Some misinformation here as usual. Yamaha probably has the most comprehensive Performance Bulletins for their engines. Pick a technology (2 or 4 stroke, pick your boat, pick your manufacturer and if they tested the exact boat you are interested in there will be test data for it. Fuel economy numbers at cruise is are valid ONLY for your boat, your water conditions, and what you consider cruise. The only number that is relatively firm in all of this is wide open throttle and as the performance bulletins point out, it doesn't matter what technology you pick, WOT fuel consumption is going to be the same or very nearly the same. Again, look at the data -- in most cases a boat is not even on plane with 4-stroke power until something close to 3000 RPM. Two strokes will usually better that. Therefore, when comparing 2-stroke & 4-stroke fuel consumption, use speed as the measure, not RPM and you will find the numbers come much closer than you've been told or thought. Why? Because a 2-stroke is generally pushing a boat faster at any given rpm than the 4-stroke. Those numbers are much closer with the very high HP engines but fuel economy at WOT is still 10% of rated HP across the board assuming all else is equal (that being engine HP, boat type size and rigging, and test conditions.
