Join Date: Nov 2009
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The rule of thumb for 2 stroke outboards is 1 gallon per hour for every 10 applied horsepower. I've had 3, 4, 6, 7 1/2, and 150 hp OB motors over the years and this rule seems to hold. I have a 4 stroke 9.9 hp and a 150 hp direct injection 2 stroke, which get better fuel consumption, but nowhere near that of my 27 hp inboard diesel.
If you are concerned about range, stay below your displacement speed (1.25 x sq. rt of your waterline length).
So, I agree with dabnis: if you are operating at full throttle with an 18 hp motor (2 stroke?) you should be getting about 2 gal/hour and be pushing your 25 ft boat past displacement speed of ~6 kts, assuming you are not pushing into high winds and seas. You should do quite well at half throttle.