Fuel Economy - Same boat, same speed, different motors at different hp ratings?

[KC8QVO]

If you have a boat with the same load and run at a modest cruising speed in the same water and weather conditions - what is going to get better fuel economy? A smaller HP motor running at higher RPM/engine load, or a higher HP motor running at slower RPM/less engine load? Say, the difference between a 75hp, 90hp, and 115hp?

 

[Frank Acampora]

Once you get onto plane it takes the same amount of horsepower to move the boat at a certain speed. It takes a given amount of fuel to generate a given amount of horsepower. So, unless you are using two engines with radically different cubic inch displacement, fuel consumption should remain relatively close with most engines. Of course, this does not include different engineering designs which would lead to overall better gas mileage. For example, a newer loop charged OMC 115 should get better gas mileage than a old school early Force cross flow 75.You need to compare apples to apples, not apples to oranges.

 

[KC8QVO]

Though, with a higher HP motor you can turn a bigger prop - either larger diameter or higher pitch, or both. Would that not come in to play also? A 115hp motor at 3000rpm is going to push a boat a different speed than a 75hp motor at 3000rpm, no?

 

[Chris1956]

There is a school of thought that says that a larger HP motor turning less RPM will use less fuel than a smaller HP motor turning faster on the same boat, same speed, same load. The difference is that the smaller HP motor is running at or near WOT, thereby incurring a fuel penalty. I have never seen real data on this, however, I do know that cutting any motor back from full throttle, looses a small amount of speed, but elevates the economy significantly.

 

[Faztbullet]

Prop sizes,pitch and gear ratios have to be figured also. So a 115 on boat at 2/3 throttle will get better economy than a 75 on same boat as it will need to run at near WOT to maintain same speed as 115. This is due to gear ratio difference and prop pitch...

 

[steelespike]

I chose a 60 and a 90 Etec motor because you can find them on boats that are essentially the same.
Using an arbitrary 4000 rpm the 90 on a 1040 lb. alum craft uses 4.49 gph at 27.2 mph 6 mpg.
At 4000 the 60 on a 1040 lb. Alumacraft uses 2.29 gph @21 mph, 9.2 mpg.
At 28 mph( close as I could come to the 27.2 of the 90) the 60 is at 3.38 gph and 8.3 mpg.
The 60 and 90 are inline motors 2 cylinder and 3 cylinder.

 

[KC8QVO]

steelspike - where did you get that data? Those are interesting numbers - that is almost a 30% difference in fuel usage. Also, what props?

 

[Silvertip]

The 90 Etec numbers are very close to the numbers I provided in my now rather dated study of 75 HP Etec and other engines on an Alumacraft Navigator 165CS. Only 75 HP engines were tested (carbed Yammy, Etec, Merc 4-stroke and my 75 Merc carbed). My boat had a fuel flow monitor so numbers are accurate. Others came from Manufacturer web site.

 

[steelespike]

My numbers came from the Etec site. They have extensive tests from 25 on up with very complete results , boat, motor and conditions.
Dry weight, load Fuel,prop etc.Boat and motor specs.

 

[KC8QVO]

Thanks for the info. I went searching the reports on the Evinrude site too. I couldn't find the same type boat on the 115, but fudging the numbers a bit with a smaller/lighter boat it looks like at 30mph I could get right around 6mpg with the 75, 90, and 115.

For reference, I am looking at the Princecraft Nanook DLX WS - 16'6", dry weight 1040lbs - smaller/lighter than any of the boats in the below reports, but at least they are a baseline for comparison.

See links below in order 75, 90, 115.

http://www.evinrude.com/Content/Pdf/...orts/PE927.pdf
http://www.evinrude.com/Content/Pdf/...orts/PE926.pdf
http://www.evinrude.com/Content/Pdf/...orts/PE648.pdf

I would be curious how the 90 High Output compares to the 90 in-line and 115 v4. If the theory is the same as between the 75 and 90 the 90 H.O. should get better economy than the v4 115. That may even get better mileage at the target 30mph than the 75 or 90? Hard to say without numbers to see.

 

[Faztbullet]

You need to compare apples to apples as a Etec is a DI motor and will get better economy than a carb,4 stoke or EFI engine...

 

[KC8QVO]

You need to compare apples to apples as a Etec is a DI motor and will get better economy than a carb,4 stoke or EFI engine...

All of the 3 motors I referenced are eTecs - the 75 and 90 are in-line 3 cylinders, the 90 High Output and 115 are v4's. All of the eTec line motors are 2 strokes. Would that not be apples to apples? As opposed to a 75hp eTec and a 90hp Merc 4 stroke?