Diesel Fuel Tank Volume - 1987 MK 1

[Jon W]

I originally posted this as a reply on the Fuel Calibration message. I deleted it from there, and moved as it's own topic to make searching easier for anyone that may be interested in this type data in the future.

I've removed my diesel fuel tank to clean the inside and check for any corrosion of the aluminum inside the tank that may lead to a future leak. Since I have everything apart, clean and at home I thought it would be worthwhile to determine the volume on a per inch basis inside the tank. I marked off inch increments externally on all sides, subtracted the material thickness as appropriate. This should provide the best estimate of the internal volume at each 1 inch section. I then converted that volume number to the number of gallons in the tank per inch. Sum up the gallons at each inch increment and I get 25.16 gallons. Considering there is a lot of distortion from the tank fabrication process, I was very surprised to get this close to the advertised 25 gal capacity. I plan to do an actual measurement with gallons of water per inch to compare the two.

The main reason for this exercise is to have a story pole with number of gallons per 1 inch increment. The story pole will be kept on the boat as a manual back up to the fuel gauge. If needed, I can dip the tank through the port cockpit locker using my new tank access plates to check how much fuel I have in the tank.

So for what it's worth, the attached PDF is my attempt to characterize this odd shaped fuel tank. I'd be interested in comments to make this a better document.

[britinusa]

I'm pretty sure my tank label read 23 gallons.

Rather than complicate the dip stick, how about marking it in gallons rather than inches. That way you could read the amount of fuel rather than the depth of fuel and then convert it.

When we purchased Eximius, the fuel sender had been removed and replaced with a simple analog model ie. dial showing fractions of full. To read it involved diving into the aft berth where 2 holes about 4" were cut in the partition next to the tank. With one hand holding a mirror, look through the other with a flashlight to read the dial.

I installed a new no moving parts sender very early on in my upgrades.

Paul

[Stu Jackson]

Great job, Jon.

Are the changes in the B & C dimensions due to the shape of the tank?

As far as rank volume, I've always worked on the 23 gallon capacity, left three in reserve, and used 20 as "useful."  At 0.50 gallons per hour consumption for 20+ years of record keeping in my Fuel Log spreadsheet, I fill up with 10 gallons every 20 hours.  All sorts of ways to do it, but this is a truly helpful effort.  Thanks.

[Jon W]

Hi Paul, The label on my tank reads 25 gallons. The number 5 in 25 is tough to read, but it's a 5. Regarding labeling gallons on the story pole instead of inches, it will depend on what I use for a story pole. It will need to be legible. I am replacing my current swing arm fuel sender with a reed switch type.

Hi Stu, Thanks. The topic of tank capacity seems to come up once a year on the message board so thought I would take the time to answer the question (at least for myself). Ironically the hardest part was putting a sensible spreadsheet together. Yes the B and C dimensions change due to the tank shape on the outboard side of the tank. I use the .50 gph method too, and use the electric sender/gauge as a reference. Now I'll have a manual method if all else fails.

[Stu Jackson]

>>>>>>>>>>>>>>>>>
Ironically the hardest part was putting a sensible spreadsheet together. Yes the B and C dimensions change due to the tank shape on the outboard side of the tank.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>

That's what I figured, thanks for confirming it.

[Noah]

Good math/geometry exercise. But, not sure I would relish pulling the inspection port to sound the tanks with a dipstick. On the 220 ft. oil rig supply ships I ran in my youth, the engineers used to sound the tanks with a roll-up tape/plumb bob, but we carried 250,000 gals of diesel and were NOT concerned about each gallon. On a C34 it has got to be pretty flat water and a cleared out locker, and still not pleasant.  Stu's keeping an engine hour log and filling up with 10 gallons every 20 hours seems like "good practices."  Perhaps carrying a jerry can(s) on deck or a 1-1/2 gallon jerry jug lashed in my anchor locker for an emergency, would be my solution on a passage—where I thought tank capacity/range might be stretching it. I would also figure 3/4 hr. per hour at cruising on the M25XP, to be super safer cushion.

[Jon W]

The story pole is when all else fails. The aft deck plate in the port locker is easier to access than the forward one by the level sensor. I have diesel jerry cans.

[Ron Hill]

Guys : I always went below and hit the calculator with .5 gal/hr for 28 years and 5000hs running!!  It was right ON!!
Why screw with other methods of fuel consumption????

A thought

[Jim Hardesty]

Ron is right again.    The hour meter is the most accurate fuel gage.  When I fill up the date and hours go on a post-it note at the nav station (and of course the log book).  My fuel usage is about .75 gal/hour.
Jim

[Jon W]

What I said was that I use gph as my primary method of monitoring fuel consumption. The fuel gauge is used as a reference. Gallons per indicator mark on the fuel gauge face will be correlated to gallons available when I fill the tank. The story pole is a back-up, not the primary method. Don't understand why any of this is controversial.

[britinusa]

There's probably a good chance that the tanks were not exactly standard.

Here's my tank label.

[Ron Hill]

Paul : My 1988 tank stated 25 gal on it's data plate.  I'm sure that all C34 tanks are made to the same exterior Catalina dimension specs.  I believe that maybe different manufactures measure differently??   total capacity/usable capacity??
 
I'm sure that with the tank vent being on the top of the back side that my real capacity is only 23 gal!!  My 25 gal would "over flow" down to only 23 gal!

My Guessing  !!

[Noah]

Ron and Paul- The vent on both my old tank and my new tank were/is located on the top of tank, not the back side.

[britinusa]

Ron and Paul- The vent on both my old tank and my new tank were/is located on the top of tank, not the back side.

Mine too.

[Jon W]

For anyone interested - I filled the tank with water to compare the actual gallons/volume per inch to the measurements I made earlier. I updated the original summary chart to reflect the actual volume and gallons per inch.

With the tank filled to the level sender, the total capacity is 26 gallons and ~40 ounces (but who's counting). 25 gallons would be a full tank with no diesel overflow into the vent line. There would be a little diesel sitting in the 1 1/2" diesel fill hose.

Next step is to check the CD SST deck fill cap to make sure it doesn't leak water. It doesn't feel like the o-ring is sealing like the plastic cap did. Final step is re-install the tank into the boat, fill with diesel while documenting gallons per indicator mark on the fuel gauge.

A couple photos and updated PDF diagram/chart are attached.