RPM's And Fuel Used Per Mile

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Those fuel burn numbers are for hypothetical propeller curves. The real curves can look very different. Also, at a given rpm setting the engine may be able to produce say 240 hp while consuming 12.2 gph. That does not mean that the engine will always use 12.2 gph at that rpm setting. If the demand on the engine is much less than 240 hp, the fuel consumption will be proportionately less as Eric said. Furthermore, if the transmission is in neutral, the engine will use very little fuel no matter what the rpms are since the engine will be producing very little power.
 
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"It is an apples to apples comparison provided you remember that the RPM is the constant as it's a fixed/constant rate fuel flow at that given RPM and the variable in this experiment is speed."


Fuel burn is never a constant vs rpm on a diesel engine as you are not controlling the fuel load at all you are controlling a governor which is setting the fuel load to match a desired rpm.
One example - our 4 stroke diesels will burn about 1.2 gph at 3,000 rpm when they are in neutral. At 3,000 rpm with a 'full load' they will burn about 16.75 gph.
Same rpm but with drastically different loads and the difference is 14X.

Similarly your 'plugged' number of 1gph for the genset on that boat is likely very high. I would expect to see about 0.1 gph for every 1,000 watts of load give or take given your sized boat and likely loads.

MPG varies much more than most folks care to think about and efficiency speeds begin much lower than the old 'hull formulae' that floats around. I have taken just a little liberty in rounding the numbers below so that an 'older' boater like myself can remember the numbers but they are very close to the actual numbers for our past boat - a 47' long 15' beam 34,000# boat with twin inline 6 cyl 4 stroke turbo diesels.
4 nmpg - 6 knots
2 nmpg - 8 knots
1 nmpg - 10 knots
0.95 nmpg - 16 knots


Your boat will get similar increased efficiencies by running at lower speeds.
 
I think therin lies the problem, there is only one point at which all (fuel, torque, rpm, propeller) curves will intersect yielding the ever elusive efficiency spot. This spot also will vary with wind, waves, amount of cargo,persons and fuel onboard at any specific time on a specific boat. If the naval architect did a good job all these curves should intersect at the boat and the owner's happy spot.

To the original poster's point less speed usually equates to increased range but even that can vary.

I'm just about to do another 200 hour interval oil change and I've burned 1040 gallons since the last change which equates to 2.9 gph per engine at mostly 45% to 50% load and less than five hours of idling. Logbook shows we've covered roughly 1500 miles from block island to Miami dodging a couple of hurricanes along the way. Might not be our most efficient speed but it is pretty much in line with our grand average over the whole boats life and one I have felt comfortable using for planning purposes.
 
Nomad Willy said:
(HS) is a relationship between wave lengths and hull lengths. It's a numerical thing. It's just a point on the speed scale whereas the length of the hull matches the length of the wave a boat is making. No magic at all.
Click to expand...

Eric, I guess I should have qualified my comments about hull speed by saying they applied to my boat, not all boats. Here's a photo a friend took of my boat when it was running right at my hull speed. As you can see, the bow wake is very small and the wave the boat is making is almost the length of the boat. This is a very comfortable speed to cruise at and likely at or near the most efficient speed in terms of fuel economy.

img_491319_0_f067ddbcb92d360b113d124a891cd407.jpg
 
k9medic said:
If you look at page 2 under E rating you will see the fuel burn for the engine. Since I have two engines the rate is doubled.
Click to expand...

Then where did your 1100 RPM number come from as that chart just goes down to 1400 RPM from what I can tell?
 
BandB said:
Then where did your 1100 RPM number come from as that chart just goes down to 1400 RPM from what I can tell?
Click to expand...

Correct. I had to extrapolate the fuel burn at 1100rpm.

The extrapolation process came from fwebster on another forum who has put a lot of work into stuff like this and knows his way around a 3116.
 
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k9medic said:
Correct. I had to extrapolate the fuel burn at 1100rpm.

The extrapolation process came from fwebster on another forum who has put a lot of work into stuff like this and knows his way around a 3116.
Click to expand...

So the entire point you were proving was proved in your mind by extrapolating information from another source and not at all based on the CAT charts you referred to as your source.

None of it would be meaningful without load and props and all the other factors, but even your comparison might have made some sense, if you hadn't pulled the most important numbers to prove your case from two different sources.

I stick to what I would have assumed and that is at 1100 RPM you will get better nmpg than at 1400 RPM.
 
k9medic said:
As another, simple example, I fly for a living. At a particular power setting it does not matter if my aircraft is at gross weight or nearly empty. I still burn the same amount of fuel.
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Diesel engines work a little differently than you describe above. There is no "throttle". You are setting the governor. And the engine will try to maintain the RPM set and will add fuel to maintain that setting. Therefore, load definitely does cause a higher fuel burn. If you are a pilot you should know how a constant speed propellor works. THe governor on that propellor works very similarly to a governor on a diesel engine(flyweights opposing speeder spring pressure). Except the governor on a prop regulates oil pressure to set the speed of the prop. On a diesel engine, it uses that pressure to operate the fuel rack on the engine. IOW, as load increases and the engine begins to slow down, fuel is added in an attempt to maintain set speed. A generator is a good example. It has a constant speed governor that usually maintains 1800RPM. As load increases, you can hear the RPMs drag down and then recover. That recovery is fuel added to maintain speed so fuel flow is definitely affected by load. With your engines, you are setting the speed by setting spring pressure. You are not modulating fuel directly like you would the throttle on your car. You are setting the speed of the governor. And LOAD DETERMINES HOW MUCH FUEL IS ADDED!!!!!....not power lever position nor the number that your tach needle is pointed at.

I don't know what kind of airplane you fly, but the ones I fly can vary by THOUSANDS of pounds per hour based on load.
 
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GFC said:
Eric, I guess I should have qualified my comments about hull speed by saying they applied to my boat, not all boats. Here's a photo a friend took of my boat when it was running right at my hull speed. As you can see, the bow wake is very small and the wave the boat is making is almost the length of the boat. This is a very comfortable speed to cruise at and likely at or near the most efficient speed in terms of fuel economy.

img_491348_0_f067ddbcb92d360b113d124a891cd407.jpg
Click to expand...

The commonly used 1.34 figure people use is an approximation. By definition, the 'hull speed' for your waterline length will be when the length of the bow wave is exactly the same as your waterline length. And the use of 1.34 in the formula gives a maximum speed only for displacement hulls.

For semi-displacement or planing hulls the number to use (to get maximum speed in displacement mode) is higher than 1.34, depending on the calculated SLR which you use instead of 1.34. For my hull and displacement, SLRMax is 1.45. At SLR of 3 and above you are planing. Chapter 2 in Dave Gerr's Propeller Handbook is well worth reading. https://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/0071381767

And Baker, nice explanation of diesel fuel consumption vs load. I was going to try and clarify, but glad you did and made it so simple.
 
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Propellers move boats, engines just turn them

This may help some understand how I look at engine loading vs vessel speed... It's not an easy read, but this is how it works ..

Propellers Move Boats, Engines Just Turn Them - Seaboard Marine

As to hull speed, I call it the magic speed of the vessel and each vessel, depending on the size, weight, LWL, and conditions at the time, all have this "Magic Speed' to where the vessel travels easily with the engines just loafing.. It's real typical to see 2-3 MPG of many slippery hulls in the 30-45Ft range travelling in the 7-10Kt. range. Plus, as a bonus, you can typically enjoy that wine a cheese along the journey..

Cheers,

Tony
 
BandB said:
So the entire point you were proving was proved in your mind by extrapolating information from another source and not at all based on the CAT charts you referred to as your source.
[...]
I stick to what I would have assumed and that is at 1100 RPM you will get better nmpg than at 1400 RPM.
Click to expand...

BandB, the extrapolation was only to get the fuel burn rate for 1100 rpm. The rest of the information is posted by CAT so I I don't see how you could assert the the the rest of the data did not come from the charts.

If the math that I did to obtain the fuel burn at 1100 rpm is off then it is not of any significance to skew the data.

Here is the question that I ask - If the CAT numbers are not correct and there are too many variables, then why would the numbers in my logbooks that I have kept over the past few years show that my real world consumption versus NM traveled are similar?

Baker, I overly generalized that statement in an effort not to greatly confuse folks. Just like we use 6.8lbs as an approximate weight for Jet A since we all know that it is variable given the temperature.

To keep things simple and on a level that more people can relate to I'll use an example of a Piper Cherokee's performance chart. You can see from the chart that there are rpm settings and airspeed notations. This graph is based on a test aircraft configured at max gross weight. Will there be small variable in RPM and speed based upon weight changes, of course, but they are not if statistical significance in this case.


Engine%20Perforamnce%20Gaph,%20Langley%20Flying%20School.gif
 
Hello K9medic -

Please follow this logic if you will - find any boat test online for any boat roughly in the same length. Many of these boat tests will include fuel burned vs speed base upon fuel totalizers utilized during the tests. Find any boat within your size range perhaps 35-45' and see what happens between 7 knots and 10 knots of speed. I think you will see about double the efficiency when changing from 7 to 10 knots on any boat that is tested whether it is gas or diesel and independent of which diesel you are running.


"If the math that I did to obtain the fuel burn at 1100 rpm is off then it is not of any significance to skew the data."


Your fuel data per rpm is not representative of your specific load it is just a static chart based upon a propeller exponent. Once again the chart will not work if the boat was in neutral at that same rpm so how can it work for most any other load factor except for the exact load factor that the exponent was calculated on.


There are some great books that you can get and read on diesels and loading vs fuel burn one of the best for a real life description is "Marine Diesel engines" Calder , ISBN #0-87742-313-X
 
I would have to argue that the Cherokee 140 example as it relates to this discussion is almost completely irrelevant.
 
"As another, simple example, I fly for a living. At a particular power setting it does not matter if my aircraft is at gross weight or nearly empty. I still burn the same amount of fuel."


The airline industry , weight loss, and fuel....


"Researchers at MIT estimate that that cost of each passenger carrying a cellphone costs Southwest Airlines $1.2 million annually in weight-related fuel expenses."

The entire article is here...


Airline Industry Shaves Weight to Save Money


Many more articles on weight costs of fuel in aviation just google them up.
 
The main reason American Airlines left their airplanes unpainted treated aluminum was to save weight...of the paint!!!! The reason they have recently changed their paint scheme is because unpainted carbon fiber(787) doesn't exactly look good.
 
Remembering this is a boy's school experiment --

Are there any TF ites brave enough to post how they would do this fuel burn experiment? Keeping in mind that an audience of perennial naysayers awaits, some of whom need help keeping track of holding tank levels. :banghead:
 
k9medic said:
BandB, the extrapolation was only to get the fuel burn rate for 1100 rpm. The rest of the information is posted by CAT so I I don't see how you could assert the the the rest of the data did not come from the charts.

If the math that I did to obtain the fuel burn at 1100 rpm is off then it is not of any significance to skew the data.
Click to expand...

Not of any significance? The entire point you were trying to make was that the nmpg at 1400 RPM was the same it was at 1100 RPM and yet you've pulled those numbers from different sources and extrapolated and then you're comparing apples and oranges. Your data is skewed. If you think that your nmpg at 1100 and 1400 RPM is the same, which is what you came into this asserting and claiming proof of, then your data is skewed and you are wrong.

This is without getting into load and why all your numbers are in a theoretical world and don't necessarily represent any boat in the real world. Sure, the CAT charts are useful places to start. They may or may not approximate your boat at your normal load, but that is not the way to go about figuring fuel consumption. We've built charts for our boats just like Boattest and PMY and the others who run tests do, with integral digital fuel flow meters or systems such as Floscan. We've also compared our numbers to those of the factory and of other testing available publicly. Others are very happy with estimates based on their normal speeds run and simply dividing gallons used by distance traveled which gives them what they need to project their normal range.

As an aside, our manufacturers have all been conservative in their estimates of speed, range, and fuel consumption. Universally, we have done better in actual usage than their charts. They are intentionally conservative because they don't want to have customers complaining they can't reach their numbers.

This is unlike the lake in which everyone claimed speeds they couldn't reach. Our boat ran 50+ knots, a top of about 60 mph (63 mph with light load and smooth water and trimmed all the way out) and it's amazing how many boats claiming to run 60 mph on the lake couldn't come close to keeping up with us. Most ran closer to 50 mph. The traditional speedometers on runabouts the the tube on the transom are notoriously inaccurate. Small boat manufacturers were very reluctant to go to GPS which would then bring the truth of speed out.
 
sunchaser said:
Remembering this is a boy's school experiment --

Are there any TF ites brave enough to post how they would do this fuel burn experiment? Keeping in mind that an audience of perennial naysayers awaits, some of whom need help keeping track of holding tank levels. :banghead:
Click to expand...

Yes, Floscan or other equivalent equipment.

The problem wasn't the boy's, but the information supplied by the father as to usage.
 
Is there a formula to calculate what your fuel burn would be if you shut down one of your engines?
And is there a formula to calculate how much the load would increase on the engine that remains running?
 
BandB said:
Yes, Floscan or other equivalent equipment. /QUOTE]

Yup, several thousand dollars of equipment will do the trick. Some of this equipment may even be more accurate than extrapolating a manufacturers curve.
Click to expand...
 
k9medic wrote;
"As another, simple example, I fly for a living. At a particular power setting it does not matter if my aircraft is at gross weight or nearly empty. I still burn the same amount of fuel."

Yes but your airspeed will drop. And your angle of attack (wings) will increase. And the drag of the wings will remain the same .. in balance. So that's true only if you loose airspeed. Same can be said about a boat. But that makes the statement meaningless.

However if you increase the weight of a boat w/o touching the throttles your speed will decrease and the prop/s will reduce rpm and fuel consumption will actually decrease. Just fewer piston strokes to inhale fuel.
 
FOG said:
Is there a formula to calculate what your fuel burn would be if you shut down one of your engines?
And is there a formula to calculate how much the load would increase on the engine that remains running?
Click to expand...
too many variables, not only with different type boats, but even with an individual boat.
 
"Are there any TF ites brave enough to post how they would do this fuel burn experiment? Keeping in mind that an audience of perennial naysayers awaits, some of whom need help keeping track of holding tank levels"


12 gallon remote fuel tank , 3 way "T" valve, bathroom scale and a few hoses and clamps.....


- Put clean diesel fuel into the 12 gallon remote tank
- Weigh the tank
- "T" into your fuel supply and return lines on the upline side of your filters (one engine) to the bottom of the remote tank
- Switch the one engine to the remote tank and operate the boat at the desired measured speed for a finite period of say 10 minutes. Stop the engine.
- Weight the tank and record the difference in weights
- Do this for varied rpms either in gear or out of gear as you see fit.
 
sunchaser said:
BandB said:
Yes, Floscan or other equivalent equipment. /QUOTE]

Yup, several thousand dollars of equipment will do the trick. Some of this equipment may even be more accurate than extrapolating a manufacturers curve.
Click to expand...

Except it's not several thousand dollars of equipment. ($700-1800 as a range). Also, standard with many boat set ups today. And it's definitely more accurate than an engine manufacturers curve with no boat involved. It likely is not much different than a boat manufacturer's tests and most boat builders do have actual curves from live tests which also show the load at which they were run.
Click to expand...
 
BandB said:
sunchaser said:
Except it's not several thousand dollars of equipment. ($700-1800 as a range). Also, standard with many boat set ups today. And it's definitely more accurate than an engine manufacturers curve with no boat involved. It likely is not much different than a boat manufacturer's tests and most boat builders do have actual curves from live tests which also show the load at which they were run.
Click to expand...

All for a kid's science experiment. I do like the $700 - 1800 dollar range estimate, for a 42' twin engine Searay that would be a real bargain assuming you hired a tech to install it.
Click to expand...
 
sunchaser said:
BandB said:
All for a kid's science experiment. I do like the $700 - 1800 dollar range estimate, for a 42' twin engine Searay that would be a real bargain assuming you hired a tech to install it.
Click to expand...

The question wasn't what about for a kid's science project, it was how to get better information. I was simply responding to that and then responding to the several thousands of dollars comments.

For the kid's science project, don't furnish the kid with fabricated numbers based on multiple choices. Don't pull 1400 RPM and above from a CAT chart and then extrapolate 1100 RPM from some other chart of someone else's that likely isn't comparable and doesn't work. Then also, if the answer doesn't make sense, go back and question your assumptions and data. In this case we all know the answer was incorrect. Now we know why.
Click to expand...
 
Insequent said:
The commonly used 1.34 figure people use is an approximation. By definition, the 'hull speed' for your waterline length will be when the length of the bow wave is exactly the same as your waterline length. And the use of 1.34 in the formula gives a maximum speed only for displacement hulls.

Not arguing but not so. There is a Willard 30 on WBO that has a top speed of 8 knots. With a 27.5' wll the hull speed is 7 knots. Again .. Just a numerical point re wave lengths and speeds.
For semi-displacement or planing hulls the number to use (to get maximum speed in displacement mode) is higher than 1.34, depending on the calculated SLR which you use instead of 1.34. For my hull and displacement, SLRMax is 1.45. At SLR of 3 and above you are planing. Chapter 2 in Dave Gerr's Propeller Handbook is well worth reading. https://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/0071381767


Nomad Willy wrote;
"Only FD boats "function" in a displacement mode. SD and planing boats just go faster .. and slower. Their fuel consumption goes up and down. The stern of the boat needs to be shaped to take advantage of the (match up) of hull shape and wave shape. The FD hull surfs, to an extent pushing the boat along. A bit like the SanFrancisco Bart subways. The subway train uses it's motors to propel the train along on the level and up hills. Going down hills the motors switch to a generator function and retreve some of the energy lost going up hills. Same w a FD boat. The stern wave pushes the FD boat fwd to help it along so less power is required to go fwd and create the bow wave.
SD and planing hulls have the wide and deep transom that causes lots more drag that cancels out what I call the Bart Boost.
Of course there's no black and white. This function happens in degrees. Boats that are nearly FD in form function nearly as a FD boat. And boats that aren't don't."



And Baker, nice explanation of diesel fuel consumption vs load. I was going to try and clarify, but glad you did and made it so simple.
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Gotta go look at this.
Sure, as long as the speed is maintained consumption will be reduced if the plane/boat are lighter.
 
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"All for a kid's science experiment. I do like the $700 - 1800 dollar range estimate, for a 42' twin engine Searay that would be a real bargain assuming you hired a tech to install it."

Most of the newer Sea Rays have an engine load gage on the boat already - find a similar boat or a published test of one of the boats and extrapolate from there - It will be 10X more accurate than the figures that K9medic currently has for the comparison between 6, 7 and 10 knots.
 
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