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Old 10-14-2020, 10:11 PM   #1
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Fuel tank estimate

We bought our boat with no contact with the previous owner. The specs said we had 300 gallons of fuel (150 per side). We had ports put in and our tanks drained and cleaned, with 1/2 on the sight gauge the fuel cleaner drew out 285 gallons. Clearly we have more than 300 gallons. Does anyone have any good ideas on how to figure out capacity? These tanks are not rectangles, getting larger at the top. My best (but still not great) idea is to use fuel to near the bottom and then fill. But if anyone has some clever math or other means to estimate capacity Iíde love to know.
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Old 10-14-2020, 10:18 PM   #2
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Is that a twin engine or single? Can you run the tanks separately?

If the answer is yes to both questions I would run port engine to port tank with PLENTY of fuel in it.

THEN try to run the SB down to "low" and fill....

Point is if you're gonna accidentally run it dry do only one side.

Oh, and have the bleeding procedure/tools ready.
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Old 10-14-2020, 11:05 PM   #3
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If you can get enough access to measure the size both top and bottom you can average it and get a reasonably close number.
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Old 10-15-2020, 05:37 AM   #4
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If you can get enough access to measure the size both top and bottom you can average it and get a reasonably close number.
I agree. I haven't seen many, but the saddle tanks I've seen are roughly triangular with a relatively small flat bottom in cross section. Not a lot of effort to contour to the hull. You should be able to measure the height and length of tank. If you can approximate the width of the tank across the top, then assume the bottom is 20% of that width, you should be able to get pretty close to overall size. From memory, there are about 7.5 gals per cubic foot of volume.

If course, since you have sight glasses, best would be to start with an empty tank and calibrate at every 25 gals or so. But starting with an empty tank is difficult.

Good luck

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Old 10-15-2020, 05:51 AM   #5
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Can always burn it down low, pump out till sucking air...

Mark that on sight tube (or add fuel till it becomes visible and you know that is reserve).

Then add and mark accordingly.

You can either stop at the top of the sight gauges or go till venting (oil pads under vents of course.

That way you are sure, not "estimating".

You may have had full tanks and the sight gauges were blocked or "shut off" ( hopefully not insulting your intelligence but there are some unusual or broken valves out there).
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Old 10-15-2020, 08:39 AM   #6
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Depending on the age of the vessel, you might want to look for stampings on the tanks that show the specifications and volume.
Another idea is to install a fuel pump in the crossover line (assuming you have one) and pump one tank into the other to completely drain it before refilling.
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Old 10-15-2020, 09:18 AM   #7
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Our PT38 Sedan has 150 gallon tanks on each side, over the last 25 years we have basically found that to be true but in imperial gallons........
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Old 10-15-2020, 10:18 AM   #8
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You know that 1/2 on the gauges is 285 so just get to 1/2 and fill up to see how much more fits.
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Old 10-15-2020, 12:08 PM   #9
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You know that 1/2 on the gauges is 285 so just get to 1/2 and fill up to see how much more fits.



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Old 10-15-2020, 12:26 PM   #10
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Originally Posted by Mengering View Post
the fuel cleaner drew out 285 gallons
Is there a chance that the fuel cleaner charged based on the total of the fuel that was in the tanks plus the contaminated wash water they drew out during the cleaning process? Both need recycling or disposal.
That said, measuring your tank or emptying one and calibrating while refilling is a necessary step. Knowing how much is left is a lot more important than knowing how much it holds! If you do the empty one and refill process, keep track of the list as it changes during filling. Your calibration of the sight gauge should be corrected for list.
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Old 10-15-2020, 12:33 PM   #11
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IF you can draw a picture of the tanks, with measurements, the math isn't tough. And free 3D software can do the calculation for you.
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Old 10-15-2020, 12:50 PM   #12
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Rectangular tanks,
LXWXH= cubic inches ????
1cubic inch = .004329 gallons
Or take the height from the lower sight glass to the upper sight glass.
What I did was take the advertised volume divided it into inches, marked a strip of blue tape one inch apart labeling it gallons.
My only concern is over filling it and I have a trap on board that will hold a little bit of fuel if over filled. You can also install a whistle on the vent line and listen.

To be honest, I dont intend to rely on the last 3 or 4 inches in the tanks. I am in real trouble if I am down to that level.
I guess you could make a mark the current level with blue tape, fill the tanks all the way up and measure between the mark and the full gallons convert from inches to gallons...... measure from the blue tape down and figure out how much fuel you had when you started. Then, calibrate the tank via blue tape and a black marker.

I think I said all that right.
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Old 10-15-2020, 02:30 PM   #13
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Volume can be calculated mathematically with virtually any shape.

For yours - Bottom length x bottom width x height = volume A.

If the top width or length is bigger, consider this a triangle attached to the rectangle. Calculate area of triangle x height = Volume B.

A+B = Total volume.
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Old 10-15-2020, 02:46 PM   #14
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Our PT38 Sedan has 150 gallon tanks on each side, over the last 25 years we have basically found that to be true but in imperial gallons........
With the help of the computer, 150 Imp gallons = 180 US gallons.

What did we do without computers? Rely on our memory? Ya surrrrreee
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Old 10-15-2020, 02:47 PM   #15
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That said, measuring your tank or emptying one and calibrating while refilling is a necessary step. Knowing how much is left is a lot more important than knowing how much it holds! .
Yup, nailed it.

Also, effective empty is when the fuel pickup is sucking air. This is extremely easy to determine if you have a fuel transfer pump hooked into the fuel pickup side of each tank.
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Old 10-15-2020, 02:53 PM   #16
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Yup, nailed it.

Also, effective empty is when the fuel pickup is sucking air. This is extremely easy to determine if you have a fuel transfer pump hooked into the fuel pickup side of each tank.
I have been told, the AT supply is a butt weld at the bottom of the tank. hmmm, one day I will go verify that. WINK

Basically, anything approaching 1/4 tank (2 tanks X 200gal/4= ABOUT 100 gallons) means I need to refuel at first opportunity.
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Old 10-15-2020, 03:01 PM   #17
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The tanks are supposed to be marked with the material, size and testing pressure. Mine also says what thickness the material is.
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Old 10-15-2020, 03:11 PM   #18
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Please let me emphasize, electronic gages are a "suggestion." Go look at your sight glasses.
In fact, may I suggest putting the return on one tank and carefully monitor the level in the feed tank. When it drops to the bottom of the sight glass, draw and return from the other tank (isolating the non-feed tank) This will allow for sloshing if in other than calm water.
Of course, you might develop a list.
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Old 10-15-2020, 03:17 PM   #19
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You should have placards somewhere on the tank like this:
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Old 10-15-2020, 03:19 PM   #20
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You should have placards somewhere on the tank like this:
LOL, in my case, it must be on the outboard sides of my tanks.
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