Fuel Consumption Curves?

**Mike Keers** · 05-09-2006, 12:21 PM

Hey gang,
My Yawnmore shop manual gives performance curves, including fuel consumption for the GM/HM series diesels. The fuel consumption curves are puzzling to me; data is given in two ways, "g/kW.h" and "g/HP.h".

(Actually, the period between kW or HP and the 'h' is in the middle of the letters, not low like a period, so possibly a 'times' thing.)

I know the kW stands for kilowatts and the HP for horsepower, but can anyone translate this into something non-engineers can understand? How does one arrive at a simple gallons per hour? We're talking graph numbers of around 200-280 at cruising speed, and I KNOW that ain't 'gph'!

Thanx! mike

**paladin** · 05-09-2006, 12:29 PM

which engine.....what is the boat.....do you know the prop/tranny info?.......at what speed does the boat travel...or what is the most efficient speed/rpm etc....

**Gary E** · 05-09-2006, 12:57 PM

Diesel engines use pretty close to 1 gal per hour when producing 20 HP....

Lets say you have a 200 HP engine...
200 divided by 20 will be 10 gal per hour...

Lets say youy have a 20 HP engine....
20 / 20 = 1 gal per hour

Spose you have a 20 HP eigine but only use 5 HP because you dont "push it"
5 /20 = 0.25 gal/ hr or aprox 1 quart/hr

Reduction gears help because the allow a larger prop which is more efficient, but the engine still burns the above fuel.

As for.... "g/kW.h" and "g/HP.h".

the first could be grams per KW hr

the second could be grams per Horsepower hour...

then again the little "g" might be something else...

how many g's in a gallon? ....I dunno

**RonW** · 05-09-2006, 01:08 PM

Your math got fuzzy gary,- --Spose you have a 20 HP eigine but only use 5 HP because you dont "push it"
5 /20 = 2.5 gal/ hr---

It should be 1/4 of a gallon or only a quart.....
I thought for the most part diesels will average about 16 horses per gallon, and the real little ones, even a little better...

---and your modern 4 stroke outboards, about 12 horses per gallon....

---go to westerburke homepage, they have charts showing gallons and liters per rpm levels and what horsepower you are producing at these rpm's.

**Lew Barrett** · 05-09-2006, 01:09 PM

The curves I've found most useful are called "Propellor Demand Curves." I've posted them at the helm and they read out (in my case) as gallons per hour at so many engine RPM.
They indicate both the H/P output (at the shaft) and what the fuel requirements are, assuming "proper" loading of the engine; that you are correctly wheeled and geared. Should read out in liters or gallons per hour. These were provided with my Cats, Perkins also gives them as does Deere, but I don't know about how Yanmar specifies it.

Lew

**Gary E** · 05-09-2006, 01:14 PM

Ron...
your right I missed a decimal pt....
5/20 is .25 gal per hr

I'll fix it now
Thanks

Ron, the 16 hp/gal is close to 20/1
I think it depends a lot on the specific engine...

**Lew Barrett** · 05-09-2006, 01:14 PM

As I get it, all predited curves and the actual consumption is dependant on loading. If you are underwheeled or overwheeled, you can't read HP out directly based on turns so all the other assumptions become thrown off. An important consideration. I have also heard it as 1 Gal/Hr per 10 HP ratio as a rule of thumb; some disagreement here? Maybe not important as efficiency of new engines is improved and more latent energy of the fuels is turned to work.
Lew

**Gary E** · 05-09-2006, 01:26 PM

Lew,

You have twin Cat's ??

You NEED this...
http://www.pyacht.net/cgi-local/Soft....htm?E+scstore
That will pay for it self in less than a year..

"FloScan Fuel Computers show exact fuel use
FloScan Fuel Computers show exactly how much fuel you've used, and how fast you're using it. Rather than trying to measure what's sloshing around in your tanks, FloScan's electronic sensors determine fuel flow right at your engine. And the computerized display indicates precise consumption totals. There's no guesswork. Unlike fluctuating fuel tank float gauges, you receive immediate, accurate data. No matter how rough the seas, no matter how many tanks or engines in your boat."

**RonW** · 05-09-2006, 01:27 PM

The westerburke homepage has a lot of good and easy to read charts, on their plain diesels and also their diesel generators. You can see some difference in fuel consumption on some of the generators if you study hard, as to the load they put on them. But really surprisingly not all that much. It really seems to be more of a rpm deal. Very roughly speaking, the transmission and prop and so forth will have a big effect on the boat speed and so forth, but the rpm;s at which the engine is run seems more critiical, and there you need to match engine, transmission and prop to get the performance and fuel economy..I think the gal to 16 horse is a pretty good rough figure over all..And I am sure this can go to hell with too small of a diesel being run at too high of rpm's so as to be able to produce horsepower needed...

---bigger engine loping along...there also is some fuel variance between the low rpm's such as a 1800 rpm diesel and the higher rpm diesels like the 3600 and so forth.....

**mmd** · 05-09-2006, 01:52 PM

g/kW.h

where:
g = grams
kW = kilowatts
h = hour

Read: "grams per kilowatt-hour"

Definition: A measure of how many grams of fuel is burned per kilowatt per hour.

Example: The Yanmar 3YM30, turning at 2500 rpm delivers 18 KW at the crank. Fuel consumption is 128 g/kW.h; therefore, fuel consumption is 128 grams multiplied by 18 kilowatts = 2304 grams per hour. A gram of fresh water has a volume of 1 cubic centimetre (cc), and 1000cc = 1 litre, so the engine burns 2.304 litres of fuel per hour. But wait! Diesel fuel is lighter than water, so we have to adjust for specific gravity. Diesel fuel has a specific gravity of about 0.85, so the fuel consumption is 2.304 divided by 0.85 = 2.71 litres per hour. Since there are 3.785 litres per US gallon and 1 kilowatt is equal to 1.341 horsepower, then 2.71 l/hr @ 18 kW is equal to (2.71 / 3.785) US gallons per hour at (18 x 1.341) horsepower, or 0.72 gph @ 24 hp.

Clear? <grin>

**Mike Keers** · 05-09-2006, 01:53 PM

OK, Thanx for the comebacks so far.

Paladin, I'm not asking specifically about a particluar engine or application, but rather what the letters in the formulas indicate, or how to translate those particluar formulas into something we Merkins understand--GPH. I've seen the 'Specific Volumetric Fuel Efficiency' equations, and they are either in litres or gallons (per HP*hrs). This formula states something along the lines of 12 to 21 HP*h/gal (quite a range). So it's the format as given in my Yani manual that I'm curious about, since it's specific to my engine.

Gary, I suspected the 'g' might be grams, but how that works out or in to the formula, who knows (that's why I'm asking!)? I've never seen fuel consumption expressed this way, is the real issue here.

Lew, I've already done some calculations using one of these generic formulae or another (memory fails me), and estimated about 3/4 GPH for my 3HM35 (rightly or wrongly). That was part of my prop selection process, using all the usual suspects including the fuel graph (you can still use the graph without knowing what the numbers mean, it's got curvy lines!), engine output / propeller power curve, engine torque, etc. I balanced all that out to arrive at the optimum fuel efficiency versus both happy engine and boat speeds. The boat is far from completed, so actual sea trials are a ways off.

But that ain't the issue here. In talking with an engineer friend yesterday, he quoted some other formula for engines (weight of fuel versus HP), and his estimates was considerably higher. So I went back to the manual, and wa-la, here we are back at the beginning again.

Ron, let's see, all my graphing ended up showing the engine would be pushing 25 HP at my happy confluence, so that would approximate about 1.5 GPH. This is similar to what my ME friend guestimated. Actually, he said 2 GPH at first, but he was assuming the engine at maximum output, not something one would normally run at.

Edited to add--several more of you have written while I was typing this....let me digest.

**Mike Keers** · 05-09-2006, 02:00 PM

Ahhhh!
MMD, you've cleared the muddy waters. Thx! I'll work out the calculations and report back, just to close the loop here.

**Gary E** · 05-09-2006, 02:03 PM

Clear? <grin>

Yes.... I think we are all in agreement....just that I never heard of anyone asking the pump jocky to "put in 20Killagrams" or whatever of fuel... but what do I know...Fill it up....it's cheaper TODAY...

**RonW** · 05-09-2006, 02:26 PM

By the way diesel today is $2.79.....someone else can figure out how many pennies to gram or vice versa....lieters, grams, I am with gary, just fill it up

Mike if you come up with 25 horses at 1.5 gallons..that is pretty close to one gallon per 16 horse...

**Gary E** · 05-09-2006, 02:38 PM

Remember when it was cheap??? like 15 cents/gal ??

On TV the other day they showed a co-op in MinneSoota... that let you buy fuel for the future... Some people bought 100's of gals when the price was under a buk... and he is still working on that.

**Mike Keers** · 05-09-2006, 03:25 PM

Diesel here in SE Aridzona is more than regular unleaded, and we're running around $2.90-$3 a gallon for gas. In a previous life I ran a service station and I recall selling gas for $.32 for regular and $.36 for 'hi-test'. $3 filled a car, not a milk jug. Sigh. Progress.

OK, I punched in MMD's figgers for my 3HM35, and it comes out to about 1.09 GPH at my optimum numbers. Which BTW, are remarkably similar to MMD's, 2500 RPM and about 18KW. My engine just uses more fuel per hour to get there. This is a bit of a disappointment to my anticipated range if true, but certainly better than the 1.5 GPH guestimate of my ME friend. I guess, having previously owned a sailboat with a gasoline-fueled Atomic Bomb (Atomic 4 for the unfamiliar) that used a solid 1 GPH at least, I'm no worse off, as I'll be moving a knot or more faster.

**sv Lorelei** · 05-09-2006, 04:57 PM

Hey Mike! Great to see you here, though you do proceed me by five or so years. How are things progressing on the stinkpot? We've still got Lorelei though we're building a 16 foot Islay Skiff right now and are about halfway through. Ever hear from JM these days?

Tim Foley

**Figment** · 05-09-2006, 06:02 PM

If you back off that last knot of speed, might you recapture a considerable amount of range?

**Lew Barrett** · 05-09-2006, 06:52 PM

Gary,
Yeah, twin Cats, a bit hard to admit on a sailboat message board eh? Two 3056 with ZF gears; new in 2004. Not too bad below 8 knots. Above hull speed it gets uglier. Actually, for a 50' 22 ton power boat Rita is kindly on fuel as long as you don't go over 9 knots. I'd like a flowscan setup (for a week or two...that could be handy), but once you have your curves, well.....you're cool

Anyway, I don't reckon they'd fit in well with the depression era decor. I believe Cat's propellor demand curves work out pretty accurately for me based on my observations. We can go all summer, half way to Alaska and back, on a couple hundred gallons as long as we're not in a hurry. The last fifty or so miles can be tense though

Mike: Ah...selecting the wheels...is it art, is it science, is it something in between? I had an engineer from Cat, a marine architect friend, and " the guy at the prop shop" all involved; came up with three different numbers for pitch. I split the difference. Everyone thought that was a good first pass, and I could pitch up or down to get any of the calculations covered from there. It worked out fine.

Added: I'm lying like a car salesman about the "couple hundred gallons"...probably closer to 250 gallons for the trip up and back to Desolation from here after all the side trips, speed runs and such. Still, nuthin' compared to those Bayliners.....
Lew

Originally Posted by Gary E

Lew,

You have twin Cat's ??

[/FONT]

**Gary E** · 05-09-2006, 08:58 PM

Lew,
Not familiar with that Cat number... straight 6 or V8 ?
3126 is a straight 6..3208 is a V8
You might still save enough to justify the expense of those flowscans. But as you know plenty of people got along without them for a long long time.

Ok...so you could do what we did when we went to the canyon which was way out and wanted to stay an overnight or two before comming home.
2 55 gal drums on deck, when we had room for 55 in the tanks, we pumped it in then tossed the drum overboard. Yeah yeah I know, but that was 40 yrs ago...today I dont think we'd admit it, besides todays boats have bigger tanks.

**Peter Malcolm Jardine** · 05-09-2006, 09:07 PM

I'd like to think I get a mile to the gallon

**Lew Barrett** · 05-09-2006, 09:25 PM

This is a relatively new thing for them; the first year was 2003 or 2004. I really was hot for Deeres but they didn't have anything in a six in my power range. Cat also make a 4 cylinder version (the 3054) that's essentially identical but for two less jugs. Mine's a 6 liter straight 6, naturally aspirated, 125HP. They make a turbo version, about 210 HP. Mine are simple, straightforward but big for their rating (1300 pounds) and efficient though I'm sure a common rail system would be even more so, and probably burn cleaner; nothing like that available just a couple of years ago. My choice was dictated, as I sort of imply, by the desire to have the smoothest possible results; that dictated a six rather than a four in my desired power range and their unit fit. I've been happy with them, no trouble. By the way, I understand they quit making the 3208. Sure do see a lot of those, though not as many as I do of 671s.
We carry three hundred gallons, same as she did in 1938; it's going to be more painful at Ballard Oil this year for me, but it could be worse I guess. That 300 will take us about 900-1000 miles if we're prudent, more if I'm willing to travel at the speed of a sailboat and bother to check the tides and currents

Being a rude power boater, how often is that going to be the case

Lew

Originally Posted by Gary E

Lew,
Not familiar with that Cat number... straight 6 or V8 ?
3126 is a straight 6..3208 is a V8
You might still save enough to justify the expense of those flowscans. But as you know plenty of people got along without them for a long long time.

Ok...so you could do what we did when we went to the canyon which was way out and wanted to stay an overnight or two before comming home.
2 55 gal drums on deck, when we had room for 55 in the tanks, we pumped it in then tossed the drum overboard. Yeah yeah I know, but that was 40 yrs ago...today I dont think we'd admit it, besides todays boats have bigger tanks.

**Mike Keers** · 05-09-2006, 09:37 PM

Tim check you PM. Peter, I think there's a gallon per mile of diesel exhaust in that wake alone! hahaha! My need for speed is more modest, judging by that pic. Moving up from a snailboat, I figgered a steady 5-7 knots in the actual direction I wanted to go beat 3 knots of wandering all over the ocean tryign to get close to where I wanted to go. Lew, I used Dave Gerr's Prop book as well as let Michigan Wheel do their mathmagics, and the results were within an inch. Since I actually did prismatic co-efficients and other esoteric stuff, I went with my calculations, figgering an inch was close enough--since it's all voodoo anyway. Figment--I did all my calculations to balance everything out to get the best economy versus speed, so not sure going slower would be more efficient--the consumption curve actually goes up going slower (or faster). Must be a sweet spot in there around 2500 rpm.

BTW, I just talked on the phone to the friend I bought the engine from (Tim, that would be SG), and he said he averaged about 1/2 to 3/4 GPH at 'modest speeds', in his 31' centerboard sailboat. The engine was overkill in that application, it replaced an A4. He replaced this 3HM35 after only about 135 hourswith a more suitable 2 cylinder Beta Marine, around 18 hp. Everyone is happy. So far; my boat ain't launched yet!

**PeterSibley** · 05-10-2006, 02:43 AM

Originally Posted by mmd

g/kW.h

where:
g = grams
kW = kilowatts
h = hour

Read: "grams per kilowatt-hour"

Definition: A measure of how many grams of fuel is burned per kilowatt per hour.

Example: The Yanmar 3YM30, turning at 2500 rpm delivers 18 KW at the crank. Fuel consumption is 128 g/kW.h; therefore, fuel consumption is 128 grams multiplied by 18 kilowatts = 2304 grams per hour. A gram of fresh water has a volume of 1 cubic centimetre (cc), and 1000cc = 1 litre, so the engine burns 2.304 litres of fuel per hour. But wait! Diesel fuel is lighter than water, so we have to adjust for specific gravity. Diesel fuel has a specific gravity of about 0.85, so the fuel consumption is 2.304 divided by 0.85 = 2.71 litres per hour. Since there are 3.785 litres per US gallon and 1 kilowatt is equal to 1.341 horsepower, then 2.71 l/hr @ 18 kW is equal to (2.71 / 3.785) US gallons per hour at (18 x 1.341) horsepower, or 0.72 gph @ 24 hp.

Clear? <grin>

yup ..clear ...good explanation ,thanks

**PeterSibley** · 05-10-2006, 02:44 AM

Now my Bukh DV 20 running at 1500 rpm ?

**Lew Barrett** · 05-10-2006, 05:15 PM

Originally Posted by Mike Keers

Lew, I used Dave Gerr's Prop book as well as...

Likely to be pronounced "Gair," but I like Gear if taken in context....
Lew

**Gary E** · 05-11-2006, 11:26 AM

Originally Posted by Lew Barrett

This is a relatively new thing for them; the first year was 2003 or 2004. I really was hot for Deeres but they didn't have anything in a six in my power range. Cat also make a 4 cylinder version (the 3054) that's essentially identical but for two less jugs. Mine's a 6 liter straight 6, naturally aspirated, 125HP. They make a turbo version, about 210 HP. Mine are simple, straightforward but big for their rating (1300 pounds) and efficient though I'm sure a common rail system would be even more so, and probably burn cleaner; nothing like that available just a couple of years ago. My choice was dictated, as I sort of imply, by the desire to have the smoothest possible results; that dictated a six rather than a four in my desired power range and their unit fit. I've been happy with them, no trouble. By the way, I understand they quit making the 3208. Sure do see a lot of those, though not as many as I do of 671s.
We carry three hundred gallons, same as she did in 1938; it's going to be more painful at Ballard Oil this year for me, but it could be worse I guess. That 300 will take us about 900-1000 miles if we're prudent, more if I'm willing to travel at the speed of a sailboat and bother to check the tides and currents

Being a rude power boater, how often is that going to be the case

Lew

Lew,
Anytime you can get ~3 mi/gal with a 50 ft built that heavy that is good economy. Have you ever run it wide open? If you have what is the burn rate and boat speed?

**Lew Barrett** · 05-11-2006, 01:31 PM

Gary,
She makes 14 Knots WFO. Completely useless speed, the bow points up, makes a huge bow curl, digs a hole, all that plus rising noise and vibes (cavitation, I think, reduced by careful syncronization). Backing off to 2300RPM makes a big difference and 12 Knots is useable but not economical. I don't have observed data to verify consumption at that speed, since I don't use full turns to travel, but the propellor demand curves indicate a shade over 16 GPH between the two.
Twelve hundred turns gets about 7.5 Knots, and uses 1.1 GPH per engine; you can travel at that speed and not feel too bad, 8 is nice and gets about the same consumption. Consumption doesn't reeally change radically until above 8.4 knots, and then you are over hull speed. My avitar shows the boat at 12, bow curl and all. The Old Man (Monk) knew his game, I think.

Here are the curves for Cat marine engines.

http://www.cat.com/cda/components/fu...5&x=7&id=99026

Added: Also I observe that these newer units seem to do better than the 10Gal/Hr/100HP ratio by some margin. In practice I have found Cat's specs to be pretty reliable for my motors. Also, in case we haven't killed this and hammered the coffin shut, the relationship between kW/H/ fuel consumption (metric spec) and HP is pretty well demonstrated in the Caterpillar spec sheets.

Lew

**RonW** · 05-11-2006, 03:09 PM

You might note that in the link above that lew gives for cat.
In looking at the 100hp. model the fuel burn rate for horsepower produced is-16hp.-1.2g. / 32 hp-2g. / 46hp-2.8g. / 64hp-3.8g. / 100 hp-6g............I will say again, it works out very close, give or take a little to a burn rate of one gallong per hour for every 16 horses produced.....
And again a modern 4 stroke outboard is producing 12 horses for every one gallon per hour...roughly..

---think I missed that you are running dual diesels, ....that changes things....

**Gary E** · 05-11-2006, 03:13 PM

That's a pretty boat... got a bigger picture ....

I know you do

"So backing off to 2300 RPM "
Well, that tells me you probably have close if not the exact right size wheels. Reduction gears? 2:1 ?
And at 125HP each, you cant expect to go like it had twin 450 Hp
I'l look at the charts
Thanks

**Lew Barrett** · 05-11-2006, 06:41 PM

Gary,
Here are a few; I can't access my favorites from this computer; these will have to do for now.
Added: I don't want or need more than 10 Knots, so 125 HP per side is fine. Mostly an 8-9 knot deal; actually a lot of time at 7 or less if I factor in Lake Union time. Gears are 2.4:1 (I think yikes I forgot!

)! Wheels are oversquare at 22X24, that I do remember!

Lew

Ron said:
---think I missed that you are running dual diesels, ....that changes things....

It seemed like a good idea at the time. Now I'm in too far to get out...
Lew

12 Knots

On the hard

[IMG]

[/IMG]

Helm (with demand curves taped to binnacle!)

[IMG]

[/IMG]

**Lew Barrett** · 05-11-2006, 06:56 PM

Apologies to you Mike. I think I've hijacked the thread.
Lew

**Mike Keers** · 05-12-2006, 11:30 AM

Lew,
Hijack away my friend, that's one beeee-yoooo-teee-full-lish boat! Magnificent!

**Lew Barrett** · 05-12-2006, 09:48 PM

Originally Posted by Mike Keers

Lew,
Hijack away my friend, that's one beeee-yoooo-teee-full-lish boat! Magnificent!

OK then Mike! Here are some images that make me happy! (Shamelessly I must point out; they've been seen on these pages before)
Lew

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