No Fuel Gauge -- What Approach Would you Take?

[model 10]

Originally Posted by Nicholson58

Day tank is 70 gallons. The resistance gauges are junk. The manometer feed-back is maybe the most reliable other than a stick. My stick is notched in 6 gallon increments & labeled.

A capacitance fuel quantity system is the most accurate and reliable as proven by just about any decent aircraft in the last 40 years.

[Zai]

On our boat we have a fuel gauge, but the tank is rather flat and as such the reading is not reliable.
I used the engine manufacturers fuel consumption chart and then the first few times we were out cruising compared the calculations with the actual usage (must run a consistant rpm for this).
After a few trials I was able to comfortably calculate our fuel consumption with remarkable accuracy.
I did these test durring several trips at different rpms.

As a side note I try never to run the tank all the way down (calculate a good reserve).

[Ex-Calif]

Lest we lose sight of the big picture we are sailboat drivers, right?

I mean can't we just sail to a gas station if we run out or have we forgotten what those big triangular napkins are for - LOL...

[Stu Jackson]

Originally Posted by Cheechako

Option B: (don't spend money and time on something you don't need)
Fill your tank. keep a log of engine hours and fills. (Date, gal's to fill, engine hours) Soon you will know your burn rate.
Sailboats use fuel so slowly the problem is more how to use it to get fresh fuel in than running out!
You will burn probably somewhere between, .6 to 1 gal per hour.
So if you have a 100 gal tank and burn .75 gal per hour at 7.5 knots.
That's roughly a 1000 miles or 135 hours of motoring full speed.

Delightfully simple.

You even get to use high school math!

It's pretty foolproof, too.

Except for us fools who use it.

16 years now and still workin' just fine.

[Nicholson58]

Originally Posted by model 10

A capacitance fuel quantity system is the most accurate and reliable as proven by just about any decent aircraft in the last 40 years.

Do you refer to the probes mounted outside the tank wall? Won’t work on metal tanks. Mine are 3/16 inconel.

None of my fuel or water gauges have ever worked. I use a graduated stick.

[wingssail]

Originally Posted by Cheechako

... Fill your tank. keep a log of engine hours and fills. (Date, gal's to fill, engine hours) Soon you will know your burn rate...

Yes, Cheechako, and with that information you can calculate how much fuel is being (or has been) used and I built a spread sheet.

Being a bit of a nerd, and also being in the habit of keeping a good log in which I record the rpm on all passages (short or long, in which the engine was run), and knowing the burn rate, I created an excel spread sheet which shows the current level of fuel in each of my two fuel tanks.

Say I start off with a full tank of 30 gallons. I record the engine hour meter.
Usually motoring at a standard speed of say 2400RPM my spread sheet knows that the consumption is .65 gal/hour. At a later date I note the engine hours and which tank was used and the spread sheet calculates the amount of fuel I've consumed and updates the balances. When we put fuel in the calculation adds that amount to the total. This has proven to be very accurate, and we find that when refueling, we know how much it will take, to the nearest liter.

The key is building the table of burn rates for each RPM band.

And if you change propellors, you need a new burn rate table.

[Cheechako]

Originally Posted by wingssail

Yes, Cheechako, and with that information you can calculate how much fuel is being (or has been) used and I built a spread sheet.

Being a bit of a nerd, and also being in the habit of keeping a good log in which I record the rpm on all passages (short or long, in which the engine was run), and knowing the burn rate, I created an excel spread sheet which shows the current level of fuel in each of my two fuel tanks.

Say I start off with a full tank of 30 gallons. I record the engine hour meter.
Usually motoring at a standard speed of say 2400RPM my spread sheet knows that the consumption is .65 gal/hour. At a later date I note the engine hours and which tank was used and the spread sheet calculates the amount of fuel I've consumed and updates the balances. When we put fuel in the calculation adds that amount to the total. This has proven to be very accurate, and we find that when refueling, we know how much it will take, to the nearest liter.

The key is building the table of burn rates for each RPM band.

And if you change propellors, you need a new burn rate table.

What engine do you have in your 43?
The boat in my avatar, an Yves Marie Tanton 44 (very much a canoe shape) had a Perkins 4-108 and the average burn over my ownership was also .65 gallons per hour.
I never worried about rpm etc, my usage was pretty consistent. But I had a dipstik in the tank If I needed to double check.

[wingssail]

Originally Posted by Cheechako

What engine do you have in your 43?
The boat in my avatar, an Yves Marie Tanton 44 (very much a canoe shape) had a Perkins 4-108 and the average burn over my ownership was also .65 gallons per hour.
I never worried about rpm etc, my usage was pretty consistent. But I had a dipstik in the tank If I needed to double check.

We have a 34hp Yanmar 3JH2E. Our burn rates vary from .25gal/hour (at 1500RPM) to 1.75 gal/hour (at 3200RPM). We often run at 2200RPM with is .67 gal/hour in the latest variation of the burn rate table (which I often update when I get a new benchmark). This is slightly more than 6.0 knots.

Another complication with using the hour meter is that the majority of our engine hours are for charging the battery without the propellor engaged. In this case I record it at 1500RPM even when the engine is running at 2200 for charging. We use less fuel at 2200 when charging than we do when running at 6 knots.

With two tanks we always have a reserve, so no dipstick is needed. With a dipstick you need an opening into the tank, which is messy at best and at worst, can leak, so I never liked them.

[ramron67]

I also use the estimated burn rate, which I supplement with a dip-stick, but I am considering retrofitting a mechanical fuel gauge.

Has anyone tried this?

[flightlead404]

I didn't read the whole thread, but operating without a fuel gauge is perfectly fine. Maybe better than relying on a gauge.

Fuel gauges on boats, like on aircraft, are "advanced guessing" at best. They are notoriously unreliable and inaccurate. Even if they are accurate, they lack precision.

First, obtain the operating manual for your engine which should give you good based fuel consumption numbers in volume/time by rpm. e.g. gallons per hour.

Now, fill your tank to the maximum. If you don't have a good view of the tank level, then pay attention to the behavior of the fuel, and foam, tank vent etc when full.

Now, go motor somewhere a half day away, spend a night on the hook, enjoy it, then motor back to the same fuel dock. Carefully record your run time and rpm settings. Fill the tank again, replicate the level from when you originally filled it, observe the behavior of the fill/vent etc.

How many gph did you use? For planning purposes use the greater of the number in the manual vs your actual number.

The next thing to do is figure out the actual usable tank volume. This is a step most people don't seem to do, but most tanks useable volume will be less than the nominal volume, perhaps significantly less. The way we do this on aircraft is to "burn one dry", maybe you can do this on your boat but if you aren't sure the fuel is dead clean at the bottom, or worry about being somewhere with the engine dead for a few minutes while you add some from a jerry can then all you can do is use the tank nominal volume minus a fudge factor. I'd subtract at least two gallons. While you might be able to get more out, think about what happens if you're in rough sea with only two gallons in there, probably pick up some air.

After that its simply a matter of a little math and keeping a log of engine run times. tbh the fuel consumption of our little diesels is low enough that I just do "back of the envelope" calculations. I check the numbers at each fill to make sure the fuel consumption is accurate. It always is.

Managing your fuel this way, rather than relying on an inaccurate, unreliable, and insufficiently precise fuel gauge is actually a better option IMNSHO.