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Old 04-01-2022, 02:37 PM   #1
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Fuel mileage efficiency

Iíve got a 120HP ford Lehman in a CHB 34, trying to optimize fuel economy. Everyone seems to agree that 1800 RPM is the way to go. I get about 7.5kn, lots of time I prefer to slow down to 1500rpm and still get 6.5kn with very low noise and minimal wake. Iím curious if there would be much savings on a MPG standpoint or if itís basically the same. I know diesels do like a workload. Would 1500rpm cause any kind of engine wear that 1800 wouldnít l? maybe due to slightly less Efficient combustion?
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Old 04-01-2022, 02:39 PM   #2
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As long as everything gets up to temperature and it doesn't get smoky after a while or show other signs of poor combustion, I highly doubt running at 1500 will hurt anything. If you're worried, just bump it up for a few minutes here and there to get things good and hot.
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Old 04-01-2022, 02:40 PM   #3
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Either speed will be good. Under 2 gpm. Why don’t you do a comparison and report what you find?
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Old 04-01-2022, 03:00 PM   #4
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Fuel economy always gets better the slower you go. The limit is usually 1,200-1,500 rpm to give the engine enough load to burn cleanly and not soot up. The Lehman is probably good to 1,200 rpm.

And for your boat and motor running at 1,200 vs 1,500 will drop the fuel consumption nearly in half I would suspect.

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Old 04-01-2022, 03:25 PM   #5
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My simple rule of thumb:
"It takes a lot of diesel fuel to make water stand up".

I can slow my John Deere down to 1,200 RPM make 6 knots at 5 GPH. My guess is that somewhere between 1,000 and 1,200 RPM, your Lehman should be able to properly splash lubricate itself internally, reach operating temperature, and maintain reasonable output on the alternator. I have my doubts about long distance / time cruising below 1,000 RPM. Also don't think you will see a big consumption difference per mile below 1,000 RPM.

Remember to look out back. When your wake becomes a ripple, that's probably about as good as it's going to get.

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Old 04-01-2022, 03:26 PM   #6
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Quote:
Originally Posted by Randomwake View Post
Iíve got a 120HP ford Lehman in a CHB 34, trying to optimize fuel economy. Everyone seems to agree that 1800 RPM is the way to go. I get about 7.5kn, lots of time I prefer to slow down to 1500rpm and still get 6.5kn with very low noise and minimal wake. Iím curious if there would be much savings on a MPG standpoint or if itís basically the same. I know diesels do like a workload. Would 1500rpm cause any kind of engine wear that 1800 wouldnít l? maybe due to slightly less Efficient combustion?
Well, not everyone. Actually, many Lehman owners run at 1,600-1,650 RPM.
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Old 04-01-2022, 06:44 PM   #7
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I used to run my single Lehman 120 at 1700/1750 and woulld use about 1.75 gph at 6.5/7 kts. Engine would easily stay up to temp at 180 f
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Old 04-01-2022, 07:07 PM   #8
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Originally Posted by O C Diver View Post

I can slow my John Deere down to 1,200 RPM make 6 knots at 5 GPH.
You don't use that much do you?
5 gallons (22.7 litres) for 6knots?
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Old 04-01-2022, 07:50 PM   #9
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I run my single Lehman at 1720 rpm, 8 knots, 1.85 gph, 4 mpg, 180 degrees.

Waterline length of 39' single engine, 2.8 : 1 transmission, 24X22 4 blade Dynaquad.
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Old 04-01-2022, 07:51 PM   #10
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I'd bet that you'd go 50% further on a tank of fuel at 1500 vs 1800. And you won't be harming the motor running at 1500.

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Old 04-01-2022, 08:03 PM   #11
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Well, not everyone. Actually, many Lehman owners run at 1,600-1,650 RPM.
Yup, optimum rpm will depend as much on the engine/hull combo as the engine itself.
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Old 04-01-2022, 08:48 PM   #12
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Greetings,
When we had our Marine Trader 34' DC I ran our 120 Lehman at either one of the two "rivets" on the Motorola tach. One was 1250 RPM and the other was 1750 RPM. She seemed to like 1750 and as I vaguely recall we burned about 2 GPH @ 7 knots. Can't remember fuel usage or speed @ 1250 RPM.
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Old 04-01-2022, 08:52 PM   #13
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Originally Posted by Simi 60 View Post
You don't use that much do you?
5 gallons (22.7 litres) for 6knots?
My mistake 1.2 GPH, 5 MPG.

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Old 04-01-2022, 09:27 PM   #14
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Over the years I learned that a prop will make a difference. The standard prop v. stainless on a runabout for instance, same size, pitch and rpm will produce more speed.

Using that thought, without knowing what others have for a prop does not help me compare very well. IMHO Please list your prop size.

I have 24x18 & 24x17 on the twins, 2.1:1 & 1.91:1, yes over propped. I started at 1800 rpm based on opinions here and was averaging 9-9.5 knots, with 13 knots at wot. Cut back to 1650 because it sounded better and did 7.5-8. Cut back to 1500 and doing 7-7.5
Burn was 9L for twins or 1 gallon CAN or about 1.2 US each over one year.
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Old 04-01-2022, 09:34 PM   #15
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I have a 24X22 4 blade Dynaquad and edited post 9
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Old 06-22-2022, 12:38 AM   #16
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My Defever 49, which I purchased last year, has 2 Ford Lehman 120's and first thing I did was to have both engines cleaned completely. All cylinders had thick carbon layers, valves did not close well anymore, injectors were leaking.
Now they both run as smooth as can be, but am not familiar with the optimum rpm range. I never had Ford Lehmans before, these are my first.

I did go through the different rpm's to see what speed I would get and basically for us she runs fine at 1350 rpm. That gives us about 5.8 to 6 kts, which is good enough for us, we are not in a hurry. Increasing rpm to 1600 rpm will only give us 1 kt extra, basically not worth the effort in our opinion.

However, I have no clue about the fuel usage at those rpm settings. Can anyone shed a light or otherwise give an advise which FF meters would work best ?



And one other question. I also have no idea of the max cruising rpm of these engines. The engines are 45 years old and I don't want to create more wear than necessary on them by running them too hard.



Appreciate all the info. Thanks already.
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Old 06-22-2022, 01:31 AM   #17
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Quote:
Originally Posted by Mambo42 View Post
My Defever 49, which I purchased last year, has 2 Ford Lehman 120's and first thing I did was to have both engines cleaned completely. All cylinders had thick carbon layers, valves did not close well anymore, injectors were leaking.
Now they both run as smooth as can be, but am not familiar with the optimum rpm range. I never had Ford Lehmans before, these are my first.

I did go through the different rpm's to see what speed I would get and basically for us she runs fine at 1350 rpm. That gives us about 5.8 to 6 kts, which is good enough for us, we are not in a hurry. Increasing rpm to 1600 rpm will only give us 1 kt extra, basically not worth the effort in our opinion.

However, I have no clue about the fuel usage at those rpm settings. Can anyone shed a light or otherwise give an advise which FF meters would work best ?



And one other question. I also have no idea of the max cruising rpm of these engines. The engines are 45 years old and I don't want to create more wear than necessary on them by running them too hard.



Appreciate all the info. Thanks already.
There is an older thread here where someone posted pics of Floscan readings for the Lehman 120 at various RPMs (intervals of 100 RPMs). It corroborated what the PO of my boat said and our own fuel burn estimations. What we get is .9 GPH at 1400 RPMs (and 7 kts). Can't recall the other settings with certainty, but I think it showed 1.2 GPH at 1500 RPMs.

I spent about 10 minutes searching for that particular thread, but could not find it. The search function can be a little frustrating...
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Old 06-22-2022, 01:38 AM   #18
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Thanks Darren for the quick reply.


I have gone through most of the topics in this part of the forum, but did not find anything either. Really would like to see those estimations.

.9 at 1400 and 7 kts is a very good result. I will be very happy if I can reach that.
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Old 06-22-2022, 08:20 AM   #19
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Quote:
Originally Posted by Moonfish View Post
There is an older thread here where someone posted pics of Floscan readings for the Lehman 120 at various RPMs (intervals of 100 RPMs). It corroborated what the PO of my boat said and our own fuel burn estimations. What we get is .9 GPH at 1400 RPMs (and 7 kts). Can't recall the other settings with certainty, but I think it showed 1.2 GPH at 1500 RPMs.

I spent about 10 minutes searching for that particular thread, but could not find it. The search function can be a little frustrating...
Is: "What we get is .9 GPH at 1400 RPMs (and 7 kts)." - Meaning fuel used per engine is .45 GPH at 7 knots ... or ... is that referring to .9 GPH per engine for a total GPH of 1.8 GPH [which is still really thrifty]!
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Old 06-22-2022, 09:01 AM   #20
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Oil temperature is a key measure for low RPM operation. Measure the temperature on the pan with an IR gun with anything below 175F or 80C raising concern. Minimally run at 180F for optimum engine ďcleaningĒ for 30 to 40 minutes each day. Iíd guess on your engines that would be 1600 or so RPM. Also, avoid idling for too long for same reasons.
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