Fuel mileage efficiency

I’ve 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. I’m curious if there would be much savings on a MPG standpoint or if it’s basically the same. I know diesels do like a workload. Would 1500rpm cause any kind of engine wear that 1800 wouldn’t l? maybe due to slightly less Efficient combustion?

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.

Either speed will be good. Under 2 gpm. Why don’t you do a comparison and report what you find?

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.

David

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.

Ted

Randomwake said:

I’ve 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. I’m curious if there would be much savings on a MPG standpoint or if it’s basically the same. I know diesels do like a workload. Would 1500rpm cause any kind of engine wear that 1800 wouldn’t l? maybe due to slightly less Efficient combustion?

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Well, not everyone. Actually, many Lehman owners run at 1,600-1,650 RPM.

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

O C Diver said:

I can slow my John Deere down to 1,200 RPM make 6 knots at 5 GPH.

Click to expand...

You don't use that much do you?
5 gallons (22.7 litres) for 6knots?

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.

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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catalinajack said:

Well, not everyone. Actually, many Lehman owners run at 1,600-1,650 RPM.

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Yup, optimum rpm will depend as much on the engine/hull combo as the engine itself.

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.

Simi 60 said:

You don't use that much do you?
5 gallons (22.7 litres) for 6knots?

Click to expand...

My mistake 1.2 GPH, 5 MPG.

Ted

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.

I have a 24X22 4 blade Dynaquad and edited post 9

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.

Mambo42 said:

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.

Click to expand...

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...

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.

Moonfish said:

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...

Click to expand...

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]!

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 “cleaning” for 30 to 40 minutes each day. I’d guess on your engines that would be 1600 or so RPM. Also, avoid idling for too long for same reasons.

Mambo42 said:

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.

Click to expand...

How was this done?

Though this thread started out being about the FL120, in the OP's boat that is CHB 34,a boat that is a very different weight, different hull shape and different WL from the more recent poster Mambo's Defever 49.

IMHO, to estimate fuel mpg, those factors, though still relevant, are less relevant than the height of the stern wave. If your boat creates a ripple, and you can still get the speed that you want, you will get very good mpg. If you aren't getting the temp that Tom (Sunchaser) mentioned, you can afford to crack it up a bit.
As far as measurement goes, you are in this for a relatively long haul, so record the fills and hours and be consistent on the speed (height of stern wave). Then, at the end of the season you will have your averages, mpg, gph oil temp or whatever other measure you have recorded.
What you should see at the end of each season, is a consistent mpg year to year, provided that the wave height is kept consistent. This should apply across all engine brand names and sizes.

Portage_Bay said:

How was this done?

Click to expand...

The boat is lying in Corfu, Greece and after some searching I found a superb British mechanic who had learned his craft back in the 80's on exactly these engines (in the UK they were used in trucks as well).

So in March / April he basically took the engines apart as much as he could, cleaned everything, sent the head off to a specialized shop. I took all the coolers to a specialized shop in the Netherlands. Then we replaced all the valves, valve seatings, valve seals, injectors etc etc.

The mechanic in the mean time cleaned the pistons, cylinder walls, even the oil pan was taken off (no idea how he managed to get it from under the engine, but he did).

It was the plan to also overhaul the minimecs, but he advised against it. As long as they are running fine I should not touch them, just keep changing the oil on time. According to him, and the shop in the Netherlands, overhauling minimecs is like opening a can of worms which you should only do when necessary. So that was good news and saved a ton of money.


The exhaust manifold was completely cleaned as well, the raw water pumps overhauled (new bearings, seals, cover plates, impellers, even repaired some corrosion on the shafts).



Lastly all the oil and water hoses were replaced, coolant tank cleaned on the inside, new thermostats installed (where the company in the Netherlands drilled a 3 mm hole in each one to prevent failing in the future), new fuel lines, new fuel return lines and then the engines were put together again.


Next thing I did was to filter all the fuel in the tanks 3 times by transferring the fuel, that was still in the boat, from one tank to the other and cleaning the tank that was empty before transferring the fuel back into that tank, while filtering as it was transferred.



The whole operation did cost a ton of money, but I did not want to take any short cuts with the engines. After all, they are basically the lifeline at sea, they need to work. Since the former owner had died and had not written down anything on paper, nor had he labelled anything, it was impossible to figure out what needed to be done.

So in my opinion there was only one thing left and that was to do as much as possible, short of taking the engines out completely.

I was lucky to find a mechanic who knows these engines inside out, that knowledge is priceless. He actually found out that the rocker assembly of the valves had been put together incorrectly by an unknown mechanic during a prior maintenance project. My mechanic told me that there had been a chance the rocker would have slid off the valve stem at high rpm with lots of consequences after that.

We are planning to spend about 7 months per year on the boat, travelling from island to island, which means lots of engine hours, so in all am very happy I did this maintenance and now the engines are running like new.



Now it is on to the next project (engine room temperatures) and many other maintenance issues that need to be addressed. But finding out the fuel consumption is important to me. It can tell me the health of the engine, but also makes fuel planning for a trip a bit more accurate.



Hope this answers your question.

Mambo42 said:

..... I found a superb British mechanic who had learned his craft back in the 80's on exactly these engines (in the UK they were used in trucks as well).

.........

Hope this answers your question.

Click to expand...

Yes it does. Thank you!



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sunchaser said:

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 “cleaning” for 30 to 40 minutes each day. I’d guess on your engines that would be 1600 or so RPM. Also, avoid idling for too long for same reasons.

Click to expand...

I have zero knowledge on this, but would expect a bit higher temp for oil to drive off moisture. If I recall chemistry correctly, residual sulfur will combine with water to form an acid.

Just curious.

Peter

mvweebles said:

I have zero knowledge on this, but would expect a bit higher temp for oil to drive off moisture. If I recall chemistry correctly, residual sulfur will combine with water to form an acid.

Just curious.

Peter

Click to expand...

Water will evaporate from oil at 175*, just not as fast as it would at higher temps. As long as things are running long enough, 175* will avoid most issues (although a bit hotter oil temps would be good).

koliver said:

Though this thread started out being about the FL120, in the OP's boat that is CHB 34,a boat that is a very different weight, different hull shape and different WL from the more recent poster Mambo's Defever 49.

IMHO, to estimate fuel mpg, those factors, though still relevant, are less relevant than the height of the stern wave. If your boat creates a ripple, and you can still get the speed that you want, you will get very good mpg. If you aren't getting the temp that Tom (Sunchaser) mentioned, you can afford to crack it up a bit.
As far as measurement goes, you are in this for a relatively long haul, so record the fills and hours and be consistent on the speed (height of stern wave). Then, at the end of the season you will have your averages, mpg, gph oil temp or whatever other measure you have recorded.
What you should see at the end of each season, is a consistent mpg year to year, provided that the wave height is kept consistent. This should apply across all engine brand names and sizes.

Click to expand...

@ Koliver,



Thanks for the info. Am not a naval engineer, but understand that I should avoid having a deep wave behind the boat ? Basically go up to a speed where the wake is max a ripple ? I know that Art Defever designed the hull in a way that the stern rises a bit, so as to keep the stern wave minimum. So far I have not seen any deep waves behind the boat and we are happy at 5.5 - 6 kts, it is fine for us. We have a cup of coffee, enjoy the view, sit in the morning sun and relax.
But if I understand you correct we could keep pushing up until the moment a deep stern wave starts to form ?


As for registering the rpm and speed in the engine log. I did not do that up until now, but will start to do it. Great tip.



We will keep track of the fuel in the tanks as much as we can, but I don't really trust the indicators at this time. The indications change for no reason at all. One moment it indicates one tank is full, next moment I get an indication the tank is almost empty. The boat was sitting for quite some time with tanks at perhaps 1/3, so fear that corrosion of the measuring equipment in the tank may have caused this. I will need to address the tank level indicators, but it is a project of lower priority at the moment.


One item that will screw up the calculation are the generator and the heating system. They are drawing fuel from the same tanks and there is no fuel flow meter for neither one of them. Since my brand new batteries were destroyed by the marina I will have to run the generator most of the time while on anchor and that is going to screw up all fuel calculations.

Unfortunately

Generally when you hit the point that an extra 100 rpm on the engines produces a smaller increase in speed and your wake starts to increase noticeably, that's when you're getting into the steep part of the fuel consumption curve (where fuel consumption will go up significantly with increased speed). I'd expect a Defever 49 to start to hit that wall somewhere in the 7.5 - 8 kt range. Going slower than that will burn even less fuel, but the difference (in miles per gallons) between 6 and 7 kts will likely be smaller than the difference between 7 and 8 kts. And pushing for 9 kts will take a whole lot more fuel than 8 kts.

rslifkin said:

Generally when you hit the point that an extra 100 rpm on the engines produces a smaller increase in speed and your wake starts to increase noticeably, that's when you're getting into the steep part of the fuel consumption curve (where fuel consumption will go up significantly with increased speed). I'd expect a Defever 49 to start to hit that wall somewhere in the 7.5 - 8 kt range. Going slower than that will burn even less fuel, but the difference (in miles per gallons) between 6 and 7 kts will likely be smaller than the difference between 7 and 8 kts. And pushing for 9 kts will take a whole lot more fuel than 8 kts.

Click to expand...

@ rslifkin


Thanks, really appreciate the info, it gives me great insight on my engine and boat performance, what I can expect and what to look for.


So far we have not gone over the 7 kts mark, I did not want the engines to run that hard after the extensive work we did. First wanted to make sure everything worked and we had to get to know the systems on board. After that we devoted a lot of time to learning how to dock this boat, it was quite a step up from what we had before. So spent at least 2 weeks practicing all kinds of docking manoeuvres , MOB, anchoring etc. We even had SuperYachtCaptain Tristan Mortlock on board for 4 days, teaching my wife how to dock and handle the boat, it will be in one of his upcoming episodes.

Even practiced storm procedures when we were caught in an unexpected storm.

I did make a small note of engine rpm's and speed, which came down to increasing rpm by 100 would lead to an increase of speed by about 0.4 kts. From 5 to say 6.5 the increase was the most, after that it started to decrease. At 7 kts an increase of 100 rpm would give us about 0.2 kts extra.


The hull has been cleaned, the props have been cleaned, so that should not be a factor at this moment.

Next time we are on the boat, in August, I will start pushing her beyond the 7 kts and see where it ends. Any idea what max rpm I should look for with the Ford Lehman 120's ?

The brick wall you face w a typical rec trawler is hull form and weight.
When trawler hulls become much more like FD fish boats and sailboats much will be gained in efficiency. Short of that … not much.

Another road to efficiency would be rebuilding the stern of your rec trawler bringing the transom up and out of the water.

And of course smaller boats and smaller engines.