# fuel consumption gas vs diesel



## jklewissf (Nov 21, 2002)

A couple of weeks ago there was a message string going about gas versus diesel engines and I was challenged when I made the comment that you get more hours of operation out of a gallon of diesel.

I just stumbled across the following on a web site called boatsafe.com.

How much do gasoline and diesel engines consume?

Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.

Note: An engine at cruising speed usually uses only about two-thirds of its maximum available horsepower. Most marine engines are designed to run continuously at between 60 and 75 percent of maximum speed. Diesels tend to be more toward the top of the range.

Gasoline four stroke inboard engines need about 1 gallon per hour for every 10 hp used. The number of gallons consumed per hour can be estimated by multiplying horsepower used by 0.100. (see note above)


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## roline (Apr 7, 2000)

*fuel consumption*

I've always used 18 HP per hr per gallon for diesel and 14.5 for unleaded gas. This assumes internal combustion engines, 4 stroke, normally aspirated. I did some research on small engine fuel consumption and it goes like this:

Diesel17-20 hp-hr/gal

Natural gas9-11 hp-hr/ccf

LP (propane)9-11 hp-hr/gal

Gasoline10-14 hp-hr/gal
The size of the engine, operating temperature, RPM, and Load all have an effect on the efficiency of energy conversion.


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## wwilson (Jul 7, 2000)

jklewissf said:


> Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.


I am not surprised that diesel has a higher power density than gasoline, but have empirical reason to question the formula.

I have no reason to think there is anything at all special about my 55-Hp diesel that pushes 10-tons of fiberglass and lead around. It has routinely averaged 1 gallon per hour running at 2400 RPM of its design WOT of 3000 RPM.

Do I misread the formula?

respectfully,
Wayne


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## JohnRPollard (Mar 26, 2007)

An old thread, but what the heck...

Over the course of 8 seasons, our current 3 cylinder 27hp diesel has consistently burned about 1/3 gph during normal sailboat operations. We push it hard (2700-3000 rpm whenever we're motoring, except when docking and during warm-up/cool down).

Our previous 2 cylinder 18hp diesel burned about 1/4 gph during normal sailboat operations.

So the figures cited in the initial post do not sync with our real world experience. I know my brother's gasoline A4 has a burn rate about 2-3 times our engine. It's putting out a bit more hp (30, I think), but it's pushing a smaller boat too.


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## Maine Sail (Jan 6, 2003)

*Why not..*

Old thread but why not.. My 44hp Westerbeke consistently burns .72-.74 GPH at cruise speed which is 80% of WOT and I measure it at every fill up. My old A4 used to burn well over a gallon per hour pushing a boat 9,000 pounds lighter..

I can't imagine anyone questioning diesels ability to sip fuel in comparison to gas engines guzzling it.


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## tommays (Sep 9, 2008)

The Beta 2 cylinder gets 14 MPG pushing the C&C 35 at 6 knots 

The 4 Hp almost NEW 2 cycle that pushes my J24 6 knots is not even close


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## roline (Apr 7, 2000)

I found an efficiency graph to add to the subject.


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## dieselboy (Aug 29, 2009)

There should be 4 lines for a Diesel.

PC or Non direst injected Diesel Engines.. PC stands for a pre cup injected 
Di or direct injected
NA or naturally aspirated Diesel Engines
and finally..
DI / turbo.. 

The most efficient and powerful is the Direct injected/turbocharged engine
you can also add a sub line for inter-cooled.. 

trying to calculate the range of a Diesel with out this information is use less.
A Pc is the least effective at turning heat in to usable power. 
the DI/T-I is the best at varying load and the most efficient at fuel-hours of operation. Its in the 90% range while a pc is more like 50%.
So when i see the term Diesel engine i just cringe.. 

Sorry, i will get off my sop box now and go back to my little corner of the world.


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## Stillraining (Jan 11, 2008)

According to the calculations then..my 64 hp Perkins should be burning 3.56 gph...It is not as far as I can tell ...but I will have to have more opportunity to check it out over time to be sure ..so far I have only ran it about 2 hours...I cant ignore this feeling in my gut telling me it will come in closer to under 1/2 of that..I will be quite surprised if it does not...as using various other engine consumptions ratios I have around here to compare it with says that calculation is all wet.


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## christyleigh (Dec 17, 2001)

dieselboy said:


> the DI/T-I is the best at varying load and the most efficient at fuel-hours of operation. Its in the 90% range


Gee... you just made me feel good about my 75hp yanmar


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## jrd22 (Nov 14, 2000)

As others have mentioned I don't think the formula holds true in the real world. Our 62hp Volvo (non turbo) has averaged 1.0 gph consistently over the years (1988 model). At 6.5 knots this equates to - ta dah! -6.5 mpg. A guy in our marina just had a new 32' aluminum catamaran power boat built, he burns app. 25 gph (two 370hp Volvo diesels, turbo charged+intercooled+electronic) and cruises at 30 knots so he is getting 1.2 mpg (and spending $75/hour),ouch!


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## bubb2 (Nov 9, 2002)

There is 112,000 btu's in a gallon of gas. There is 130,000 btu's in a gallon of diesel. Given the same efficiently of engines the diesel would use less fuel to do the same work. However, diesel engines are also inherently more efficient than a gas engine as they burn more of the available fuel and less goes out of the exhaust.


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## negrini (Apr 2, 2008)

Guys, the fuel burn per hour seems very rasonable, considering the btu's diesel provide and also the average efficiency on engines nowaday. But seems mostly of you are taking your engine MAX RATED HP in consideration and not the real HP you're using. In other words, my boat has a 54HP Yanmar, and when cruising in calm waters at 6knots, I use barely 1600 rpm, this mean roughly 27HP out of my engine. At this HP, it burns almost what the previous table say. So, if you're measuring you consumption, please note the rpm and go to your engine graph to show you how much HP it puts at such rpm.


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## TSOJOURNER (Dec 16, 1999)

Stillraining said:


> According to the calculations then..my 64 hp Perkins should be burning 3.56 gph...It is not as far as I can tell ...but I will have to have more opportunity to check it out over time to be sure ..so far I have only ran it about 2 hours...I cant ignore this feeling in my gut telling me it will come in closer to under 1/2 of that..I will be quite surprised if it does not...as using various other engine consumptions ratios I have around here to compare it with says that calculation is all wet.


If your 64 HP Perkins was actually generating 64 HP I expect it would eat up about 3 GPH. You'll notice that during rough weather in a head wind your fuel consumption will increase even though you're running the same RPM as you always do.

In calm weather with no wind it just doesn't take all that much muscle to move a sailboat around.


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## Stillraining (Jan 11, 2008)

jrd22 said:


> As others have mentioned I don't think the formula holds true in the real world. Our 62hp Volvo (non turbo) has averaged 1.0 gph consistently over the years (1988 model). At 6.5 knots this equates to - ta dah! -6.5 mpg. A guy in our marina just had a new 32' aluminum catamaran power boat built, he burns app. 25 gph (two 370hp Volvo diesels, turbo charged+intercooled+electronic) and cruises at 30 knots so he is getting 1.2 mpg (and spending $75/hour),ouch!


Thats great to hear John...Im counting on mine being no more then 1.5 and I will be estacic if it is as low as yours.....PS: Thats better then my Dump truck gets..I get about 4.5 MPG....but thats 444 hp...ya baby!!

Dully noted Selkirk...I suppose you are right about that..I never run my Heavy Equipment more then 75% throttle and most of the time less then that...per the calculations my Dump Truck would only get 2.5 mpg


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## motion300 (Feb 12, 2009)

My 120 hp lehman burns 2.3gph no matter the weather as long as I keep it at 1650rpm


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## JimsCAL (May 23, 2007)

One thing to keep in mind: the power doesn't vary linearly with speed (and RPM), it varies with about the cube! So at 3 knots, you need about 1/8 the power you do at 6 knots. Reduce the speed by only 20% and the power is only about half of what it was. That explains the differences between the calculated fuel consumptiong based on rated engine power and actual consumption at cruise speeds.


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## negrini (Apr 2, 2008)

JimsCAL, very good point to remember. Also, many new diesels does provide an average chart for rpm/HP (like you say, clearly not linear), so you can beter estimate fuel consumption if you balance against speed.


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## jrd22 (Nov 14, 2000)

Negrini- I realize that I am not using all 62 hp when at 2500 rpm on an engine that will do 3500, but do the math, it still does not equate. Even if I were only using 40 hp to push Laurie Anne (40' - 24,000 disp.) by that formula I would be burning 2.2 gph which is over twice what in fact it uses. By this formula I should only be using 18 hp at cruise speed since I only use 1 gph. In fact, using that formula it is wrong for any of the scores of diesel engines I have owned and have paid for the fuel bills on, so I know how much they use. My current crab boat, for just one more example, has 200 hp, 190 at the prop, and I run it at 3400 rpm. WOT is 3800 and I average betweeen 6 -7 gph at that speed(I log every gallon that goes into it). According to this formula it should be burning close to 9 gph. The Volvo fuel consumption graph is close to my actual burn rate, a little higher. All I am saying is that this does not seem to be a reliable formula to use to estimate anticipated fuel consumption, and therefore required tankage.


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## negrini (Apr 2, 2008)

... gallons, littlers, miles, kilometers ..... I just messed up as I don't think imperial. John, redoing the math to my engine, you're correct. Shame on me.


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## bubblerings (May 10, 2013)

JohnRPollard said:


> An old thread, but what the heck...
> 
> Over the course of 8 seasons, our current 3 cylinder 27hp diesel has consistently burned about 1/3 gph during normal sailboat operations. We push it hard (2700-3000 rpm whenever we're motoring, except when docking and during warm-up/cool down).
> 
> ...


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## miatapaul (Dec 15, 2006)

bubblerings said:


> JohnRPollard said:
> 
> 
> > An old thread, but what the heck...
> ...


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## jameswilson29 (Aug 15, 2009)

bubblerings said:


> Hello, new to your forum...
> By "normal sailing operations", did you mean your motor is running to assist your sails, or running alone, under full load???
> Is your diesel motor really that efficient?
> 
> ...


Yes, diesels are that efficient. My 3 cylinder 24 h.p. burns about half a gallon per hour running alone under full load:


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## SimonV (Jul 6, 2006)

My Perkins 4.108 should burn about 2.5 l/h @2400 revs giving 6.5 knots. When trip planning, I use 3 l/h and a speed of 5 knots, and a max running time of 100 hrs. As I carry 420 litres, my workings should give me a decent safety net.


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## copacabana (Oct 1, 2007)

My Yanmar 3GM30F burns 2 liters/hour at 2500RPM, pushing the boat at 5.5 to 6.5 knots, depending on sea state. Does that jibe with other owners of 3GM30F engines?


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## bubblerings (May 10, 2013)

Thank you, everyone!
I will have to seriously consider a small diesel, now... I had no idea!!!
And, yes I will eventually be sailing, one day.
Up until now, my life revolved around being IN the water... freediving.
I do love ON the ocean, as well... maybe I just won't have a car!
I need a boat more than wheels.


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