Fuel gauge is a poor guess

Davidasailor26 · Nov 12, 2019

Yeah, common problem on many different tanks. Check your hours when you get fuel, and use that for the calculations rather than the gauge. We burn about .7 gph With the 3YM30 on our 37. You can probably use that as a starting point for your estimates and refine once you have some data from a few fill ups.

JoeWhite · Nov 12, 2019

Yes, the guage is not linear and does not reflect the number of gallons accurately. You can find out by trial and error. Ask me how I know.

SoSound · Nov 12, 2019

on that. The bottom half of the tank goes very quickly. Our usage is about the same rate as @Davidasailor26. We find that estimating how much we will need based on hours of usage and consumption rate is a reasonable starting point. And don't rely on the gauge for estimating how much time you have left in the tank. We find that it takes forever for the gauge to move from full, and by the time it's at half, we should definitely be thinking about where and when our next fuel stop will be.

AaronD · Nov 12, 2019

JoeWhite said:
Yes, the guage is not linear and does not reflect the number of gallons accurately. You can find out by trial and error. Ask me how I know.

Entirely unrelated to your big boats, but here's how I learned that lesson on an outboard tank. For your amusement... Leaving the Dock (Fail)

jssailem · Nov 12, 2019

In some ways I envy your boats set up with 30 gallon tanks. Always able to have fuel consumed in a single season. Clean tanks.

On the other hand, the ability to cruise at full power during periods of windless weather for 1300 miles also has it's appeal. My boat came with two 85 gallon diesel tanks positioned port and starboard on either side of the keel. Only issue is one needs to be cognizant of the fuel age and maintain a cleaning additive procedure. I have found the suggested Practical Sailor mixture of Biobor JF and Star brite Biodiesel biocide do a good job of attacking the strains of fungi or bacteria that infect our fuel. Minimizing water intrusion if primary.

Also as @LeslieTroyer observed (seeing my stash of Racor 30M filters), having a supply of Water Separator filters on board is a good idea.

Stu Jackson · Nov 12, 2019

SoSound said:
We find that estimating how much we will need based on hours of usage and consumption rate is a reasonable starting point. And don't rely on the gauge for estimating how much time you have left in the tank. We find that it takes forever for the gauge to move from full, and by the time it's at half, we should definitely be thinking about where and when our next fuel stop will be.

If you do a search here on "fuel consumption" you will find many, many posts about this issue. Boat fuel gauges are NOT car fuel gauges, and boat gauges are horribly unreliable. Doesn't matter how big or small your tank is.

jssailem · Nov 12, 2019

Stu Jackson said:
Boat fuel gauges are NOT car fuel gauges, and boat gauges are horribly unreliable. Doesn't matter how big or small your tank is.

True words spoken by a master fuel manager. Cruising from San Francisco Bay to Vancouver Island and calculating fuel consumption to less than a 1/2 gallon error. Never stranded at sea.

Pizzazz · Nov 12, 2019

Accurate level measurement depends on the tank shape and the boat motion, of course. However, most of the time these factors can be accounted for. My fuel tank is rectangular in shape and the gauge is pretty accurate, to within a gallon or 5%. My water tank has an odd shape, following the hull, so you need to calibrate it in your mind. Best way is to start with a nearly empty tank (you can take a bottle of diesel just in case you run out), then go to the pump station and start filling it in, one gallon at a time, noting the level indicator. Then you can either draw a custom scale that you stick on the gauge or just keep a mental note of it. You will soon realize, it is more accurate than you think.

Using hours is only approximate because it depends on load. A better indicator will be based on rpm's but I have not seen an rpm totaling counter that can be easily programmed. Should exist though. Diesels, when normally loaded consume fuel in very close proportion to rpm's.

I would rather have a smaller tank for everyday use and get a bladder for long distance cruising when needed. But if you cruise long distances more often then large tankage is always good to have.

Stu Jackson · Nov 12, 2019

Pizzazz said:
Using hours is only approximate because it depends on load. A better indicator will be based on rpm's but I have not seen an rpm totaling counter that can be easily programmed. Should exist though. Diesels, when normally loaded consume fuel in very close proportion to rpm's.

We hear this all the time during these discussions, and my reply is: while true in concept, it is ridiculous in practice.

Why?

Unless you are using your engine at 1/3 speed for extended times, most skippers are using their engines at rated cruising speeds (not usually WOT).

Therefore, the hourly use eventually and actually averages out all of the short times one uses the engine at lower rpms.

And that's ALL one needs to know: average fuel consumption.

For your boat, not mine.

In fact, if ya think about it, Joe Blow who uses his engine at 1/3 speed for two hours and then cruising speed for the next two hours, ON A REGULAR BASIS, will use less fuel per hour than I will, 'cuz I rarely idle along when the engine is running.

Joe needs to know what HIS consumption is. My info is useless to him.

Mine is 0.493 gallons per hour, over 21 years carefully kept in a spreadsheet.

If Joe had the same boat and engine I do, he could maybe get down to as little as 0.400 gallons per hour, IF Joe repeatedly ran his boat that way.

In the real world, arguments about different fuel consumption at varying rpms simply don't apply. It's true that lower rpm uses less fuel, but the actual number one needs for their boat is the average fuel consumption based on how you motor your sailboat around.

Pizzazz · Nov 12, 2019

It is a fair point that we just need an average or a rough guess. We have all tried to get a better estimate of fuel flow and fuel consumption for our small boat engines (larger engines are easier with a flow meter). There are three options I see:

- Record the hours, calculate average consumption for a particular cruising lifestyle and use that. It requires keeping a regular log and regular usage pattern. The fuel consumption is different if you idle, if you maneuver in a marina or if you are making a passage at 80% throttle.

- Install a totalizing rpm meter that shows you both rpm's and total rpm, multiplied by a scale factor. This will give you a much better approximation as it will adjust for the different rpm's but it will still be subject to load variations (i.e. fuel consumption will be different if you are idling charging the alternator or idling making progress. However, the error will be much smaller. I will be experimenting with a meter like this one:

Red Lion CUB5R Electronic Dual Counter and Rate Indicator with Reflective Backlight, 8 Digit LCD Display, 20 KHz: Process Controllers: Amazon.com: Industrial & Scientific

www.amazon.com

- Install two small fuel meters in the feed and return lines and then take the difference of the two. Here you will run into issues with inaccurate flow metering and low flow volumes typical in small diesel engines plus you will need the electronics to take the difference of the fuel flow meters.

Either method has advantages and disadvantages but at least they give you a better indication than a simple fuel gauge. In addition, the cost is not that great. The totalizing meter above is $130. Just installing a fuel gauge will run you that much.

shemandr · Nov 12, 2019

For my former Yanmar 3GM (J?) that .7gph was a pretty good number. I rarely ran it less than about 80% power, even if I was just going a mile to stop and raise sails. I think I learned that on this site. It was a great engine.

Davidasailor26 · Nov 12, 2019

Pizzazz said:
It is a fair point that we just need an average or a rough guess. We have all tried to get a better estimate of fuel flow and fuel consumption for our small boat engines (larger engines are easier with a flow meter). There are three options I see:

- Record the hours, calculate average consumption for a particular cruising lifestyle and use that. It requires keeping a regular log and regular usage pattern. The fuel consumption is different if you idle, if you maneuver in a marina or if you are making a passage at 80% throttle.

- Install a totalizing rpm meter that shows you both rpm's and total rpm, multiplied by a scale factor. This will give you a much better approximation as it will adjust for the different rpm's but it will still be subject to load variations (i.e. fuel consumption will be different if you are idling charging the alternator or idling making progress. However, the error will be much smaller. I will be experimenting with a meter like this one:

Red Lion CUB5R Electronic Dual Counter and Rate Indicator with Reflective Backlight, 8 Digit LCD Display, 20 KHz: Process Controllers: Amazon.com: Industrial & Scientific

Red Lion CUB5R Electronic Dual Counter and Rate Indicator with Reflective Backlight, 8 Digit LCD Display, 20 KHz: Process Controllers: Amazon.com: Industrial & Scientific

www.amazon.com

- Install two small fuel meters in the feed and return lines and then take the difference of the two. Here you will run into issues with inaccurate flow metering and low flow volumes typical in small diesel engines plus you will need the electronics to take the difference of the fuel flow meters.

Either method has advantages and disadvantages but at least they give you a better indication than a simple fuel gauge. In addition, the cost is not that great. The totalizing meter above is $130. Just installing a fuel gauge will run you that much.

I do keep a log of all my trips, but for the fuel calculation you really just need to record the hour meter and gallons taken each time you fuel up. I also don’t have a perfectly constant usage pattern, but I can still get sufficiently accurate consumption projections. If I fuel up after a few months of mostly motoring in and out of the marina I’ll get about .65 gph. If it’s after a longer trip with lots of cruise RPM motoring it’s closer to .75. I find both of those are close enough for me to know when to refuel, and when to start listening for the vent sound to change during the refuel.

Stu Jackson · Nov 12, 2019

Pizzazz said:
The totalizing meter above is $130. Just installing a fuel gauge will run you that much.

And for NO $$ you can simply keep track. Like this:

Fuel Log 101

Fuel Log

SoSound · Nov 12, 2019

I don't keep a spreadsheet, but I do record engine hours, fuel (gauge) level, fuel (gallons) purchased, and usage rate in our log book. I compare current usage to previous usage. I'm most interested in knowing when I need to start paying attention at the fuel dock.

Our fuel consumption has been relatively stable for the last year or so. Except for a recent surprise where lower engine RPMs and favorable currents may have had us over estimating our consumption. Luckily, it was just a surprise, and not a spill. @Dr. D, our fuel consumption during the engine break-in period was higher than what we see at present. Instead of ~.65 gph, we were seeing consumption numbers closer to 1 gph until we were out of the engine break-in period.

Benny17441 · Nov 12, 2019

Indeed they are, fuel gauges are basically useless. The tried and proven method of determining quite accurately how much usable fuel is in the tank is by calculating the average burn rate of the engine, using an hour meter to determine the time the engine has ran and keeping a simple log of the number hours when the tank was last filled. It takes a little practice but after a while you can glance at the hour meter and do some simple calculations in your head and come up with a good enough approximation of the fuel on board. I regularly can tell how much fuel it will take to top off the tank to Plus or Minus 1/4 of a gallon. The burn rate is a simple formula BR= GPH (gallons used per hour) It has nothing to do with the boat, currents or wind. Do not worry about measuring time at idle and then at cruising RPM since you are going to be using an average figure. Fill up the tank, connect an electric hour meter to the key switch and after you have ran some 4-5 hours go fill up the tank again. Divide the number of gallons taken in (same as used) by the hours and you will have your Average Burn Rate. Now you want to repeat this calculation every time you fill up and you will notice that after a number of times your calculations will settle into a pretty consistent Average Figure. No need to go into too many decimal points just round up to .25, .50, .75..... The other piece of information you will need is the size of your tank in gallons. Since not all the fuel in the tank is usable you may want to substract a 10 to 15% to keep as reserve so a 30 gallon tank may be deemed to have 25 gallons of usable fuel. The log should consist of the date you topped off the tank and the reading of the hour meter. When you first get to the boat refresh your memory on the reading in the log, check the hour meter and determine the difference or the time the engine has ran since the last fill, lets say if it has ran 5.5 hours and your average burn rate is 1/2 gallon per hour then you have used 2.75 gallons. Substract the 2.75 gallons from the 25 usable gallons and you can be fairly certain you have 22.25 gallons left on board to start the day. Knowing the amount of fuel on board at any time allows you to calculate range according to the prevalent conditions and will allow you to anticipate if you can reach your destination or if you need to deviate to an alternative destination. It basically allows you to plan your refueling stops. my fuel gauge stopped working years ago and I have not even bothered fixing it. I round up the numbers to do good enough calculations in my head, but if I need more precise calculations I will use a calculator, especially if a decision to divert course or not needs to be made. Even if you keep your gauge practice the described method and you will see how easy and accurate it is.

FastOlson · Nov 12, 2019

When cruising we always hedge our bets by keeping two full 'jerry cans' lashed to the lifelines by the fill fuel on the side deck. No spreadsheet, altho that certainly sounds like a Good Idea.

I had occasion to pump out our boat's ten year old tank a couple months ago -- tracking down a pinhole leak caused a teeny tiny bit of water along a bottom seam. I found that the indicated level on our WEMA electric gauge is very accurate. Given the slope to the bottom of the tank this surprised me.

This tank was built in 2008 by Coastline Tanks in Bellingham WA, and they installed the new sender and provided the gauge for mounting in my nav station area.
It was quite a leap into modern times from the old smaller stock tank with its direct-reading dial and float-on-an-arm inside. It was helpful to have a general knowledge of the level, but not as useful as the electric version in the new tank. I suspect that the builder calibrated new gauge.
Checking the pumped-out amount of diesel against the gauge showed good accuracy by boating standards.

With the old 3 cylinder Universal and now the new 3 cylinder Beta, consumption is still about 3/4 gallon an hour, and that's at cruising rpm's of 2500 to 2600. With clean bottom, that's 7 kts for our particular boat.

View this as just another 'data point', as they say.

DougM · Nov 14, 2019

I never did trust the fuel gauge. The 17 gallon relatively flat tank in my boat typically registers ”full” all summer. Then starts to register between 3/4 and 1/2 after relatively little use. My rule of thumb for usage is a half gallon per hour after 14 years of use. ... I filled it for winter storage in September with the gauge showing 3/4 and it took all of 1.9 gallons to fill, which would suggest that I ran the engine about 4 hours. That is just about right for the amount of use for this last summer.

FastOlson · Nov 14, 2019

Reading of all the 'real world' problems with measuring fuel in a sailboat, gets me to pondering why we all, collectively, put up with this.

It's sort of like a revisit to those little analog oven clocks in 1950's and 1960's households.... that mostly all stopped working within the first year!

Everyone joked about it and accepted it as 'normal'. (sigh)

sail sfbay · Nov 14, 2019

Fortunately my 2005 Hunter 46 came with a fuel gauge in the cockpit. When I first filled it up last winter I had slightly less than 50 gallons by the gauge on a 100 gallon tank and took slightly over 50 gallons. So in my case, the gauge was "relatively" accurate.

I top off the fuel tank every 3 months from one or two 2.5 gallon fuel containers since the fuel dock is 30 - 90 minutes away and add Stanadyne 43566 to clean the diesel engine injectors and Star Tron Enzyme Fuel Treatment to stabilize the fuel by mitigating phase separation by dispersing water throughout fuel.