At economical speed, the engine of a large container ship consumes 1,660 gallons (6,284 liters) of heavy fuel oil an hour. The answer varies enormously depending on the vessel.
A large container ship like the Emma Maersk is powered by a Wartsila Sulzer RTA96-C 14-cylinder diesel engine, the largest single diesel unit in the world, delivering a maximum power of 108,920 hp. The thermal efficiency of the engine is over 50% at maximum economy. Fuel consumption of large marine diesel engines would be more than 100 metric tonnes per day, depending upon the load, speed, and the weather and sea conditions.
Cruise ships operate differently from cargo vessels. A medium-sized cruise ship carrying 1,500 passengers, running on three diesel generators at 18.5 knots, burned 3.75 tonnes of heavy fuel oil per hour, or 38 US gallons of fuel oil per mile. A large container ship burning the low-sulfur diesel equivalent would see usage jump to 1,880 or more gallons per hour, since heavy fuel oil carries significantly more energy per gallon than road diesel.
Warships burn fuel at different rates depending on speed and power plant configuration. The USS Midway burned about 260 gallons per mile at cruising speed, and at maximum speed of 33 knots that figure would quadruple.
A large ocean liner like the Queen Elizabeth II consumed a gallon of diesel fuel for every thirty inches of passage at maximum speed in rough seas. At its maximum speed equivalent of 34 knots, it would have consumed over 82,000 gallons of fuel over the course of one hour, though it generally cruised at 28.5 knots, burning a gallon of fuel to go about 50 feet.
The easy conversion for estimating purposes is horsepower multiplied by 0.055 equals gallons per hour at full throttle, so an 80,000 hp engine consumes roughly 4,400 gallons per hour, not including generators.