Fuel Mileage

[Dom61]

I have a 34 Trader with a 120 any ideas about the best RPM to run for best fuel mileage , we are always with or aginst a curant so M/N per hr* will not work

 

[coyote454]

Dominic,

Here's another area where everyone has their own opinions. *My 34' CHB, with the same 120 Lehman as you, normally will do about 8 knots (9.6 mph) with a fuel burn of 2.5 to 3 gallons per hour. *Of course, current, wind and general sea conditions will all affect these numbers.


Your own numbers should be close, but the size and pitch of your prop may change things a bit.


The only accurate method I've heard of for your best RPM is with a FloScan instrument, but they are a little pricey.


http://www.floscan.com/html/blue/seriesdetail.php?sid=14&catid=2




Mike
Brookings, Oregon

 

[Marin]

Fule Mileage

Mike's suggestion of something that measures the fuel flow is about the only sure way of matching economy to throttle setting.

But I'm not sure in the case of a trawler-type boat with an FL120 the knowledge is worth the cost. The FL120 uses so little fuel--- perhaps 2.5 gallons an hour at something like 1600 rpm--- the fuel use difference within the "normal" range of power settings for this engine, which is generally considered 1500 to 1800 rpm-- is not going to be enough to care about. Running the engine hard--- 2000 rpm and up--- will result in a higher fuel burn but even then it's not going to be a staggering increase. Backing the power down below 1500 will probably drop the fuel burn a fair amount but now you're running too slow to maintain proper temperature so unless you've got a prop with a massive amount of pitch in it, it's not something you want to do anyway.

For the last twelve years we've run our two FL120s at about 1650 rpm (the SW tachs on the boat are not anything you want to set your watch by so 1650 is an approximation) and that gives us about 8 knots at a combined fuel burn of between 5 and 6 gallons an hour. This is with a 36', 28,000 pound boat and 23" diameter, four-bladed props with a 16" pitch.

-- Edited by Marin on Friday 3rd of September 2010 07:02:27 PM

 

[Delfin]

I don't think this is quite as complicated as it would appear.* If you can find the power curve data from Lehman, they'll show 'prop demand data', indicating how many gph are burned at different rpms generating 'x' horsepower.* Just build a spread sheet that has rpms in the first column, prop demand fuel burn in the second column, and speed in knots absent current that you experience in the third column.* From that it is easy to calculate nm per gallon burned, and you can pick your most efficient balance between burn, speed and distance traveled.

For Delfin, we burn 2.75 gph at 7.5 knots or so, giving us a range of 6000 miles plus.* If I want to get there faster, I can go 8.25 knots at 4 gph, but for long distances, the increase in speed isn't worth it.

You indicated that NM/gph 'won't work'.* Regardless of the current, the water you travel through will be the water you travel through, even if you don't get anywhere because of current.* The efficiency calcs are the same, with the choice being do you want to travel through more water per unit of time, or travel the maximum distance regardless of the time it takes.

 

[Daddyo]

I know a guy who has a Flow Scan on his FL120 on a MT 40 sedan.

.75 gph @1400 rpm
1.5 gph @1600 rpm
2*** gph @1700 rpm

 

[Dom61]

Thanks for the info I know the 1400 is not good for the engine 1600 sounds about right from what i have been hearing** Thanks Dom

 

[RT Firefly]

Hiya,
** As mentioned, a fuel burn curve should answer your question.* From MY experience with a FL 120, to the best of my memory, 1750 RPM would give me about a 2 GPH burn and about 7 knots.* It seemed the 2 "sweet spots", that area where the FL "sounded" the best, were right in line with the two rivets on the face of the old Motorola tach'.* The*rightmost upper*rivet was 1750 RPM and I can't remember the RPM where the lower spot was.* I mentioned this to Bob Smith at one point and he agreed that the FL seemed most comfortable @ 1750.* But slowing down a bit WILL give you better "mileage".**
**Ocasionaly I would pin the throttle producing copious amounts of black smoke and get to 8 knots (2200 RPM) but I knew I was over propped.* You should be able to reach close to 2400 RPM (WOT) with the appropriate prop.
* As also mentioned, prop size will also determine what your burn will be at any given cruising RPM's.
** From Daddyo's post it would seem 1600 RPM's is what you're looking for.

 

[jleonard]

"You should be able to reach close to 2400 RPM (WOT) with the appropriate prop"

You need to reach 2500 rpm.
I can do that with my Albin 40 and get up to a whopping 8.3 knots.

Normally I run at about 1750 and burn approx 1.5 gph as close as I can figure.

 

[Dom61]

I can hit 2400-2500 and just suck the fuel right out of the tanks* Dom

 

[Baker]

Dom, I don't think they are suggesting you do that on a regular basis. But that is a good measure of whether you're properly "propped".

 

[jleonard]

"But that is a good measure of whether you're properly "propped"."

Exactly. I run it up to WOT maybe half a dozen times thruout a season. Always as I am approaching the harbor entrance on my return trip., just in case something goes wrong. I run at WOT for about a minute, maybe 2. This lets me make sure I get to 2500 rpm, make sure I do not have black smoke, and let's me check out my cooling system to make sure there is no overheat.

 

[amdavis]

fuel milage

We took our Tung Hwa Clipper 30 out today for a jaunt around Vashon Is. from Tacoma. It was the first time out with her for an extended run (6 hrs.) I ran at 1750 rpm for 2 hours using aprox 3 gals. or 1 1/2 gph at 7+ kns. Then ran 2 hours at 2000rpm using aprox.6 gals. or 3 gph at 7.5 to 8 kns. Its a fl 120 with 3380 hours on the clock and seems to be propped about right as it will wind up to just shy of 2500 rpm

 

[Peter B]

That sounds about right. I find our sweet spot about 1750-1800 rpm.

 

[motion30]

I used to run my empty at 1650 rpm. 2.5- 2.8 gph. I slowed down to 1400 rpm and burned 1.75 gph. lost about 1 mile per hour

 

[yo]

We also run at 1750 --1800 every thing feel right at that speed.

 

[Nomad Willy]

I think this "sweet spot" stuff has little to do w the engine and all to do with how the boat was built. I also assume "sweet spot" specifically is the engine speed (rpm) that the least amount of vibration and noise is experienced. That mostly has to do w the dimensions and stiffness of bulkheads, hulls, cabin roofs, floors, fuel tank panels or the hull just above the props.

The engine "excites" such structural things, causing vibration and resulting in noise. Not too much unlike a tuning fork.

Everybody responds differently to noise and vibration. Some avoid it like the plague and others actually like it. I'm a "sweet spotter" too but Willy seems to have very little resonance. Smaller boat w possibly heavier structures and a much smaller engine. I suspect lots to most of the noise on Willy comes from the engine air intake. The intake of an engine needs muffling to a surprising extent like the exhaust. Haven't built a resonator intake box so I guess Willy seems quiet enough. Bigger engines (all other things being equal) make more noise.

 

[SomeSailor]

I run at 1650 most times. It seems to be the spot where the boat "feels" right and nothing is working too hard. I have digital tachs and getting everything sync'd to within a few RPM really seems to cut down on the resonance and noise.

 

[Ben]

A fellow boater just posted his readings on another thread. I was compelled to graph them. His engine is a Perkins 6.354 (twins) which is roughly the size of my ADC Ford Lehman 135. These readings should not be far off from your FL120 in terms of the curves and sweet spots in the graph. His are twins, but as I read his numbers, fuel use figures appear to be per engine. So what I can extract is a sweet spot around 1800 RPMs.

All current, prop pitch/diameter, hull shape parameters will impact this, but use as you wish as a guide.

 

[Blue Heron]

Ben,

Yes they are twins and usage is per engine. Glad I could help.

 

[FlyWright]

If that's the case, shouldn't the GPH be doubled and the NM/Gal be halved? The sweetspot of 1800 RPM remains, but the data plots shift.

 

[CaptTom]

Regardless of the current, the water you travel through will be the water you travel through, even if you don't get anywhere because of current.

All true, but let's try an extreme example.

Suppose my most efficient speed is 3 knots through the water, and I decide I'm going to save money by never exceeding that.

I head upriver and encounter a 3 knot foul current. My speed over ground drops to zero, my NMPG drops to zero and my ETA becomes infinite. Obviously I'm not saving any money, even if I'm in no hurry to get anywhere.

In fact, the faster I go, the less time I'll spend fighting the current before I get there.

Somewhere, there's a "sweet spot"; a trade-off between minimizing the time spent fighting a foul current and burning more fuel.

 

[psneeld]

All true, but let's try an extreme example.

Suppose my most efficient speed is 3 knots through the water, and I decide I'm going to save money by never exceeding that.

I head upriver and encounter a 3 knot foul current. My speed over ground drops to zero, my NMPG drops to zero and my ETA becomes infinite. Obviously I'm not saving any money, even if I'm in no hurry to get anywhere.

In fact, the faster I go, the less time I'll spend fighting the current before I get there.

Somewhere, there's a "sweet spot"; a trade-off between minimizing the time spent fighting a foul current and burning more fuel.

And that is where a flo scan or similar fuel monitor hooked to a gps and spitting out a NMPG number would be the use I would most benefit from.

Hand crunching the sweet spot would probably give me a headache if I had to do it all the time.

 

[Codger2]

And that is where a flo scan or similar fuel monitor hooked to a gps and spitting out a NMPG number would be the use I would most benefit from.

 

[twistedtree]

This may just be about terminology, but I feel compelled to jump in here. When we talk about "efficiency", the scientific meaning is work done per unit of time. In a car, it's measured in MPG. Similarly, in a boat, it's measured in nMPG. GPH, by itself, is a meaningless number when looking at efficiency.

A point of maximum efficiency would be some sort of local maximum on the nMPG curve. It would be a point where going either faster or slower yields worse nMPG. Such a point doesn't exist for a displacement boat.

When one talks about an "efficiency sweet spot", I start looking for a point where there is some maximum nMPG value, but for a boat that exists only at the boat's slowest speed. For every increase in speed, the nMPG number drops. For a displacement boat, it never even levels off. It's just all down hill the faster you go.

But there is also a subjective definition for a "sweet spot" which would be the point where the operator is happiest with the tradeoff between speed and nMPG. Going slower will always yield better nMPG, but most of us want to eventually get where we are going. Going faster will always yield worse nMPG, but we are willing to accept some degree of that in the interest of time. We all have that spot where we are operating at a happy tradeoff between nMPG and speed. Calling that a "sweet spot" is as good a term as any, but technically speaking, it's not a point of maximum efficiency. Not even close.

 

[psneeld]

I think "sweet spot" is now confusing in this thread...some I think were referring to where the engine and boat "sound" or "feel" in harmony...but little to do with efficiency...then some started using the term to mean where the boat is "relatively" efficient...but we all know that's a fleeting concept because of environmentals.

CaptTom made the only real point that I can see as far as looking for the most efficient rpm/hull speed translated to SOG...but I never have used the term sweet spot when referring to anything but when the engine sounds purdy .

 

[Ben]

I think "sweet spot" is now confusing in this thread...some I think were referring to where the engine and boat "sound" or "feel" in harmony...but little to do with efficiency...then some started using the term to mean where the boat is "relatively" efficient...but we all know that's a fleeting concept because of environmentals.

I started it! I am guilty! on Sweet Spot. My intention for my own selfish purposes was to name the general (you must pick what works for you) convergence of speed, fuel use, and RPMs so that the boat moved through the water at best speed with least fuel use.

As for the current, that is out of anyone's control.

"Sweet Spot" Ah ha ha ha ha ha! Now you must go crazy.

 

[CaptTom]

Maybe I did throw around words like "efficiency" and "sweet spot" loosely.

Still, if you go 3.1 knots against that hypothetical 3 knot current, you're making .1 knots of headway. You're burning fuel for one hour to go just over 600 feet.

Bump it up to 4 knots. That's one knot over ground. Now it takes you 6 minutes (1/10 of an hour) to go that same 600 feet.

There's no way running the engine for an hour at 3.1 knots burns less fuel than running it for 6 minutes at 4 knots.

Hence the made-up and un-scientific concept of a "sweet spot".

 

[Ben]

Maybe I did throw around words like "efficiency" and "sweet spot" loosely.

Still, if you go 3.1 knots against that hypothetical 3 knot current, you're making .1 knots of headway. You're burning fuel for one hour to go just over 600 feet.

Bump it up to 4 knots. That's one knot over ground. Now it takes you 6 minutes (1/10 of an hour) to go that same 600 feet.

0.1 knots is not sweet in anyone's book. Not sure I'd want to cruise the ICW doing 3.1 knots in a good current in any circumstance. That puts me at about 1050 RPM. Nice to pull in the slip with, but not much else. I'll run about 1750-1950 RPMs and do 6.7-7.5 most days. I'm not picky, but I do want to know how best to treat my engine and fuel burn.

The knowledge of the engine performance is the value. Now I just got to degrease, change pipes, and clean my heat exchanger! (I digress)

 

[FlyWright]

This may just be about terminology, but I feel compelled to jump in here. When we talk about "efficiency", the scientific meaning is work done per unit of time. In a car, it's measured in MPG. Similarly, in a boat, it's measured in nMPG. GPH, by itself, is a meaningless number when looking at efficiency.

A point of maximum efficiency would be some sort of local maximum on the nMPG curve. It would be a point where going either faster or slower yields worse nMPG. Such a point doesn't exist for a displacement boat.

When one talks about an "efficiency sweet spot", I start looking for a point where there is some maximum nMPG value, but for a boat that exists only at the boat's slowest speed. For every increase in speed, the nMPG number drops. For a displacement boat, it never even levels off. It's just all down hill the faster you go.

But there is also a subjective definition for a "sweet spot" which would be the point where the operator is happiest with the tradeoff between speed and nMPG. Going slower will always yield better nMPG, but most of us want to eventually get where we are going. Going faster will always yield worse nMPG, but we are willing to accept some degree of that in the interest of time. We all have that spot where we are operating at a happy tradeoff between nMPG and speed. Calling that a "sweet spot" is as good a term as any, but technically speaking, it's not a point of maximum efficiency. Not even close.

Well said, twistedtree. No problem with the term sweet spot from my perspective. I suspect most know we're talking about a subjective, personally-accepted compromise between noise, engine health, speed and fuel consumption. That was the point of the discussion here and seems to have carried through on this thread as well.

Nothing says that sweet spot can't change with the conditions of the day, i.e. working against a strong current.

What's the confusing part?

 

[psneeld]

Maybe I did throw around words like "efficiency" and "sweet spot" loosely.

Still, if you go 3.1 knots against that hypothetical 3 knot current, you're making .1 knots of headway. You're burning fuel for one hour to go just over 600 feet.

Bump it up to 4 knots. That's one knot over ground. Now it takes you 6 minutes (1/10 of an hour) to go that same 600 feet.

There's no way running the engine for an hour at 3.1 knots burns less fuel than running it for 6 minutes at 4 knots.

Hence the made-up and un-scientific concept of a "sweet spot".

I understand completely...you are giving great examples.

Unfortunately it is a lot of tables or graphs or hand calculations..or just a good fuel monitoring system that includes SOG for an accurate, up to the minute KMPG for day to day use...

In general...confusion over sweet spot?...not in my mind...as I've been around boats in general for over 50 years and marine engineers pretty much for the last 35 years.