This leaves about 13-14 percent of the original energy to turn the propeller. Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055. An engine at cruising speed usually uses only about two-thirds of its maximum available horsepower. Most marine engines are designed to run continuously at between 60 and 75 percent of maximum speed.
Diesels tend to be more toward the top of the range.
Gasoline four stroke inboard engines need about 1 gallon per hour for every 10 hp used. The number of gallons consumed per hour can be estimated by multiplying horsepower used by 0.100. Outboards might use considerably more since two stroke motors seem to have a greater thirst than four stroke motors. Any attempt to force a displacement hull beyond its maximum theoretical hull speed brings exponentially higher fuel consumption for minimal gain in speed. On the other hand, slowing down a little almost always is rewarded by gains in economy, and thus range.