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Author Topic: Fuel gauge calibration  (Read 104 times)

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Breakin Away

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Fuel gauge calibration
« on: March 02, 2019, 11:31:47 AM »

For the last couple of seasons I have kept some logs of fuel gauge level vs. engine hours. I also keep track of exactly how much fuel we put into the boat, and top off completely once on the hard at the end of the season. My consumption has ranged from 0.55-0.60 gal/hr (depending on what date range I select). I typically cruise at 2200 RPM, though sometimes motorsail at a lower RPM.

One of the interesting results of all of this is a fuel gauge calibration chart. Once I've topped off and calculated average consumption, I can use that to back-calculate an estimate of how much fuel was in the tank at each fuel gauge reading. The end result is a calibration which is pretty much linear, but clipped at both ends. I recorded the reading in "eighths", in other words, 0=empty, 4=1/2 tank, 8=full, and 9=pegged above full. The points extrapolate to about 2.5 gallons in the tank if I ever let the gauge get all the way to "E", 10 gallons at 1/2, 17 gallons in at "F", and 18-25 gallons when pegged above F. (I assume here that Catalina's spec of 25 gallons for the MkII boats is correct, and includes all the volume in the vent and fill hoses when I top off completely.

I'm not sure if any of you have done something similar, so I'm offering it up for your comments:

« Last Edit: March 02, 2019, 01:32:39 PM by Breakin Away »
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2001 MkII Breakin' Away, #1535, TR/WK, M35BC, Mantus 35# (at Rock Hall Landing Marina)

KWKloeber

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Re: Fuel gauge calibration
« Reply #1 on: March 02, 2019, 12:25:37 PM »

BA

Very interesting, couldnít get more linear than that data shows!
Odd though w/ the tank shape - Iíve seen arguments made on here that the FG isnít linear.
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Ron Hill

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Re: Fuel gauge calibration
« Reply #2 on: March 02, 2019, 12:32:07 PM »

Breakin : If you've read any of the posts or articles on our C34 fuel tank, I find your research interesting.

Our tanks are asymmetrical with the widest portion of the tank at the top and the narrowest portion at the bottom - wider in the front and narrower in the rear.  So when the float on the fuel gage arm is indicating that the tank is "half full"; in actuality the tank (gallon wise) is really about @ 1/3 full (8.4 gal not 12.5 gal) remaining.
I wouldn't term that as linear - as far as the gage reading against actual volume.  The MKI data plate on the top of the tank is stamped 25 Gallons so I assume it is correct.

When I first got my C34 and until the time I pulled and cleaned that tank, I only somewhat believed in that fuel gage. My 30 years of military flying experience taught me to always use fuel consumption against engine time (except for the 20 minute Warning Light - that light I BELIEVED!!).  Then I pulled the tank for cleaning and saw that the tank was an asymmetrical design with a float ("powered") switch. That confirmed to pay little attention to using the fuel gage and went to the straight fuel consumption calculated against engine running time.  I stuck with the .5 gal/hr (M25XP) .55 gal/hr (M25XPB) and seldom had run over when topping off during a refill because my 1st Mate called off the Gallons and Tenths to me - if I couldn't see the fuel pump.  I usually ran at 80% power.

A few thoughts

« Last Edit: March 02, 2019, 01:52:59 PM by Ron Hill »
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Breakin Away

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Re: Fuel gauge calibration
« Reply #3 on: March 02, 2019, 01:31:21 PM »

I too was very surprised how linear the data looked. I expected a more parabolic shape, curved downward, to reflect the tank shape. But I did nothing to "fudge" the data to look linear. It is what it is.

And as you can tell from the points plotted, I've never run below 1/8 of a tank. I don't like seeing the needle bounce around (and the fuel slosh around) so much when it gets below 1/2 tank - though I do want to get the tank as empty as possible to avoid back-mixing of stale fuel with fresh. The one time I went down to 1/8 tank to get the data point I had a full jerry can (and extra fuel filters!) onboard.
« Last Edit: March 02, 2019, 01:34:57 PM by Breakin Away »
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2001 MkII Breakin' Away, #1535, TR/WK, M35BC, Mantus 35# (at Rock Hall Landing Marina)
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