Merc 50 4-stroke fuel economy figures

[longshanks]

Posting this, in case anyone is interested.

Using a Quicksilver stainless 11 5/8 by 11" prop, on a 15' heavy-duty aluminum skiff. Tiller Drive. Holeshot is just over 3 seconds. This propeller over-revs slightly at WoT.

Figures obtained using a gps and a Mercury smartcraft guage.

4000 RPM, 18mph, 1.8gph, 10mpg

4500 RPM, 21mph, 2.3gph, 9.1mpg

5000RPM, 24mph, 2.8gph, 8.6mpg

5500RPM, 27mph, 3.4gph, 7.9mpg

6000RPM, 31mph, 4.3gph, 7.2mpg

I was interested to note that this motor is only burning 4.3gph @ the max. RPMs, and I would have expected it to be higher.

 

[Silvertip]

Re: Merc 50 4-stroke fuel economy figures

You might actually "measure" the amount of fuel used to verify the accuracy of the Smartcraft gauge. Fuel economy depends to a large degree, on the hull design, load, prop and water conditions. In general, a 50 HP motor (2-stroke or 4-stroke, carbed or EFI) will burn very close to 5 g/hr. The 10% rule is very alive and well -- all one needs to do is visit either the Alumacraft or Yamaha web site and check their performance reports.

 

[steelespike]

Re: Merc 50 4-stroke fuel economy figures

Interesting;It is generally thought that the 4 stroke will deliver excellent GPH.
And you might do a little better with a little tweak on the prop.
I stumbled on a couple of 50 hp Etec tests While they couldn't match WOT GPH figures they were faster at partial throttle 31.9mph, 4.40 GPH 7.3 MPG
at 5,500 rpm and 31.2 3.94 GPH 7.9 mpg.at 5,000rpm. Boats were A JV17 Carolina Skiff 688 lbs dry and a 165LTD CS Alumacraft 700 lbs dry.
Max speed was 33.8 at 5,715RPM on the Skiff at 5.27GPH,and 34.9 at 5,525RPM on the Alumacraft at 5.23GPH.

 

[Texasmark]

Re: Merc 50 4-stroke fuel economy figures

Other thing you point out here is what has been repeated many times: Best fuel economy is usually just on plane where the boat drag is smallest and engine is running in an economical part of the power band.

Something else. Silver has been harping on this 10% thing at WOT for quite awhile. So, being of an inquisitive mind, I have been following fuel consumption data on numerous types of engines including marine, automotive, and heavy duty diesel. Can't say that I've found much over a 10% variance in economy, regardless of engine type......found that amazing.

But on injected engines, like the diesels, they do rate them in gallons per hour per hp and that means consumed hp, not engine rated hp. The diesel (I know about them) is very fuel efficient as long as you aren't working it. Once you put the load to her, the BTU's in the fuel (hence fuel consumption) get burned up just like any other type engine. Assume gas injected engines are the same.

Mark

 

[Silvertip]

Re: Merc 50 4-stroke fuel economy figures

The key in making fuel consumption comparisons between engine types to ensure those comparisons are made on the same boat as I did with 75 HP engines. At wide open throttle the difference is so small it becomes a non-issue. Steel -- You did make a good observation, and one that few people want to grasp and thats that the two strokes generally push a boat faster at any given RPM than a four stroke so you really need to study fuel economy to make sure you aren't fooling yourself, or being fooled by the manufacturers. Face it, an engine that pushes a boat faster at any given rpm means you travel less time to destination so in the end, even though g/hr may be higher, the total fuel consumed is the same or even less than on what might be termed a "more economical engine". Again, my charts proved that in those tests.