel useage to miles per gallon is all over the map - and each model of motor will have it's own "sweet spots".
All you can do with a given motor is -
make sure it's tuned properly (new spark plugs, fresh gas, properly adjusted carbs)
make sure it's mounted properly (correct height, trimmed properly)
make sure it's "propped" properly (running too much pitch will overwork the engine and fuel economy will suffer/too little pitch will not get you the speed for a given rpm and could tend to make the mpg suffer)
load the boat properly (too much weight in the front will cause it to plough water and waste gas etc)
and once you have done all that, you need to play around and see "where" your motor likes to run - normally somewhere in the 3500-4200 rpm range will give you the best combination of speed and fuel economy.
If you are going to do alot of trolling or "poking along", install a kicker (8, 9.9, 15 horse). It will "always" burn less fuel for the same speed compared to backing down a larger engine...
