Fuel economy is a determining factor in virtually every automobile purchase, but many people fail to consider this when purchasing a boat. Since the water obviously provides more resistance than the air, boats are less efficient at using fuel than other conveyances; only 13-14 percent of the energy the engine generates is used to power the prop—the rest is lost through heat, vibrations and friction. Most diesel engines will use about 1 gallon of fuel per hour for every 18hp used. Four-stroke inboard engines our slightly less fuel efficient, using 1 gallon per hour for every 10hp used.
Most of our readers won’t like to hear this, but two-stroke outboard motors tend to get the worst fuel economy. There are several variables for two-stroke engines—such as the ratio of fuel to outboard motor oil—so there isn’t a standardized calculation to determine economy.