Fuel Economy: 2- vs 4-stroke

[onoahimahi]

Hi - I've been unable to make sense of the claims of various owners and manufactures and decided to do some of my own research and analysis. I went to the manufacturers web sites and downloaded data from similar size/weight center-console boats so I could examine them together on the same plot. They are mostly 25-foot boats with dry weights of about 3500 lbs. Since the hulls are not identical, the absolute values may not be representative; however, the relative shapes and locations of the peaks are interesting.

The first plot shows three 2-strokes and an F250. The F250 has the highest MPG at 3000 RPM but over a fairy narrow range. All the 2-stokes peak closer to 4000 RPM at which two out of three show higher MPG then the four-stroke. At WOT, they all converge to the same value.

The second plot shows MPG vs speed for the E-TEC and the F250. The F250's sweet spot is at 24 MPH. The E-TEC has a broader sweet-spot from 27-32 MPH. There is a cross-over point at 30 MPH above which the 2-stoke is using less fuel.

The most astonishing difference between the E-TEC and the Yamaha F250, however, is the low-speed performance. The E-TEC slow-speed trolling performance is an incredible 11 MPG. I probably wouldn't believe that if I wasn't getting 8.3 MPH tolling with my Sailfish on a single engine and 6.7 MPG when using both engines. Since most outboards spend most of their lives running at 1500 RPM or less, this is an important consideration.

The boats:
http://www.evinrude.com/Content/Pdf/neu ... /PE871.pdf
http://www.yamahaoutboards.com/sites/de ... 13_OCC.pdf
http://www.mercurymarine.com/engines/en ... 9106724709
http://www.yamahaoutboards.com/sites/de ... 00txrc.pdf

Cheers,
-Scott

 

[family affair]

Interesting plots.
There are so many variables that impact fuel efficiency. I find it impressive that 2 drastically different IC engine types can perform so similarly. I'm curious to see if direct injection has as much impact on 4 stroke boat engine efficiency as it has cars. If it does, I wonder if brp will have something to match it? Time will tell.

 

[onoahimahi]

Here are some comparisons of different engines on the same hulls:

1) Dusky 252 with an E-tec and Suzuki 4-stroke

http://www.dusky.com/static/sitefile...Open_SZ300.pdf

http://www.dusky.com/static/sitefile.../252EV3002.pdf


2) Contender 31open

http://www.evinrude.com/Content/Pdf/...orts/PE398.pdf

http://www.yamahaoutboards.com/sites...f250txrd_0.pdf


3) everglades 243, the Yamaha is a 300hp compares to the Envirude 250.
http://www.evinrude.com/Content/Pdf/...orts/PE393.pdf

http://evergladesboats.com/boats.cfm...tails/id/23180

Okay - the first plot the Dusky 252 set up with a 300HP E-TEC or Suzuki 4-Stroke. E-TEC excels at the low end and catches up at about 35-53MPH. The Suzuki is most efficient at about 24 MPH.

The second plot is the Contender 31 Open equipped with twin 250s. This shows pretty much the same story with the E-TEC catching up at 40 MPH. The Yamaha F250s are most efficient about 30 MPH.

The third plot is the Everglades 243. There is a mismatch here with a F300 being compared to a E-TEC 250 but the results are still interesting. The economy story is still the same with E-TEC excelling at the low end and catching up at 35 MPH.

This example is the only place where I have seen Yamaha performance numbers for RPMs less than 1000. Numbers for 600 RPM are given and the MPG actually drops by more than 20% from 1000 RPM to 4 MPG. For the E-TEC, that number is 15.6 MPG.

Props were similar for these three examples but the gear ratio for the 4-strokes varied between 1.75 and 2. The E-TECs use a 1.85 gear ratio.

Hopefully this helps some to debunk the myth that still seems to persist about 4-strokes being hands down super efficient compared to today's clean 2-strokes. The story is not so cut and dry and will depend on how the boat is used.

Cheers,
-Scott