Measuring Diesel Usage for Range Calculations

[twistedtree]

So why not simply fill them
Go out and do a decent amount of hours at cruise rpm
Fill tanks again
Maths.

That'll give you your hourly burn and can work out range from that.
No need to muck around with day tanks etc.

I think because he wants so-called polars telling him how the boat performs at different speeds and in different conditions.

 

[North Baltic sea]

Hi, also take into account the conditions wind, waves and currents, also the cleanliness of the hull. Sometimes the wind and current give more range and sometimes they reduce it. It's different for every boat model, even if it's the same engine. for me, the conditions can be in the order of +-20% of the normal reading that I see on the trip computer.

Nbs

 

[sunchaser]

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.

Scott
Given your vessel has seen some long distance sea time over the years, how is the fuel system set up that may be of interest to the OP's questions?

 

[slowgoesit]

Scott
Given your vessel has seen some long distance sea time over the years, how is the fuel system set up that may be of interest to the OP's questions?

Tom, similar to many vessels built in the '70's through the '90's, our boat has a fuel supply manifold that allows the user to select which tank supplies fuel to the various appliances, in our case, main engine, auxiliary engine, hydraunic heat, or fuel transfer pump (to move and polish fuel from tank to tank). The fuel source is one or more of 4 fuel tanks. We only draw from one tank at a time.

The return manifold selects which tank returned fuel goes back to, as well as accepting returning fuel from main engine, auxiliary engine, and fuel transfer pump. The hydraunic heating system only takes the fuel it needs, so it has no return fuel line.

In order to get an accurate record of fuel burned, you have to tap into the fuel supply line at the latest point before the main engine, and the return line after it leaves the engine. Those two lines go into your graduated cylinder, or "day tank". Calculate fuel burn over time and distance traveled.

 

[Benthic2]

Before you actually commit resources to this, I would like to suggest an excercise. Sit down and draw out the system you intend to install and make up some hypothetical measurements. Then consider how accurate your RPM guage is and how accurately can you measure it. Do you really know the difference between 1650 & 1700 rpm ? (Some tachs have more increments than others ) Then consider you are taking a measurement of a liquid in a tank on a moving boat. What kind of accuracy will you get there.....probably within a half centimeter at best. Margin for error is cumulative. If each of the previous 2 measurements are only 95% accurate, your margin for error is now 10%. Then add in a margin of error for wind speed and direction, current speed and direction, air temp, sea state, and number of passengers. Each of those factors adds uncertainty to a number that is only applicable when conditions will be exactly the same, which they never will be. Each day is unique. The margin for error grows again. Then double all of these error margins because you will be doing the measurements twice. Once during testing, and then once when applied to your cruising. Then you are going to add a wild a#$ guess to a reserve capacity at the end that further reduces accuracy.

My point is: You can spend a lot of time and money to get a number that is probably 85% accurate at best and only marginally useful in actual usage due to different conditions......or you can spend an hour online finding a performance chart for your engine that will be just as useful.

Just something to think about.

 

[BobMc]

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
Will

There's no need by buy any expensive equipment (other than fuel). You already have everything you need.

Fill the tanks. Run at your normal cruising speed for a hundred hours or so (or 50 hours if you burn a lot of fuel), then refill the tanks. Divide fuel used by number of hours. Easy peasey. Doesn't all have to be done at once if you're not cruising long distances.

Beats any other method of determining fuel usage. We found with Sanderling, we burned 1.75 gallons per hour at our cruising speed of 6.3 knots; day in, day out, including generator usage, waiting for bridges, in and out of marinas, etc. While the fuel gauges were handy for a rough estimate, we always relied on engine hours to determine when we needed to get fuel. 500 gallons will last for quite a few miles if you know your consistent burn rate.

 

[NWpilot]

Hi Will

If your fuel tank is round you can get pretty accurate measurements of fuel usage with a dipstick. Some tanks have flat ends which wont have an affect. Bulbous or rounded tank ends will cause the readings to be skewed towards the conservative for the middle 2/3's of fuel consumption. The critical factor is that you need access so you can measure the tank dimensions.

Now for the method:

Here is a website that explains the math for calculating how much liquid is in a partially filled tank.
https://www.hagra.nl/glossary/convertors/horizontal-tank-volume-calculator.html
It also has calculators for volume, both full and partial.

Here is another website that allows you to produce a chart that allows you to read the fuel remaining based on a measurement of depth.
https://www.odayequipment.com/tank-calculator/
It can be produced in 1" or 1/8" increments.

Except for tanks with rounded ends I would think you would find using a chart to be accurate enough to be within your safety factor. Tank manufacturers commonly use this method to confirm tank capacity.

Good luck

 

[Barking Sands]

I dont know. After the discussion here it sounds like fancy electronic equipment is easier and faster. It can also give that feedback the moment you get into a particular condition or sea state right up front VS waiting to check burn rates over time.

Cost may be comparable too depending on how fancy either set up becomes.

Other than that if you just need generalized data I would use the method BobMc describes.

 

[Simi 60]

I dont know. After the discussion here it sounds like fancy electronic equipment is easier and faster. It can also give that feedback the moment you get into a particular condition or sea state right up front VS waiting to check burn rates over time.

Cost may be comparable too depending on how fancy either set up becomes.

Other than that if you just need generalized data I would use the method BobMc describes.

It's as simple, complicated or expensive as you want it to be.

Our fuel burn at various rpm matches up near enough to the published data from Cummins.
I'm sure Gardner's would as well.

6lxb below

 

[Wolfgang]

Hi Will, day tanks have almost disappeared completely from boats. Wonder why?
You have two things to solve: 1) The calibration of your horizontal tank volume converted to the use a dipstick and 2) your quest for knowing the engine diesel consumption per hour at different rpm.
There are mathematical solutions for item 1.
For item 2, if you really want to know liter/hour across the whole range of rpm’s, I suggest this: (I think every diesel driven boat should have it!) After the primary fuel filter a T (2way) valve needs to be installed. Then have a 5 gallon diesel jerry can rigged up to that valve. That way you can meter the diesel to the engine. The fuel return line needs a T-valve too to direct the return flow to the jerry can. That way , with another jerry can used to refill, you can exactly measure your fuel consumption at every rpm. This is a primitive day tank! Well, if you say so.

 

[GoneDiving]

For item 2, if you really want to know liter/hour across the whole range of rpm’s, I suggest this: (I think every diesel driven boat should have it!) After the primary fuel filter a T (2way) valve needs to be installed. Then have a 5 gallon diesel jerry can rigged up to that valve. That way you can meter the diesel to the engine. The fuel return line needs a T-valve too to direct the return flow to the jerry can. That way , with another jerry can used to refill, you can exactly measure your fuel consumption at every rpm. This is a primitive day tank! Well, if you say so.

Just thinking out loud here, but if the return line is T'ed into a valve between the filter inlet and a flow meter, then an instantaneous nett reading would be available at any time or load.

Obviously the valve couldn't be left open all the time or the cooling function etc of return fuel would be lost but for the minute or two required for instantaneous readings it wouldn't be a problem. Readings at all speeds and comparisons over time would be easy and fast.



Cheers

 

[OldDan1943]

Based upon my own experience, install a FloScan and be done with it. It will give you an instantaneous read out during the real world operations.

You can also search the internet for the engine make and model.
Finally, search the internet for forum specific to your brand of boat and engine.

 

[Captain DJ]

I have used both Floscan (twins Lehmans 120 with 4 units calibrated by the previous owner after installation) and on my previous boat, a GB 42 Classic with twin DD 4-53's, I made a 40 US Gallon and 40-inch high day tank with sight gauge viewable from master aft stateroom.
I mention this because I have way more experience (and confidence) in the use of a calibrated day tank vs expensive and sensitive Floscan gauges. The 100 hp DD used 1.35 US Gallons per hour per engine at 7 knots (1450 rpm) over flat water. The Floscan on a smaller 34-foot Cheoy Lee is all over the place, with the Port engine often using twice the fuel or more than the Starboard engine. Both "reportedly" used/using far more fuel than the norm for Lehman 120s at 1500 RPM. That being said, I know recalibration of 4 gauges and reporting tachometers is in my future.

The other MAIN advantage of a day tank is the pre-filtered diesel always available for the 10-12 hr days usage. If a filter clogs from the transfer, it is easily dealt with in the morning while filling up the day tank and recording the previous day's usage in the log. All done while still at anchor or mooring. So much easier than a clogged fuel filter/water separator during a passage.

My two centavos make the permanent installation of say a 100-liter day tank win every time.

 

[Mobcat]

KISS …., it’s not hard this answer a day is cool and I have a beautifuly calibrated day tank but in prior boats I didn’t so I just used a temporary 20lt oil drum filled from my main tank by my polishing system I simply took the fuel and returned the fuel to the temporary tank to all my calcs at different rev ranges formed a spread sheet over a 500nm journey and lived by that spread sheet for planning for the rest of the time I had that trawler which was around 15000 nm of cruising and never had a fuel problem or expensive and complicated fuel management systems ……. KISS works well for trawlers in some ways and this is one of them.

 

[OldDan1943]

KISS …., it’s not hard this answer a day is cool and I have a beautifuly calibrated day tank but in prior boats I didn’t so I just used a temporary 20lt oil drum filled from my main tank by my polishing system I simply took the fuel and returned the fuel to the temporary tank to all my calcs at different rev ranges formed a spread sheet over a 500nm journey and lived by that spread sheet for planning for the rest of the time I had that trawler which was around 15000 nm of cruising and never had a fuel problem or expensive and complicated fuel management systems ……. KISS works well for trawlers in some ways and this is one of them.

Even simpler, I called the boat builder and asked for and got the commissioning graph then adjusted for real life.

 

[Will Fransen]

Thanks Captain DJ for your thoughts. Definitely heading down the day tank path, cheers Will

 

[Will Fransen]

Hi Mobcat. yes this is pretty much what has been suggested and the way I'll go. Thanks for taking the time to reply. Cheers Will

 

[Will Fransen]

Hi Steve. Yes, on my boat the diesel return will be easy to rout to the day tank, so I'll be able to get a more accurate picture. Thanks for your message. Cheers Will

 

[Will Fransen]

Hi Bob. Thanks for your advice. I will take that on board for sure, as once fuel burn per hour/s are established it will be relatively easy - I generally cruise at 7.5 knts anyway. Thanks for your thoughts. Cheers Will

 

[Will Fransen]

Hi there. Thanks so much for your thoughts. On my boat the dipping isn't that straightforward, albeit I do so when I refueling. To gauge consumptions are varying speeds, I now intended to set up a day tank. Thanks for you reply, Cheers Will

 

[Will Fransen]

Hi Snapdragon 3. Thanks very much for taking the time to reply. I'll take your thoughts on board. Cheers Will

 

[Will Fransen]

Hi again Simi 60. Thanks so much for the graph - This is awesome! This sort of confirms my expectations, but I'm going to try to establish a bit more fact based on my boat. Very grateful thank you. cheers Will

 

[Will Fransen]

Hi Wolfgang. Thanks for your comments. Yes - will be going along the lines you've suggested. Cheers Will

 

[Whodgins]

Hi Will, you've received some excellent advise from well experienced people here on this thread and sounds like you've now got a clear idea of what will work best for you in measuring your fuel usage. We have a 6LXB on our relatively recently launched boat, a 24m all aluminium eXtreme eXploration Passage Maker and so I read your post and this thread with great interest.

If it helps you, here is how I look after fuel usage that seems to be working well for us so far.

I have both the electronic method using Maretron fuel flow meters to measure instantaneous fuel usage rates, accurately calibrated fuel tank gauges and a "manual" easy to read graduated sight gauge on large day tank. I am glad to have all of these and find their respective data points to be useful and I record them all in our engine log with typical ER inspections about ever 1-2 hours while underway.

For your stated goal of using your fuel consumption data to calculate range on passages, my experience on all our boats has been that the larger sample size is the most useful and accurate. So add my vote to those recommending that you have a way to measure a full tank refill as accurately as possible and then use your total trip distance since last fill up to determine the average fuel consumption.

The instantaneous fuel flow meter readings are interesting to watch but they are similar to the same type of digital readout instantaneous fuel use gauges you see on the dashboards of most cars these days in that the readings range and change a lot as you watch them while underway, even when sea conditions are relatively calm and consistent. You can even out these readings by adjusting the time interval these readouts display but keep in mind that they are taking relatively constant instantaneous readings of the actual fuel flow. Great data to have and helps me track the effects of changes in sea conditions, engine RPM, EGT temps, CPP pitch, etc. but I don't find this to be as helpful for determining our fuel range on a passage which I too very much want to track.

Therefore I find that having the largest sample size of fuel volume and nautical miles covered, provides the most accurate way of estimating what our actual fuel use and range will be.

As several others have mentioned, I have also found it very useful to do some of the math to calculate the more theoretical fuel consumption rates for a given engine. I’ve been able to accumulate a pretty good collection of data for the Gardner 6LXB and I’ll attach one of the more helpful which is the graph from a dynamometer test on a Gardner 6LXB. Of course it all depends on what HP rating the engine has been set up for and the graph here is for the 150HP @ 1650 RPM Continuous duty version, which is what my 6LXB is currently set at. You can confirm what your engine is by the data stamped on the brass plate attached to the fuel injection pump body. Sorry this graph is not the best quality but I have found it to be very useful. If it helps you do some of the math around this, the specific fuel consumption rate of the Gardner 6LXB when directly coupled to the dynamometer is 0.330 lb./per b.h.p./hour which represents an overall thermal efficiency of 39.73%. From what I’ve read over the years, this is apparently one of the highest efficiencies of any production diesel engine in these size ranges. But of course, this is all based on laboratory conditions on a dyno and it will be the data you collect in your very real world use of your boat and engine that will provide the most accurate and useful information.

I would be glad to share any of the other data points and experiences I have with our 6LXB so just PM me if there is more that you’d find helpful.

Hope this helps you in your quest for better estimates of your fuel use and range as you venture out more and more. My wife and I spent many years in your fabulous part of the world cruising in our previous sailboat from your home base up through Fiji, Vanuatu, Marshall Islands and more and we are already looking forward to getting back there on our new boat so we’ll hope to one day be able to share an anchorage with you.

-Wayne

 

[mvweebles]

Therefore I find that having the largest sample size of fuel volume and nautical miles covered, provides the most accurate way of estimating what our actual fuel use and range will be.

Not necessarily. This gives accurate average consumption, but leaves you guessing if you encounter non-average/adverse conditions during a long passage.

The small, highly calibrated tank is the only way I can think of to get confident and actionable readings of consumption vs rpm. While I have no experience with Maretron systems, the OBD computers in vehicles are often 10% or more off in measuring MPG. My guess is onboard measurements from diesel OEM are not any better.

Peter

 

[Whodgins]

Not necessarily. This gives accurate average consumption, but leaves you guessing if you encounter non-average/adverse conditions during a long passage.

The small, highly calibrated tank is the only way I can think of to get confident and actionable readings of consumption vs rpm. While I have no experience with Maretron systems, the OBD computers in vehicles are often 10% or more off in measuring MPG. My guess is onboard measurements from diesel OEM are not any better.

Peter

Good point and quite right about the accuracy and usefulness of these small calibrated tanks Peter. On long week+ passages though, I have found the very short duration of small calibrated measurement tanks can make it challenging to figure out how to factor this into the overall passage fuel usage. But I'm a big fan of having all the data I can get and so appreciate having as many different measurements as possible onboard.

I tend to keep a running tally on the volume of fuel actually consumed from the tanks at each point on longer passages and have found this to be very good at updating how much we've gone through at any point and then extrapolate that to estimate how much will be left in reserve when we get to our destination. By using the calibrated sight glass with each refill of the Day Tank you get a pretty accurate fuel consumed total each day.

I have also been fortunate to have relatively large total fuel tankage volumes which increases the range available if you ever did get into a situation of getting close to that point. Avoided that scenario so far and hope to continue!

 

[OldDan1943]

So many things effect actually fuel consumption. The best we can hope for is average.
Planning for distance/gallon is almost foolish but we keep trying.
If you get down to 100 gallons, we better start hunting a fuel stop.
A down side of a day tank is, people get distracted and have been known to forget to fill the day tank. Perhaps it is a good reason for an low level alarm on the day tank?

 

[Hippocampus]

Tend to look at things like W. Your interest is whether you can complete the passage safely. So you figure out food and add at least a 1/3rd. You have dry stores hidden away in case the freezers fail. You carry at least 3 of consumable spares and more of key filters. Would have the same attitude towards fuel. You don’t need to know instantaneous nor average short period of fuel use. You need to know worst case fuel usage. Perhaps average fuel usage over a long period of mixed conditions plus a reasonable reserve is a safe way to approach this calculation.
Think the question is what’s a safe range. Not how far you can go in mild conditions using every last drop.

 

[mvweebles]

Good point and quite right about the accuracy and usefulness of these small calibrated tanks Peter. On long week+ passages though, I have found the very short duration of small calibrated measurement tanks can make it challenging to figure out how to factor this into the overall passage fuel usage. But I'm a big fan of having all the data I can get and so appreciate having as many different measurements as possible onboard.

I tend to keep a running tally on the volume of fuel actually consumed from the tanks at each point on longer passages and have found this to be very good at updating how much we've gone through at any point and then extrapolate that to estimate how much will be left in reserve when we get to our destination. By using the calibrated sight glass with each refill of the Day Tank you get a pretty accurate fuel consumed total each day.

I have also been fortunate to have relatively large total fuel tankage volumes which increases the range available if you ever did get into a situation of getting close to that point. Avoided that scenario so far and hope to continue!

Avoiding the fuel scenario with ample capacity and reserves is of course the best course for range, though as a boat with more range than I need (approx 2500 nms of range), keeping stored fuel clean is a problem.

When PAE built the N40 and decided on the Round-the-World event, they designed the tankage to be adequate for the longest let - California to Hawaii. By this point in time, PAE's pricinpals had a lot of offshore experience (they started their boat-building career with the Mason 43 sailboat). Being a 40-footer, they didn't have the luxury of over-capacity (I think the boat carried 950 gals). But still, what happens if, during the 12+ day passage, they encounter adverse weather? Knowing fuel burn at each RPM in open-ocean conditions was incredibly useful information, which is exactly the question the OP asked.

So a small, carefully calibrated 'day tank' was installed (as TwistedTree clarifies, 'day tank' is a bit of a misnomer). I forget exactly, but I think they made at least two trial runs from Dana Point to Seattle (>1000 nms to weather) to establish the fuel burn. On their leg to Hawaii, they were able to power-up for the last couple days to increase speed. They arrived with a remarkably small amount of fuel - something under 75 gals as I recall. All because they had carefully calibrated burn.

Now, this is not common amongst most owners, but if you're going to run at the outer bands of range, it's pretty useful. I cannot think of any other device or mechanism that would give me the confidence to do what the N40 did.

Peter

 

[JD Ray]

This is going to sound silly, but akin to Bob’s suggestion about filling the tank, going on a cruise, and refilling the tank, a viable (though not necessarily cheap) way to handle the problem without modification to the boat is to have the boat weighed (on a lift), go out for a cruise, then have it weighed again. The difference in weights is how much fuel you burned, though that presumes you didn’t put any water overboard with… other systems. ��