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Old 04-23-2013, 06:54 AM   #1
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Lehman 120 Fuel Burn Pics

These are from a single FL120 on a Marine Trader 40 Sedan. These numbers are going to be very similar on any boat. You decide what speed/burn works for you but as you can see a little difference can be very dramatic. Moderators you might want to tag this in some way so it is easily accessible since this question comes up all the time.
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Old 04-23-2013, 10:46 AM   #2
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Quote:
Originally Posted by Daddyo View Post
These numbers are going to be very similar on any boat. You decide what speed/burn works for you but as you can see a little difference can be very dramatic.
Excellent post!
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Old 04-23-2013, 10:49 AM   #3
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Mark,
What were your speeds at each rpm?
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Old 04-23-2013, 11:14 AM   #4
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It was a friend's boat. Don't know the speeds. It would depend on reduction gear and prop.

The point is these are the numbers your engines (any diesel) burns at these rpms give or take 10%.
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Old 04-23-2013, 11:47 AM   #5
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Quote:
Originally Posted by Daddyo View Post
It was a friend's boat. Don't know the speeds. It would depend on reduction gear and prop.

The point is these are the numbers your engines (any diesel) burns at these rpms give or take 10%.

I thought that was something you just did.

thanks
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Old 04-23-2013, 12:24 PM   #6
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At 1650 each of our FL120s burns a bit over 2.5 gph. So a bit over 5 gph for both engines. This gives us 8 knots. This is why for fuel calculating purposes we use 6 gph to be on the safe side.
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Old 04-25-2013, 07:58 AM   #7
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Great post, looks like your just on or just past the optimal point. See attached image If you would be kind enough to post speed, rpm and fuel flow starting at 700 rpm the optimal rpm for maximum range could easily be found. Sorry for the non labeled spreadsheet, working on a small android system

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Old 04-25-2013, 08:12 AM   #8
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Mark, Gordon -

Outstanding!! Most of this forum is fun. This is valuable! Thank you for providing.
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Old 04-25-2013, 09:50 AM   #9
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Good visuals Daddyo. But GPH is only part of the story.

On our longer cruises in the high tidal flow PNW waters I often run at higher less fuel efficient RPMs/ gph to gain greater use of higher mpg for the days trip. Better said, 3 to 5 knots on the nose is a lot worse than 3 -5 knots on the stern for overall efficiency. So a ball busting 1850 RPM and 6 gph combined is worth the fuel "loss" - with a 3 knot tail tide at a SOG 11+ knots for hours on end - to cut 3 hours off the days trip.

Off shore or for long passages we will run faster speeds (say 9.0 vs 7.5 knots) for hours on end to avoid forecast bad weather troughs or make an inlet crossing.
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Old 04-25-2013, 10:00 AM   #10
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Quote:
Originally Posted by Daddyo View Post
... these are the numbers your engines (any diesel) burns at these rpms give or take 10%.

That would depend on reduction gear and prop, number of cylinders, size of cylinders, power output, weight of boat, condition of bottom, wind, trim ... ad infinitum.

A larger engine might burn that much at idle. A smaller engine might not burn that much if it had a broken fuel line and was on fire.
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Old 04-25-2013, 07:31 PM   #11
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Those numbers are more than i burn at those rpm....my info based on fuel added at a given number of hours, not instantaneous figures.
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Old 04-30-2013, 09:07 AM   #12
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That would depend on reduction gear and prop, number of cylinders, size of cylinders, power output, weight of boat, condition of bottom, wind, trim ... ad infinitum.

A larger engine might burn that much at idle. A smaller engine might not burn that much if it had a broken fuel line and was on fire.
I agree.
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Old 04-30-2013, 10:23 AM   #13
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Quote:
Originally Posted by RickB View Post

That would depend on reduction gear and prop, number of cylinders, size of cylinders, power output, weight of boat, condition of bottom, wind, trim ... ad infinitum.

A larger engine might burn that much at idle. A smaller engine might not burn that much if it had a broken fuel line and was on fire.
Hi Rick- all what you have said is true, but this thread started on the Ford Lehman 120 HP engine, a 6-cylinder NA diesel around 380cc (ballpark) and which has several cousins (like my ADC 136N).

The general assumption might be (please correct if wrong) that the engine is not on fire. (Call me crazy, but that's what I think it is...)



If forced at gunpoint (or under threat of beer deprivation) to guess, I would say this brand and its variants make up the majority of 7-8 knot cruising trawler engines, followed by Volvo and Perkins. And props (guessing again) in the 20-24 inch size range.

All that said (at length, I know), the data shared is a great indicator of this size and output, entirely valuable to me and other 7-knot skippers. I am quite possibly considering removing the admiral's frilly cabin decorations (under penalty of court martial) and replacing them with these data charts here.

All wry humor aside, I'll likely download these and paste them in my logbook for benchmarking. I honestly appreciate the effort and the sharing.
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Old 04-30-2013, 11:05 AM   #14
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[QUOTE=Ben;153234]
I would say this brand and its variants make up the majority of 7-8 knot cruising trawler engines QUOTE]

Alas, no more looking forward. Lehman new builds were gone a long time ago. As repower or junk the boat realities appear with greater frequency, the increasingly asked question is "what are the repower choices?"
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Old 04-30-2013, 12:09 PM   #15
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Hi Rick- all what you have said is true, but this thread started on the Ford Lehman 120 HP engine, a 6-cylinder NA diesel around 380cc (ballpark) and which has several cousins (like my ADC 136N).
Fair enough. What you wrote (plus adding - at the same rpm and power output) is what should have been stated rather than:

"... (any diesel) burns at these rpms give or take 10%."
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Old 04-30-2013, 12:33 PM   #16
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Fair enough. What you wrote (plus adding - at the same rpm and power output) is what should have been stated rather than:

"... (any diesel) burns at these rpms give or take 10%."
The analyst in me wants to overlay these fuel burn curves on various displacements of 6-cylinder engines and their power outputs, and also do the same for a single engine with various prop configurations. The goal would be to find the optimal sweet spot for each boat mass and hull configuration.

If I get around to it, I'll probably find it in some engineering research or a Nigel Caulder book.

Nothing gained for me except just knowing my engine a little deeper. With my hull, 6.5-7.5 knots is the right range with 6.7 being a reasonable sweet spot (wind and current = 0). Daddyo's data is confirmation of what I was guessing anyway.

My cousin has a Cat 3208 in some kind of picnic boat, so occasionally I get a little speed envy. Still, mine is just as fast at anchor as his is.

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