Some time ago I posted fuel economy and performance data for the 75 HP class of outboards on 17 foot aluminum side console style walleye fishing boats. The subject motors were my personal 75 HP Merc carbed two-stroke, a Yammy 70 two stroke, a Yammy 75 4-stroke, and a 75 E-tec. All boats were the same except for the 70 Yammy two-stroke which was the same style boat, just a different brand since that engine was not tested on the same boat. All data for the Yammy and E-tec came from the manufactures web site while mine came from the fuel flow meter on my personal boat. The long and short of this is that all motors at wide open throttle burned the same amount of fuel or were so very close to each other it wasn't worth discussion. However, when looking at fuel economy numbers at ANY throttle setting one must also see how fast the boat is traveling at that throttle setting. If you compare economy at the same rpm across the board the 4-stroke wins every time. Fact is, 4-strokes are generally geared with a higher ratio (lower gear) than a 2-stroke so boat speed is slower at the same rpm. This is gear ratio/prop selection issue is made necessary because four strokes tend to suffer during hole shot. This is also shown by the charts in that the two strokes are on plane at lower rpm than four strokes. The really interesting point of that study showed that the 70 two stroke yammy actually got better fuel economy than the four stroke -- although partially because of the 5 HP difference. As for top speed, the two strokes were all faster by several MPH than the four strokes. The only point where four strokes had a significant advantage was at displacement speed. Once they began to struggle to get on plane the playing field leveled. Again, compare fuel economy at speed, not rpm because the two-stroke will generally be pushing the boat faster at that rpm so distance covered, and hence MPH and GPH will be more comparable.
