if you're burning 200 litres, you're travelling a big distance. i'll put it to you in caveman speak, i can travel in my 5.3L v8 engine truck, 350km and only use about 50-60L of gas, if i was pulling a boat, might be 80-100L, now,my dad has an 86 searay, with a 4.3L V8 merc, 220hp I beleive, 21ft boat, a full tank of gas will last a little over a week, and thats boating every single day.
bass boats are feather weights compared to that, they have 200hp motors, they get to a spot quickly, and then shut down and switch to their ELECTRIC trolling motor. the big motor is used only for travelling from spot to spot, as the saying goes, time is money