Re: Gas usage??
Fuel economy on 2 stroke outboards at wide open throttle is not significantly different than it is with a carbed four stroke, and only very little different in the injected four strokes. The DFI engines get most of their economy in the midrange. That said, one need only go to Yamaha's web site and look at performance tests where you can compare two and four stroke models for speed and fuel consumption. To answer the posters original question, if you did as I suggested, you will see the 10% of HP rule applies in nearly every case but does vary somewhat for lightweight but high power boats, heavy low powered boats, heavy high power and light, low power. Here is just one example (90 HP Yamaha 2S on an 18 foot bay boat).
Note that all across the rpm range, fuel economy (MPG) varies less than 0.6 MPG at the three sweet spots. In this example, best MPG is achieved at 5000 RPM and an extra 6 MPH only costs you 0.6 MPG. So, if one juggles the numbers, at 5000 RPM (36 mph) you would need to run another 10 minutes to go 42.5 miles. So my question is, unless you just want to poke along or even cruise fairly fast, but less than WOT, is that 0.6 MPG really a significant savings? I don't think so. To make matters worse, you could cruise at 3500 RPM and again, get 5.2 MPG but you are only moving at 23.4 MPH. So at that rate you are burning 4.5 gallons of fuel per hour. Sounds economical right? Wrong? If you had to go 42.5 miles, you would be better off running wide open as the trip would only take 9.1 gallons versus 9.0 gallons at 3500. Remember, you are only going 23 MPH so it takes twice as long, hence 4.5 x 2 = 9 gph. With todays gas prices at $3.00+/gallon the savings is inconsequential. These are Yamaha's figures, not mine. By the way -- I do run a fuel flow monitor on a 75 Merc 2S and it burns exactly 7.5 GPH at wide open throttle, with two aboard plus lots of gear on a 17 footer.
