Researched this question a few years ago.
Basic rule of thumb:
Horsepower divided by 10
is the number of gallons burned per hour WOT
10 hp burns 1 gallon per hour WOT
100 hp burns 10 gallons per hour WOT
so 15 hp burns 1.5 gallons per hour WOT
If you want to get picky use these formulas:
An in-tune four-stroke gasoline engine will burn about 0.4 to 0.45 pounds of fuel per hour for each unit of horsepower.
A well-maintained two-stroke outboard burns nearly 0.6 to 0.8 pounds of fuel per hour for each unit of horsepower.
Gasoline averages 6.2 pounds per gallon (depends on type)