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Author Topic:   Fuel Consumption
Mojo posted 03-16-2003 07:10 AM ET (US)   Profile for Mojo   Send Email to Mojo  
I have a 24 Outrage, powered by 2 x 150 Johnson (2 stroke). What I would like to know is that if I cruise at a steady 3250 RPM at 25 MPH, what should the the approximate fuel consumption be. Assume the boat is carrying 3 pax,200 Litres of fuel and flat calm seas.
I am getting figures of about 1.5/2.0 litres per KM.
I apologise for all the non metric/metric conversions.
Any advice greatly appreciated
jimh posted 03-16-2003 10:34 AM ET (US)     Profile for jimh  Send Email to jimh     
As an approximation for fuel consumption, you can use a rate of 0.5 lbs per hour per horsepower.

Let's see how this works out. To use this approximation we need only to input one variable: horsepower.

How much horsepower are you using? Assume at cruise you are using about 50-percent, thus in your case about 150 horsepower.

You should burn (and burn is a good verb to describe what is happening to the fuel) about

150 x 0.5 pounds of fuel per hour, or
75 pounds of fuel.

One gallon of gasoline weighs roughly 6-pounds, so this implies a rate of fuel burn of

75 pounds X 1-gallon/6-pounds = 12.5 gallons per hour

If you are travelling at 25-MPH, your fuel consumption rate per mile will be

25 Miles / 12.5 gallons = 2 miles per gallon

This is a quite typical number for a 2-stroke outboard motor keeping a boat on plane at moderate speeds.

Note that the principal assumption this analysis makes is based on a conversion of energy from the fuel (based on its weight) into motive power to move the boat.

We also made an assumption about the amount of horsepower you were using. This is another important assumption. To get more accuracy you could have your engines tested on a dynometer at the crankcase speed you are running. Otherwise, I would suggest that 50-percent of total horsepower (150-HP) is a reasonable assumption.

If your hull form is especially easy to push through the water, you might find you were using slightly less horsepower.

And it is possible that the maker of your engine has discovered some new approach to internal combustion engines that delivers greater horsepower per pound of fuel burn. But I don't think that number will change by more than a few percent. It is not like there will be an order of magnitude difference.

Diesel fuel is often noted as having better efficiency, but it has that only when measured by volume. When compared on a pound-for-pound basis, diesel and gasoline are similar in the amount of energy they contain and can release when burned.

You can use this same approach to calculate the fuel consumption rates of diesel engines. At my workplace we have two large diesel engines and generator sets, capable of supply emergency power to sustain our operation with complete independence from the public utility. These are periodically run with a load to verify they are in good repair. We compared some rough measurements of the fuel consumption with the formula given above and had good correlation.

(Aside: As I was writing this reply I was making these calculations for the first time. I had no idea what number would result, and I was pleasantly surprised to see the final answer was in excellent agreement with the collective field experience that has been reported by many others.)

jimh posted 03-16-2003 11:07 AM ET (US)     Profile for jimh  Send Email to jimh     
A great deal of information was submitted with the initial question.

Boat = 24-Outrage
Engine = twin 150-HP Johnson 2-stroke
Crankcase speed = 3250 RPM
Boat speed = 25 MPH
Load = 3 PAX (passengers?)
Fuel on board = 200 litres of gasoline
Sea state = calm

Observed fuel consumption = 1.5 to 2 litres/kilometer

In my analysis I throw all this out and just use one input, horsepower being used, which I assume is about 150 horsepower.

Let's convert the observed fuel consumption into a more familiar dimension, miles-per-gallon.

1 mile = 1.6 kilometers

1 gallon = 3.78 litres

Thus the reported fuel burn rate of 2-litres-per-kilometer converts to:

1.2 MPG

Apparently more than 150 horsepower is being used to sustain the boat on plane at 25-MPH.

We can quickly work back from fuel burn to horsepower. The fuel economy observed implies the boat is using 250-horsepower.

Perry posted 03-16-2003 02:28 PM ET (US)     Profile for Perry  Send Email to Perry     
Jim, I did the calculations for my application and it came out pretty damn close to actual results. My Honda 90 gets a little better fuel economy than the fromula's predicted outcome but like I said, pretty close.
Sal DiMercurio posted 03-16-2003 07:32 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Jim, highly unlikely for any carbed 2 stroke to do better then 2 mpg, those engines have 12 carbs between them.
I see people posting that they get 3+ mpg with their carbed 2 stroke.
I call that wishful thinking.
It just isn't in the books for that to be true.
1 - 2 mpg is the norm on "carbed" engines.
elaelap posted 03-25-2003 09:47 PM ET (US)     Profile for elaelap  Send Email to elaelap     
I'm burning about one gallon per hour with my Yami 50 hp four-stroke...maybe 1.25 gallons, but I think less. I probably average 10-15 miles per hour in my smirkless 16/17 during the 4 or 5 trips I've kept fairly close records about -- 3 hour (average) trips in fairly lumpy seas.

Does this make any sense, jimh or anybody else? It seems much too good to be true, especially after reviewing this post.


Baseline posted 03-26-2003 02:37 PM ET (US)     Profile for Baseline  Send Email to Baseline     
Its funny how many ways, this topic can be addressed….

Years ago I read this tidbit in an old military manual in my dad’s library, also known as the bathroom. Remember this was a military manual, so it was intended to be simple and relatively conservative. The manual stated that for every 10 horsepower generated by an engine it is expected to consume about 1 gallon of gasoline per hour. This is very simple formula, but it amazing accurate.

Looking at elaelap’s example: The formula the maximum expected fuel consumption for 50 hp motor would be about 5 gallons per hour (50 hp / 10) X 1 gallon / hour. Assuming that it takes 25% of the motors output to push his boat 10 to 15 mph, that would be (50hp X .25) = (12.5 hp / 10) X 1 gallon / hour = 1.25 gallons / hour.

Looking at Mojo’s example with jimh’s assumption of that 50% of a 300 hp is required for cruising. The maximum expected fuel consumption for all 300 hp would be about 30 gallons per hour (300 hp / 10) X 1 gallon / hour. Assuming that it takes 50% of the total output to obtain a comfortable cruising speed, that would be (300hp X .5) = (150 hp / 10) X 1 gallon / hour = 15.0 gallons / hour.

Jimh’s calculation came out to 12.5 gallons per hour. So the difference is 2.5 gallons per hour difference, between the calculations. Again this formula is intended to be simple and relatively conservative. Assuming jimh’s calculation is 100% accurate, my calculation gives you a 17% margin of error or safety depending on you point of view. In other word, you might have fuel left in your tank at the end of the outing, which is not a bad thing.

The other interesting thing about this formula, variables such as number of passenger and the alike are not relevant to the fuel consumption. Running a motor at 50% of its total output will consume the same amount of fuel per hour, regardless of the load in the boat. The real variable is how fast the boat travels at given load in the boat and at a given level of horsepower. Most captains tend to increase the horsepower, as the load or number of people increase in the boat, to maintain a selected speed. To maintain a set speed with a heavier boat, you have to increase the horsepower, therefore you burn more fuel per hour.

jimh posted 03-26-2003 10:56 PM ET (US)     Profile for jimh  Send Email to jimh     

Your formula is really very similar to the one I mention.

If 10-HP burns 1 gallon of gasoline in an hour, this is just another way of saying that 1-HP burns approximately 0.6 pounds of gasoline each hour.

Bigshot posted 03-27-2003 08:48 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
At WOT. This is pretty much true with both conventional, 4 stroke and DFI outboards at WOT. My 225 supposably burned 23gph. The newer DFI's and 4 stroke 225's might burn 20 at WOT but still close. This however does NOT pertain to inboards and I/O's. My Baja does not come anywhere near 40 gph it is actually closer to what my 225 burns.
Baseline posted 03-27-2003 10:28 AM ET (US)     Profile for Baseline  Send Email to Baseline     
Jimh, I absolutely agree with you. I think the Army manual just simplified the equation a little so that the calculations could be done in the field without a calculator.
captbone posted 03-27-2003 06:38 PM ET (US)     Profile for captbone  Send Email to captbone     
Sounds like Bigshot is bragging about this 400 hp motor.
elaelap posted 03-27-2003 09:33 PM ET (US)     Profile for elaelap  Send Email to elaelap     
More good reasons, other than my current sheer cowardice, not to run at WOT, even with my little 50, outside the harbor...

Sorry, Sal, Bigshot, and captbone...I can see you speed merchants kicking the wall or howling with laughter.


Bigshot posted 03-28-2003 11:15 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
No need to brag....I am already a "Bigshot" ;)
captbone posted 03-28-2003 08:41 PM ET (US)     Profile for captbone  Send Email to captbone     
How fast, 72mph?
Bigshot posted 03-31-2003 09:52 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Sal DiMercurio posted 03-31-2003 10:49 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
BS, just wondering, what makes you think your a BS.
You know, BS can mean other meanings also.
Why in the world would you pick that as a title?
Unless your a wannabe, & the only place you think you can is on the net.
Hell, I could say i'm a brain surgeon, but that wont make me one.
The thought that you really believe your some sort of BS is really sad.
Bigshot posted 03-31-2003 12:29 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
If you knew me Sal you would NEVER ask that question, especially here. I do not brag and that was my response to Captbone, not you, it was a joke. Why would I want people to know my full name over a website? People's handle are just that, so do you think "Smirkless" can't smirk? Do you think jack is really a "Whalernut"? Do you think "Hooter" is an owl or just a boob? How about Swellmonster, is he really a monster? What difference does my handle make and why do a few have such issues with it? Does it threaten some that I have a "vanity" name? Actually you should have my handle because it would suit you better than me. I would trade but I don't think I want to be known as Sal DeMercurio.
jimh posted 03-31-2003 01:48 PM ET (US)     Profile for jimh  Send Email to jimh     
Apologies to Shakespeare:

'Tis but thy name that is my enemy;
Thou art thyself, though not a Bigshot.
What's Bigshot? it is nor hand, nor foot,
Nor arm, nor face, nor any other part
Belonging to a man. O, be some other name!
What's in a name? that which we call a rose
By any other name would smell as sweet;
So Bigshot would, were he not Bigshot call'd,
Retain that dear perfection which he owes
Without that title. Bigshot, doff thy name,
And for that name which is no part of thee
Take all myself.

Bigshot posted 03-31-2003 02:28 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Exactlly.....I think ;)
Sal DiMercurio posted 03-31-2003 02:34 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Huh ?.........whatever you say Jim.
newt posted 03-31-2003 03:35 PM ET (US)     Profile for newt  Send Email to newt     
Actually, I think Hooter is an Owl, and he says he has 4 boobs....:)

Luckily, owls don't eat reptiles or you certainly wouldn't see me everytime it rains.

whalernut posted 03-31-2003 05:59 PM ET (US)     Profile for whalernut  Send Email to whalernut     
Actually Biggie, I am a little Nutty over Classic Whaler`s :) If I could buy them all I think I would, but as it stands I am a poor Classic Whaler owner, but still Nutty of them!! So my name fits me very well :) Jack.
Bigshot posted 04-01-2003 10:01 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Yeah but there is no such thing as a Whalernut. There are Pinenuts, Walnuts, Cornnuts, but no Whalernuts :)
Chap posted 04-01-2003 10:28 AM ET (US)     Profile for Chap  Send Email to Chap     
I don't know BS......, whales are mammals.
Bigshot posted 04-01-2003 10:55 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Good one Chap....not sure if they have nuts though, nor do I really want to know ;)

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