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Old 08-07-2022, 10:12 PM   #1
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Measuring Diesel Usage for Range Calculations

Hi All. I have a Gardner 6LXB powered vessel, and without purchasing specialist electronic equipment, I would like measure my fuel consumption from my cylindrical fuel tanks so as to measure range at various speeds. I use a dip stick, when refueling as a rough guide, but am thinking about some form of inline measuring tank to calculate gallons per hour at cruising speeds. (Liters per hour actually, as I'm a Kiwi lol). I would remove the system once i have a reliable graph/table. I would be grateful for any suggestions on what you have used, and recommend?
Thanks in advance
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Old 08-08-2022, 05:46 AM   #2
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Floscan

Have a look at http://www.floscan.com for diesel applications.
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Old 08-08-2022, 06:15 AM   #3
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Greetings,
Welcome aboard. Sounds like you are looking for an accurately calibrated day tank. No need to remove after you get your sums as a few TF members have day tanks as an integral part of their fuel systems.
It would seem to be that fuel usage would also, very much, depend on sea conditions. You might be better off installing ..."specialist electronic equipment..." to be able to monitor your usage under a greater variety of situations.
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Old 08-08-2022, 06:50 AM   #4
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Thumbs up to RT Firefly's suggestion of a calibrated day tank. When PAE/Nordhavn set upon preparing a Nordhavn 40 for their around the world campaign circa 2003, fuel burn was of course a huge concern. I forget the size of the tank, but it was instrumental in planning. They did a couple test runs from Dana Point to PNW to fully test the fuel burn. The first leg of their RTW run to Hawaii would be the trips longest, and they arrived with a comfortable reserve.

Peter
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Old 08-08-2022, 10:54 AM   #5
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Find a rectangular or square tank as a temporary tank. Figure the internal liters per cm. Cruise, measure the tank at some interval and do some more math.
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Old 08-08-2022, 11:11 AM   #6
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Nice engine! First question: Do you know beyond a shadow of a doubt the actual fuel capacity of your cylindrical tank?

The only sure way is to empty it, and then fill it to the full mark. Everything else is an approximation.

The other issue is that if you are using any system that uses GPS only for speed, you really don't know what your speed through the water is. Once again, you can approximate based on tides, estimated currents, etc, but the only way to know for sure is to measure speed through the water.

There are systems out there that measure both the fuel to the engine, and the return fuel from the engine to the fuel tank, subtract, and give you actual fuel burn. They tend to be very expensive.

A less expensive, although more complicated way to do it is to build a graduated cylinder that holds, say 5 gallons. Calibrate it at every say 1/10th of a gallon by filling and marking. The taller and thinner, the more accurate the change in height of fuel is. Plumb directly into fuel line to the engine supply and fuel return. Set up your engine rpm where you want to test, and while running at that speed, top off the cylinder to the full mark (this will require two people, one running the boat, one managing the fuel), run for a predetermined time, say at least 15 minutes (the longer the time, the more accurate the measurement, mark the fuel consumed. Don't worry about the calculations now, you can do that later.
Set up your next RPM, fill to full mark, start timer, run . . . etc, etc, etc,
It's complicated in execution, but not difficult. You only need to do it once in order to have the information forever.

I'm not familiar with your boat, so I can't make estimates however you may be interested in the following:
Our boat is a Beebe Passagemaker, 50'. We are returning from a trip from Puget Sound, Washington to Glacier Bay, Alaska. When we departed, we weighed roughly 86,000 lbs. Over the course of 2,800 miles so far, we have averaged just under 3 gph, including use of generator and hydraunic heating.

Boat is powered by a Gardner 8LXB.
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Old 08-08-2022, 11:31 AM   #7
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A "burn tank" like you describe is probably the best way to do it. I would think something in the 8-10 liter range would work and allow you to run long enough to get an accurate measurement.


The tank should be taller rather than wider. That will allow you to get more accurate measurements. Then fill and calibrate it very carefully. Accuracy will really matter.


You will need both the engine draw and return to go to the burn tank, and will need a way to refill the burn tank.


Then do opposing runs, again with very accurate time and speed over ground measurements. The longer the run the better, and the calmer the conditions the better.


From there you should be able to calculate nautical miles per gallon/liter which is ultimately what you need to know for a long distance run.


You could also do some runs into head seas to see how much of an impact it has. This can be significant for a long trip.
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Old 08-08-2022, 12:25 PM   #8
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Quote:
Originally Posted by Dwk View Post
Have a look at http://www.floscan.com for diesel applications.
Glad to read, FloScan is back in business or still in business.
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Old 08-08-2022, 12:39 PM   #9
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Dipstick should be good enough
We have sight tubes so same concept.

We did ours by sliding electrical tie to level (dip stick and mark)
Add 1000 litres and mark
Note hours
Go on a cruise at normal cruise speed until back to original mark
Note hours
Maths.

Get a near enough hourly fuel burn at cruising RPM
Which for us was pretty much what the engine specs said we burn at that RPM

Spend floscan and daytank money on more Diesel for actual cruising miles.
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Old 08-08-2022, 01:05 PM   #10
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Is your cylinder vertical or horizontal? Both have online calculators that can be used to closely determine volume at various fill points. For absolute accuracy take measurements at every 50 or 100ltrs at the next fill.

I'd agree that a sight tube is easier and more accurate than a dip stick if you can get access and allows burn to be measured in all weather and sea states.
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Old 08-08-2022, 02:02 PM   #11
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I assume that you are wanting fuel consumption data in a fairly narrow range, say 60% of full displacement speed up to 120%. There are several direct ways of doing this as have been discussed above including Flowscan and a day tank.

There is an indirect way and that is to use the engine's specific fuel consumption which Gardner says is 0.330 lb./hp/hr and apply that to a hp vs speed curve that you can develop from Boatdiesel's online app.

It won't be entirely accurate as specific fuel consumption does vary with load and the Boatdiesel data is calculated not measured, but it should be ok within maybe 15%.

David
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Old 08-08-2022, 03:41 PM   #12
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Thanks for your comments David. I had no idea of the mentioned Gardner fuel consumption. Probably heading to the day tank idea, as a result of feedback. Cheers
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Old 08-08-2022, 03:42 PM   #13
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Quote:
Originally Posted by Dwk View Post
Have a look at http://www.floscan.com for diesel applications.
Thanks for your comments David
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Old 08-08-2022, 03:50 PM   #14
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Quote:
Originally Posted by RT Firefly View Post
Greetings,
Welcome aboard. Sounds like you are looking for an accurately calibrated day tank. No need to remove after you get your sums as a few TF members have day tanks as an integral part of their fuel systems.
It would seem to be that fuel usage would also, very much, depend on sea conditions. You might be better off installing ..."specialist electronic equipment..." to be able to monitor your usage under a greater variety of situations.
Thanks for your comment RTF - Heading for the suggested day tank now. Cheers
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Old 08-08-2022, 03:51 PM   #15
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Quote:
Originally Posted by mvweebles View Post
Thumbs up to RT Firefly's suggestion of a calibrated day tank. When PAE/Nordhavn set upon preparing a Nordhavn 40 for their around the world campaign circa 2003, fuel burn was of course a huge concern. I forget the size of the tank, but it was instrumental in planning. They did a couple test runs from Dana Point to PNW to fully test the fuel burn. The first leg of their RTW run to Hawaii would be the trips longest, and they arrived with a comfortable reserve.

Peter
Thanks for your comments Peter - cheers Will
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Old 08-08-2022, 03:52 PM   #16
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Quote:
Originally Posted by Lepke View Post
Find a rectangular or square tank as a temporary tank. Figure the internal liters per cm. Cruise, measure the tank at some interval and do some more math.
Thanks for your comments Lepke!
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Old 08-08-2022, 03:57 PM   #17
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Quote:
Originally Posted by slowgoesit View Post
Nice engine! First question: Do you know beyond a shadow of a doubt the actual fuel capacity of your cylindrical tank?

The only sure way is to empty it, and then fill it to the full mark. Everything else is an approximation.

The other issue is that if you are using any system that uses GPS only for speed, you really don't know what your speed through the water is. Once again, you can approximate based on tides, estimated currents, etc, but the only way to know for sure is to measure speed through the water.

There are systems out there that measure both the fuel to the engine, and the return fuel from the engine to the fuel tank, subtract, and give you actual fuel burn. They tend to be very expensive.

A less expensive, although more complicated way to do it is to build a graduated cylinder that holds, say 5 gallons. Calibrate it at every say 1/10th of a gallon by filling and marking. The taller and thinner, the more accurate the change in height of fuel is. Plumb directly into fuel line to the engine supply and fuel return. Set up your engine rpm where you want to test, and while running at that speed, top off the cylinder to the full mark (this will require two people, one running the boat, one managing the fuel), run for a predetermined time, say at least 15 minutes (the longer the time, the more accurate the measurement, mark the fuel consumed. Don't worry about the calculations now, you can do that later.
Set up your next RPM, fill to full mark, start timer, run . . . etc, etc, etc,
It's complicated in execution, but not difficult. You only need to do it once in order to have the information forever.

I'm not familiar with your boat, so I can't make estimates however you may be interested in the following:
Our boat is a Beebe Passagemaker, 50'. We are returning from a trip from Puget Sound, Washington to Glacier Bay, Alaska. When we departed, we weighed roughly 86,000 lbs. Over the course of 2,800 miles so far, we have averaged just under 3 gph, including use of generator and hydraunic heating.

Boat is powered by a Gardner 8LXB.
Hi There. Thanks so much for your detailed reply. I do have the exact max volume for the tanks, but will not normally keep them fully filled if that makes sense. On your advice and those of others, I'm going to head for a day tank, or at least a measurement system, and do plenty of trials to gain a full picture. Much appreciated. Cheers Will PS love the boat, and go the Gardner's
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Old 08-08-2022, 04:08 PM   #18
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Quote:
Originally Posted by twistedtree View Post
A "burn tank" like you describe is probably the best way to do it. I would think something in the 8-10 liter range would work and allow you to run long enough to get an accurate measurement.


The tank should be taller rather than wider. That will allow you to get more accurate measurements. Then fill and calibrate it very carefully. Accuracy will really matter.


You will need both the engine draw and return to go to the burn tank, and will need a way to refill the burn tank.


Then do opposing runs, again with very accurate time and speed over ground measurements. The longer the run the better, and the calmer the conditions the better.


From there you should be able to calculate nautical miles per gallon/liter which is ultimately what you need to know for a long distance run.


You could also do some runs into head seas to see how much of an impact it has. This can be significant for a long trip.
Hi there, and thanks for taking the time to reply. I'm going to head for a day tank and put the effort in to getting accurate measurements. cheers Will
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Old 08-08-2022, 04:11 PM   #19
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Quote:
Originally Posted by Simi 60 View Post
Dipstick should be good enough
We have sight tubes so same concept.

We did ours by sliding electrical tie to level (dip stick and mark)
Add 1000 litres and mark
Note hours
Go on a cruise at normal cruise speed until back to original mark
Note hours
Maths.

Get a near enough hourly fuel burn at cruising RPM
Which for us was pretty much what the engine specs said we burn at that RPM

Spend floscan and daytank money on more Diesel for actual cruising miles.
Thanks for your reply. Unfortunately my tanks don't have sight glasses, and the dipstick is hard to get accurate enough, so I'm heading for a day tank. Really appreciate your reply thanks. cheers Will
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Old 08-08-2022, 04:13 PM   #20
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Originally Posted by GoneDiving View Post
Is your cylinder vertical or horizontal? Both have online calculators that can be used to closely determine volume at various fill points. For absolute accuracy take measurements at every 50 or 100ltrs at the next fill.

I'd agree that a sight tube is easier and more accurate than a dip stick if you can get access and allows burn to be measured in all weather and sea states.
Thanks Gone Diving. The cylinders are horizontal, and hard to get accurate measurements from with the dipstick, so I'm heading for a day tanks. Appreciate your comments. Cheers Will
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