I'm looking at purchasing a 30ft or so size boat with dual screws. The ads typically state the size of the engine like twin Chrysler 318's etc. I'd like to figure out the cost of gas per hour, so my thinking is that regardless of whether the engine is in a car or a boat, at say 2000 rpm, you should get similar consumption. In my car, my usage is around 11 liters /100 km at 100 kph and 2000 rpm. In other words, I'll burn 11 liters in one hour. So, if this engine was put into a boat, I should burn 11 liters per hour as well; the actual distance would depend on hull shape, etc.
Is this thinking correct or am I way off? Does anyone have a list or table of hourly gas consumption by boat engine? Is there a proper way to calculate consumption?