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Old 04-10-2022, 11:45 AM   #1
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Fuel usage: "new" vs "old" diesel engines

Regarding "new" vs "old" diesel engine fuel burns.

I find it difficult to understand why some new, modern, state of the art engines burn twice as much (or more) fuel than my Gardner 8LXB, designed, I believe, in the 1950's, and built in 1978 burns.

Granted, my engine produces (from one source) 20% to 40% more pollution than the newer engines per gallon consumed, but if mine is consuming less than HALF the fuel for the same work (miles made good through the water), and the new design is 20% to 40% less polluting, but burns over twice the fuel per mile made good through water . . . . . isn't the new, modern, sophisticated, common rail, secret spy ring engine actually polluting MORE than my old engine?!?

Mechanical: 1978 Gardner 8LXB with 36" Hundestat CPP
Boat: 1982 Robert Beebe designed 50' Passagemaker
Normal cruising weight: 75,000 to 90,000 lbs, depending on fuel/water on board.
Normal cruise: 7.5kts, 1300 rpm, pyrometer (EGT) 430(ish) deg F. Typical 120v load.

Fuel burn: 2.4 gph measured, which gives me 3.125 nm/gal.

And I'm not even taking into account the pollution and expense of drilling, refining, and transporting that extra fuel in order to run the boat with a modern engine the same number of miles over the ground . . .

And I didn't factor in that my engine is also running a hydraulic pump that is running the hydraulic motor that turns my 12kw 120v generator . . . . so if I have a need for LOTS of 120 power, there is no need to start another diesel engine to provide the hyd pressure/flow.

Just thinking out loud . . . .
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Old 04-10-2022, 12:41 PM   #2
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The old Gardners are pretty efficient compared to a lot of other diesels. It also helps that your propulsion package (including the CPP) and hull are all well matched for efficiency. In general, once you get past the old Detroits, newer diesels haven't gained all that much efficiency compared to older ones. Maybe 10-15 percent at best. But the newer stuff often scales better, so efficiency doesn't change as much with RPM or load.

I'd expect that a well selected modern package could at least match your existing setup, but I'm not sure you could do a whole lot better. Most of the highest efficiency modern engines are just bigger and more powerful than your boat needs.
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Old 04-10-2022, 12:58 PM   #3
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Originally Posted by slowgoesit View Post
I find it difficult to understand why some new, modern, state of the art engines burn twice as much (or more) fuel than my Gardner 8LXB, designed, I believe, in the 1950's, and built in 1978 burns.

What new, modern, state of the art engines are you comparing to yours?

In what boats?

-Chris
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Old 04-10-2022, 01:40 PM   #4
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Clean air diesels run a richer combustion mixture to promote
more efficient combustion than their older counterparts.
Thus, they pollute less but burn more fuel doing so.

We have observed this 1st hand on several of our tugs that
have had older mechanical diesels replaced with their new
equivalent electronic sibling.
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Old 04-10-2022, 01:45 PM   #5
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All of the above is true. In addition, the Gardner operates at low rpms, so it has low parasitic loads. Also I suspect the piston and rings were designed for minimum friction but ignore fuel and exhaust leakage past the rings.

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Old 04-10-2022, 02:14 PM   #6
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It's really hard to compare boats relative to fuel burn. You can have two identical boats withe the exact same engine / transmission package, but different pitch props can require the RPM to be different enough to effect fuel economy. Obviously I'm wandering off in the weeds here. To me, the tell all is how many HP per gallon the engine generates in the peak part of the torque curve. Most fuel efficient diesels in our size range are generating around 20 HP per gallon in the peak of the torque curve. After that, it's a question of whether you can get the speed you want in that peak torque area.

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Old 04-10-2022, 02:38 PM   #7
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I think the underlying assumption is incorrect, namely that a Garner is twice is efficient as a modern day engine. If I remember correctly a Gardner consumes around 190-200 g/kw-h of output power (bsfc). My 2017 Scania consumes the same. For similar size/output engines, I just haven't seen huge variation in efficiency of the engine itself.


And comparing one boat with a Gardner to another boat with something else is a nearly impossible task since there are so many different variables. It's comparing apples to gorillas. Even accurately measuring fuel consumption is highly problematic.


Engines on their own are well characterized, and with some searching, data that is often published. BSFC is an apples to apples comparison between two engines. A Gardner is quite good, but definitely not 2x anything, let alone any current build.
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Old 04-10-2022, 08:49 PM   #8
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Quote:
Originally Posted by ranger58sb View Post
What new, modern, state of the art engines are you comparing to yours?

In what boats?

-Chris


Chris, thanks for your input. I looked at various engines, not boat specific, since I'm looking at HP output for engines at different RPM's, but even that is problematic with a CPP, since I can do 1300 rpm for instance at flat pitch (equivalent to being in neutral with a boat without CPP). or 1300 rpm at a pitch to push our boat 7.5 kts at about 430 deg F Pyrometer (EGT). Comparing manufacturer HP output at various RPM's without CPP, and associated fuel burn listed is what I looked at.

My intent is not to get down in the weeds of minutia with this, so if that is what you are after, you're barking up the wrong tree! This thread was just a casual observation regarding newer vs older engines. For instance in our GB36, re-engined with a Cummins 6BTA, fuel consumption was about 4.5 gph at 7.5kts, using around 80 to 100 hp.

Just to make it clear, I'm NOT interested in doing a doctoral thesis thesis here, just casual conversation.
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Old 04-10-2022, 08:59 PM   #9
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Originally Posted by slowgoesit View Post
Chris, thanks for your input. I looked at various engines, not boat specific, since I'm looking at HP output for engines at different RPM's, but even that is problematic with a CPP, since I can do 1300 rpm for instance at flat pitch (equivalent to being in neutral with a boat without CPP). or 1300 rpm at a pitch to push our boat 7.5 kts at about 430 deg F Pyrometer (EGT). Comparing manufacturer HP output at various RPM's without CPP, and associated fuel burn listed is what I looked at.

My intent is not to get down in the weeds of minutia with this, so if that is what you are after, you're barking up the wrong tree! This thread was just a casual observation regarding newer vs older engines. For instance in our GB36, re-engined with a Cummins 6BTA, fuel consumption was about 4.5 gph at 7.5kts, using around 80 to 100 hp.

Just to make it clear, I'm NOT interested in doing a doctoral thesis thesis here, just casual conversation.
For the GB36, 7.5 kts was likely faster than an ideal speed. Your current boat has a longer waterline, so the fuel burn cliff starts at a higher speed. Plus, the flat transom SD hull GB was likely a slightly more draggy hull, although lighter weight would have offset some of that.
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Old 04-10-2022, 09:06 PM   #10
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I think the underlying assumption is incorrect, namely that a Garner is twice is efficient as a modern day engine. If I remember correctly a Gardner consumes around 190-200 g/kw-h of output power (bsfc). My 2017 Scania consumes the same. For similar size/output engines, I just haven't seen huge variation in efficiency of the engine itself.

Not sure why you think that "the underlying assumption is that a Gardner (not Garner) is twice is (sic) efficient." I never made that statement.
Fact is that I am getting 7.5 nm through the water, using 2.4 gallons/hour.

Interesting Fact: A similar Beebe designed boat, same hull, similar loading, with a Cummins 6BTA, 215hp version, at 7.5kts, he is burning 6.6 gph. You can draw whatever concussion from that you want, but both boats, side by side, same hull design, similar loading, etc, one goes 3.125 miles on one gallon of diesel, the other, 30 year newer design engine, goes 1.36 miles on one gallon of diesel . . . . . call it whatever it is, more efficient, whatever, just curious.
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Old 04-10-2022, 09:11 PM   #11
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For the GB36, 7.5 kts was likely faster than an ideal speed. Your current boat has a longer waterline, so the fuel burn cliff starts at a higher speed. Plus, the flat transom SD hull GB was likely a slightly more draggy hull, although lighter weight would have offset some of that.

I agree, not a perfect comparison, as my GB36 had a hull speed of 7.9 kts, and my Beebe's hull speed is calculated at 9.2 kts, but it IS a comparison of sorts.
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Old 04-10-2022, 09:25 PM   #12
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I think you stepped on a hornets nest with this thread.. But love your boat, love the story of the trip and think its one of the nicest boats on the forum.. Enjoy!
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Old 04-10-2022, 11:15 PM   #13
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Not sure why you think that "the underlying assumption is that a Gardner (not Garner) is twice is (sic) efficient." I never made that statement.
Fact is that I am getting 7.5 nm through the water, using 2.4 gallons/hour.

Interesting Fact: A similar Beebe designed boat, same hull, similar loading, with a Cummins 6BTA, 215hp version, at 7.5kts, he is burning 6.6 gph. You can draw whatever concussion from that you want, but both boats, side by side, same hull design, similar loading, etc, one goes 3.125 miles on one gallon of diesel, the other, 30 year newer design engine, goes 1.36 miles on one gallon of diesel . . . . . call it whatever it is, more efficient, whatever, just curious.
That seems really high for the Cummins. Maybe a gearing/pitch issue? I agree with the previous posts that efficiency spreads between older and newer diesels should be pretty minimal *all else equal*. It's also really hard to get all else equal in practice.
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Old 04-10-2022, 11:37 PM   #14
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Quote:
Originally Posted by slowgoesit View Post
Regarding "new" vs "old" diesel engine fuel burns.

I find it difficult to understand why some new, modern, state of the art engines burn twice as much (or more) fuel than my Gardner 8LXB, designed, I believe, in the 1950's, and built in 1978 burns.
. . .
Wow, an 8lxb seems generous for a 50fter

Our sister from another father ship at 55ft has a 6lxb
They considered a 6lxb for ours @ 60ft during the rebuild but went nta855 Cummins instead
Friends with the 65 ft milkraft hull with a few feet extra beam have an 8lxb.

They report a similar fuel burn to ours @ 7.5 to 8kn
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Old 04-11-2022, 06:29 AM   #15
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Quote:
Originally Posted by slowgoesit View Post
Chris, thanks for your input. I looked at various engines, not boat specific, since I'm looking at HP output for engines at different RPM's, but even that is problematic with a CPP, since I can do 1300 rpm for instance at flat pitch (equivalent to being in neutral with a boat without CPP). or 1300 rpm at a pitch to push our boat 7.5 kts at about 430 deg F Pyrometer (EGT). Comparing manufacturer HP output at various RPM's without CPP, and associated fuel burn listed is what I looked at.

My intent is not to get down in the weeds of minutia with this, so if that is what you are after, you're barking up the wrong tree! This thread was just a casual observation regarding newer vs older engines.

Just to make it clear, I'm NOT interested in doing a doctoral thesis thesis here, just casual conversation.


Not to worry. Intent behind my question was just to set up for a subsequent comment about apples to gorillas, different results from engines, boats (lengths, weights, hull shapes), gears, speeds, etc....

And then a further recognition that we don't always have control over selection: the available boat has whatever engine it has, repowering is expensive and not usually necessary and certainly not always economical... etc etc etc.

Our boat, not at all designed or set up for "economical" actually does OK at trawler speeds. Not great, others could certainly use less fuel... but not bad. And then lots of boats with more "economical" engine won't make 30 kts on demand, either.

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Old 04-11-2022, 07:04 AM   #16
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Not sure why you think that "the underlying assumption is that a Gardner (not Garner) is twice is (sic) efficient." I never made that statement.
Fact is that I am getting 7.5 nm through the water, using 2.4 gallons/hour.

Interesting Fact: A similar Beebe designed boat, same hull, similar loading, with a Cummins 6BTA, 215hp version, at 7.5kts, he is burning 6.6 gph. You can draw whatever concussion from that you want, but both boats, side by side, same hull design, similar loading, etc, one goes 3.125 miles on one gallon of diesel, the other, 30 year newer design engine, goes 1.36 miles on one gallon of diesel . . . . . call it whatever it is, more efficient, whatever, just curious.

6.6 gph would indicate that he's using somewhere around 120 hp to push the boat through the water. Either that, or that boat has a ton of accessory drag pulling from that engine (like running with that hydraulic gen heavily loaded all the time, as 12 kw will add about 20hp of demand plus hydraulic losses).



It's quite possible that something is just very poorly optimized on that boat, as even a 2 stroke Detroit isn't inefficient enough to account for that much more fuel burn than your Gardner.
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Old 04-11-2022, 09:48 AM   #17
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For instance in our GB36, re-engined with a Cummins 6BTA, fuel consumption was about 4.5 gph at 7.5kts, using around 80 to 100 hp.
For another data point, our NT37 has a 2001 Cummins 6BTA 5.9 M3, 330 hp. Over the past 4,000 engine hours, at our cruising speed of 7.25 knots, we average 3.7 nmpg, which translates to 1.96 gph. And this is not considering that some fuel is used by the heater and the generator. Does not seem like a problem with the fuel efficiency of the engine.
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Old 04-11-2022, 10:12 AM   #18
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And comparing one boat with a Gardner to another boat with something else is a nearly impossible task since there are so many different variables. It's comparing apples to gorillas. Even accurately measuring fuel consumption is highly problematic..
Amen. I do find it curious that reported fuel economy always seems over-stated vs understated. I've only felt confident of fuel burn once. I delivered an N57 from Dana Point to Ft Lauderdale over a 25-day period. Roughly 4500 nms, 3000 gals fuel, and 500 engine hours plus 70 generator hours.

I was a guest on a Willard 40 with JD4045T from Long Beach to La Paz, about 1000 nms. From the sight gauges, appeared damn close to 1.5gph and we averaged 7.2 kts, but I didn't do the fueling and keep full records so only an informed guess.

I'm sorry, but someone has to say it. Anyone who thinks an old NA Gardner (or Perkins or Lehman) is more fuel efficient than the new stuff they're putting out is wrong. California paid millions to yank old engines out of fishing boats because they were inefficient. If truckers could improve their fuel mileage by running a 50-year old engine they'd be first in line. There are good reasons to run an old engine. Fuel efficiency is not one of them.

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Old 04-11-2022, 10:17 AM   #19
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I just looked up some data on the Gardner 8LXB. It's rated 175hp at 1500 RPM. At full power, it burns 8.4 gal/hr, giving an output of just over 20.8 hp-hr per gallon. That efficiency seems to scale very well across the whole RPM range for that engine.



FWIW, the most efficient modern small-ish diesels are in the 20 - 21 hp-hr / gallon range. Some may get as high as 22. Many of the smaller, naturally aspirated engines are down in the 18 - 19 hp-hr per gallon range. 2 stroke Detroits are typically around 16 - 18.



So the Gardner is still quite efficient by modern standards. Not meaningfully better than most modern engines, but it's just as efficient as the majority of them.
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Old 04-11-2022, 10:24 AM   #20
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My first theory was that Gardner made stationary and marine engines, which is a different beast than a "generic" diesel that may have to climb hills, shift gears, etc. I thought that maybe the newer engines had to be a jack-of-all-trades in order to make commercial sense. The difference would be a tortoise and hare kind of issue. The Gardner just plods along and it's efficiency is due to the fact that it only need plod along.

Then I looked at Wikipedia (the font of modern knowledge). It turns out that Gardner made a diesel for the Bentley in the 1930's. 80 mph and 30 mpg. So much for my hypothesis.

https://en.wikipedia.org/wiki/L._Gardner_and_Sons
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