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Nomad Willy 09-08-2012 12:45 PM

HP per gallon per hour
This thread is in response to Marin's comment about the rule of thumb that it takes 1 gallon of fuel to make 20hp for an hour.

I may have stated the function in the thread title wrong. Please correct that and any of the following that is wrong or stated incorrectly.

I think your friend Dick DOES know what he's talking about but everyone loosely refers to the hp per gallon in an approximate rounded off number for rough "in the ballpark" calculations. But if you and he were bent over his engineering desk w graphs, calculators and deep statistics at hand I'm sure your friend would say the Lehman would produce more like 16hp per gal. And that would probably be achieved at the best rpm for fuel efficiency. And when you look at a graph that shows an engine makes 60hp at a certain rpm that's at WOT and full load. That's way different than putzing around in an old trawler at 25% load. Mark has a very modern engine and he dosn't even make 20 ... maximum. When you look at a sales brochure and it says the John Deere 4045 makes 17.8hp per gallon that's at one engine speed only ... usually about 1800rpm and if so at 1500rpm the number will be different. And at WOT and full load it will be different again. And when you look at a sales brochure and it shows how much power an engine makes it's at WOT at the rpm shown. If it says 60hp at 1600rpm you can't say at 1600rpm cruising along in your boat your engine is making 60hp at 1600rpm. So when you look at a power/rpm chart all the hp numbers are at the rpm specified and at WOT. But 20hp per gallon is a rule of thumb and perhaps not a good one as I think most trawler engines are closer to 15, especially at 25% load.

I'm not sure this is all correct and if it's a bit foggy I'd like to have someone clear it up for me. Rick, Tom, FF ... what do you guys think?

Marin 09-08-2012 03:54 PM

Dick gave me the formula of 1gph per 20 hp as a rough rule of thumb. He said that at an rpm of 1600 rpm in our boat with factory props an FL 120 will develop 60 hp. (based on Northern Lights' GB re-powering data).Thus 3 gph per engine X two for 6 gph total. Which based on our fuel consumption observations over the years seems about right.

djmarchand 09-08-2012 04:45 PM

I have looked for the Lehman and Perkins performance curves and the ones I found were small scale and almost unreadable. And in one case it showed only a fuel consumption curve at wot, which as Eric notes above is very misleading.

So I use the Yanmar engine curves to see the relationship between rpm and specific fuel consumption for a NA engine. The 4JH5E engine produces 53 hp at 3,000 rpm and is suitable for small displacement speed trawlers. The curves for that engine shows power at wot and the synthetically derived (by exponent formula) prop curve. Fuel consumption is shown for the prop power curve.

So here is some data taken from Yanmar's curves:

WOT 3,000 rpm 53 hp 2.9 gph 17.9 hp per gph
Prop 2,500 rpm 30 hp 1.6 gph 18.9 hp per gph
Prop 2,250 rpm 23 hp 1.2 gph 19.2 hp per gph
Prop 2,000 rpm 16 hp 0.9 gph 17.8 hp per gph
Prop 1,750 rpm 10 hp 0.6 gph 16.7 hp per gph

Now these values are at best good to 5% and at the low end maybe only 10% due to the limited scale on the Yanmar curves. But they do tell the story. The WOT specific fuel consumption is a bit worse than lower rpms; the specific fuel consumption peaks at about 2,250 or at 75% of WOT rpm; and the specific fuel consumption drops from there, but not dramatically. I think that this data is representative of medium speed NA diesel engines.

This engine would be suitable for the Willard 30 and would cruise nicely at 2,000 rpm making 16 hp. Overpropping to achieve that 16 hp at 1,750 would save a little fuel, but at most 10%.


Nomad Willy 09-08-2012 09:27 PM

Thanks for posting DJ and for the good solid input.
Yes I did look at the JH Yanmar when I repowered. They have an excellent reputation and are known to have good fuel efficiency. The JH loading at cruise w the W30 is only about 42% (rough numbers) and I felt that was too low. I didn't feel I needed more power than the 36hp Perkins I ran before. So I bought a 40hp (approx) Mitsubishi. It's a 3000rpm engine and I've never gone over 2300rpm unless on a lark and I'm propped to rated rpm. So I felt I'd be overpowered w the JH. In seven years I've never needed more than 30hp and not quite that really. But I do remember those specific fuel consumption numbers and I think the JH scored higher than any other engine I looked at.
I suspect the Perkins I had and the Lehman would produce specific fuel consumption numbers of about 15 to 16 gphph. I had a Sumnercraft 29 w a 120hp Sabre engine. It's the same Ford engine the 120 Lehman is made from and in the book for that engine there was a synthetically derived prop curve. I only remember the 2500rpm WOT consumption ... 5.8gph. That is obviously over 20hp per gal and supports what Marin and his friend says but the curve was in the book and it probably came from Ford. This engine was supposed to have been blueprinted for military use and may not be representative of the Lehman.
But on boat diesel and elsewhere only the most efficient modern NA engines made 18hp per gal so I'm inclined to accept FF,s original statement re the specific fuel burn of the Lehman engine in that it is probably closer to 15 than 20 and Marin's friend just applied the rule of thumb without giving it much thought.
Marin, were you assuming you were loaded to 60hp because you are consuming 3gph? Perhaps you are only loaded to 50hp?

Marin 09-08-2012 10:12 PM

I'm just going on the data compiled by Northern Lights/Lugger that, according to my friend, has the FL120 burning 3gph at 1600 rpm.

FF 09-09-2012 06:09 AM

To KNOW , not guess a "fuel map" or BMEP from the engine MFG is required.

These are more closely guarderd than the T 88 small nuke weapon plans.

A fuel map looks like a series of clouds drawn one inside the other.

In the center is all the RPM / Hp combinations that are that engines highest efficiency.

Really hard to find , tho I'm told pump, compressor and genset OEM can get a factory copy.

psneeld 09-09-2012 07:49 AM

1 Attachment(s)
I'm not sure about some of these engine charts and the way they define hp...they seem to be all over the place.

here's the numbers a guy posted by pictures of his flow scan and gps a couple years back...I think he had a MT 34....

Larry M 09-09-2012 09:27 AM

1 Attachment(s)
This is for a Ford Lehman SP135. The numbers are theoretical and a little optimistic from my experience.

Nomad Willy 09-09-2012 02:37 PM

Active Caption inspired me to look up my specific fuel consumption on the internet and I did. I found the following:

My own engine, a Mitsubishi S4L2, 7 years old but unchanged as far as I know has a minimum SFC (specific fuel consumption) of "252 g per kw" at 1800rpm. That deteriorates to 270 g/kwh at WOT and at 1000rpm.

While there I discovered an interesting engine that probably would make a good replacement for the old Lehman's.
A Mitsu D04FD-TTA. Specs= 250cu in, turbo, 150hp at 2500rpm. SFC curve is almost totally flat at 235 g/kwh. From Klassen the exhaust manifold is steel ... not aluminum.

Marin, You may be pleased at the low cost of these engines compared to the NL engines. I'd be very curious to know how wide the gap would be myself. If you check please share. What does NL make their exhaust manifolds from?

Blue Heron 09-11-2012 01:42 PM

My Observations
My recent observations are as follows:

Engine is a 135 Hp Perkins 6.534 N/A

WOT 2600 RPM at 10.8 Kts for 7.02 GPH

RPM RPM Ratio GPH Speed
2500 .96 6.11 10.5 kts
2400 .92 5.26 10.1 kts
2300 .88 4.35 9.8 kts
2200 .85 3.86 9.1 kts
2100 .81 3.65 9.0 kts
2000 .77 2.74 8.8 kts
1900 .73 2.53 8.4 kts
1800 .69 2.04 8.2 kts
1700 .65 1.90 7.7 kts
1600 .62 1.69 7.2 kts
1500 .58 1.40 6.5 kts
1400 .54 1.26 6.3 kts
1300 .50 1.05 6.0 kts
1200 .46 .84 5.4 kts
1100 .42 .77 5.0 kts
1000 .38 .56 4.6 kts

all GPH is x 2 for twin engines.

Nomad Willy 09-11-2012 02:24 PM

I assume you have a Flow Scan or equivalent.
Under propped 100rpm and anything up to that I think is fine. I assume your rated rpm is 2500. What is it?
Looks like at 8.9 knots your'e at 50% load. A little over 2300 your'e at 75% load. That's where Steve DeAntonio says we should operate 75% of the time. And also I see that at about 80% of rpm your'e at about 50% load. I think I predicted that. I also see that at a bit over 6 knots your'e burning the same as my Willard even at that speed.
These numbers say that over propping shouldn't show a meaningful gain.
I also suspect that these numbers should be very close to the same for most Perkins and Lehman engines.
I think it also says that at 1600rpm this engine should be making 34hp. Assuming power output is directly proportional to fuel consumed as a percentage of maximum burnable. Does anybody else good (better) w numbers figure anything different? I'm glad you posted those numbers BH. Much can be read into the results.

Blue Heron 09-11-2012 03:02 PM


You're welcome. My Prop is 22 inch / 17 * 3 bladed.

Sweet spot as far vibrations and harmonic balance for my motors appears to be about 1700-1900 ERPM.

I attempted to provide the GPH at RPM and the observed speed. This is helpful when planning trips and / or fuel consumption for range and fuel stop.

Of course this is with full Fuel (400 Gallons) and full Water (300 Gallons) and assuming the displacement is rather heavy. Bottom is clean and fresh and props were just scraped so there is no marine growth.

honeybadger 09-11-2012 11:40 PM

Great Info guys, I know on the outboards its 150hp=15GPH 75HP=7.5 gph i have used my floscan and its pretty darn close. and the numbers you guys have posted seem to reflect what i have seen on the web about diesels. My 4-154 perkins at 75% before removing all the shrimpboat rigging was burning 1.9 GPH and its rated around 65HP.? sounds to good to be true ?

Nomad Willy 09-12-2012 03:09 PM

Burn sounds about right to me.
Perkins 107/8, 236 but never heard of a 154.
That's considered heavy loading by most here and on an old engine. How long did you run it this way and how'd it work out?

honeybadger 09-12-2012 07:03 PM

4-154 perkins it is a commerical engine that normally runs generators or large water pumps even generators. Mazda is still using it overseas for trucks and other commeical uses.I ran it for about 1 hour at 75% of max rpm it never even creeped up on the temp. It's fresh water cooled with a keel cooler and the exhaust, It only uses raw water from the elbow down and out back,the HP rating hangs on the pump you use and rating can very from 55 all the way to 85 Kiwi pump on mine claims it is set up for 75HP wot.

psneeld 09-12-2012 08:00 PM

Most diesels I know are run at 200 rpm under max RPM...for thousands of hours...that's what most commercial captains do that get no brief, no instructions before jumping on a workboats/deliveries and working all day...check the oil, coolant, start her up and run at 200 under max.

Nomad Willy 09-13-2012 12:03 AM

I don't know what the rated rpm for the Perkins 6-354 is but if it's 2500rpm like the Lehman Blue Heron is only at about 70% load at 200rpm down .. according to his numbers.
Glad you mentioned the 200 dow rule of thumb psneeld. Wish Marin was here to hear it.

Marin 09-13-2012 12:18 AM

Run an FL120 continuously at 200 rpm below max rated rpm and you will make Bob and Brian Smith at American Diesel very happy because they'll each be able to afford those new Aston Martins they've always wanted.

markpierce 09-13-2012 12:20 AM


Originally Posted by manyboats (Post 103033)
Glad you mentioned the 200 dow rule of thumb psneeld. Wish Marin was here to hear it.

Makes me feel more confident driving my John Deere 4045 at 2200 RPM (which I consider "hard") compared to its theoretical max of 2500. But fuel efficiency is much better at an "easy" 1800.

RickB 09-13-2012 05:52 AM


Originally Posted by markpierce (Post 103036)
But fuel efficiency is much better at an "easy" 1800.

That same engine turning 1800 rpm driving a 60Hz marine generator produces 148 hp and burns 7.8 gph at full load.

It will do that forever - almost.

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