Your not far off one gallon for every ten horses but that would be more or less flat out and carbe'd two strokes. Things have changed dramatically in the outboard world with the introduction of the four stroke. However having said that engines have got so much bigger in horsepower and many new boats are totally over powered .People generally have no concept of fuel usage on a boat I still shock myself sometimes and that 's after 60 years . Go to any Marina in the country and many twin engined petrols v8s never go out they can run to 40 gals an hour. Em