You probably have the first generation Mercury VERADO FOURSTROKE engines. They are notorious for their high fuel consumption.
At WOT the engines will be making 500-HP and consuming about 58-gallons per hour. That will be about 220-liters per hour--or more.
The higher-than-normal fuel consumption of the VERADO FOURSTROKE engine at its rated maximum power is probably related to the use of supercharging. The supercharger consumes a significant amount of the shaft horsepower of the engine. This means that when a supercharged engine produces, say, 300-HP at its power output shaft, perhaps as much as 30-HP was being used to run the supercharger. So the engine was actually making 330-HP to produce 300-HP useful power at its output shaft.
Mercury quickly recognized that high rate of fuel consumption of the VERADO FOURSTROKE engine was a concern, and within about two years of its introduction Mercury produced a second generation engine. The refinements were principally intended to improve fuel economy. Several years later, Mercury sought advice from VERADO FOURSTROKE owners about their satisfaction with the product, and this inquiry again lead to a second effort to improve fuel economy. Apparently there was not much room for improvement in the engine, so Mercury developed a special propeller, designed specifically for the VERADO. The new propeller was called the ENERTIA ECO propeller. The propeller has been described by one boater writer as being "a miracle." In a
2015 article published in BOATING magazine, test results showed that the ENERTIA ECO propeller improved fuel economy at fast planing speeds by a factor of 1.43:1. Indeed, if one could actually obtain those results, it would be a miracle. In addition to careful design of the propeller, the miracle propeller was made from a
Rearden Metal marketed as X7.