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 I’de love to know.

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.

If you can get enough access to measure the size both top and bottom you can average it and get a reasonably close number.

Comodave said:

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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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

Peter

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).

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.

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........

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.

bayview said:

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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Ding Ding Ding!

Mengering said:

the fuel cleaner drew out 285 gallons

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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.

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.

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.

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.

rochepoint said:

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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With the help of the computer, 150 Imp gallons = 180 US gallons.

What did we do without computers? Rely on our memory? Ya surrrrreee

Island Cessna said:

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! .

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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.

sunchaser said:

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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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.

The tanks are supposed to be marked with the material, size and testing pressure. Mine also says what thickness the material is.

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.

You should have placards somewhere on the tank like this:

Alaskan Sea-Duction said:

You should have placards somewhere on the tank like this:

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LOL, in my case, it must be on the outboard sides of my tanks.

bayview said:

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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Thanks for the good ideas. I think I’m going to go for this one as a first pass.

psneeld said:

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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+1 This is what we did on NWD. We did not have any calibrated markings for our sight gauges. After doing similar work on both tanks, we left one EMPTY. We had discovered the tanks are not symmetrical from bottom to top. We then filled it up until it started to show on the sight gauge, so I know how many gallons to the bottom of the sight gauge, which is just above the level of valve for the fuel supply. Then filled it 10 gallons at a time (we did this when the fuel dock was slow, believe me!), marking a piece of tape as we went. This is now our template to create permanent marks for our sight gauge guards, calibrated to the nearest 10 gallons (on 200 gallon tanks.) I can quickly visually estimate my gallons on board to the nearest 5 gallons, and with a tape measure reasonably measure my fuel use within a gallon or so if I really wanted to, if I'm sitting in calm conditions....for most of my sight gauges its between 1" and 1 1/8" per 10 gallons....

After we did this, once we were in a position to run the other tank down to nothing we did the same thing to mark it... It took us a while simply because I didn't want to have one tank completely full and the other completely empty at the same time... We actually went a little over 1/2 tank at a time on each side.... then the rest of the way once we'd done that.

As to comments/pictures of builders tags on fuel tanks... ha! I don't think all TT's got them. I uncovered almost every surface of my tanks during the work I did on them, and while I believe I was seeing the original surfaces (as once I removed the sound baffling I found a page of a newspaper laying on the port side tank from the year she was built!) I found no builder plates at all....My tanks appeared to have been built by a craftsman who bent and welded the tank to fit the space it needed to fit - the tanks are similar but not exactly the same.

Draining the tank and then filling is a more accurate method.

Better tanks will frequently have a screen built in above the tank bottom to serve as a sump.

This is unburnable although the math would count it as fuel.

If you know what make and model and year the boat is there is probably pretty good literature on what should be in place. If they cleaned and drained the tanks how much fuel did it take to fill it?

Tony sundeck 40 said:

If you know what make and model and year the boat is there is probably pretty good literature on what should be in place. If they cleaned and drained the tanks how much fuel did it take to fill it?

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What source of literature do you recommend?

The manufacturing data on my Albin 40 has been way off on every tank but the tub sump.

Every time I see it on the internet it seems to be just repeated, even on my surveys.

On my 1990 Albin 40 I have approx 170 each side and 50 in the keel. That is by doing the math. I should have filled it when I had the tanks cleaned but my intent was to mark my sight tubes as I was filling because of their configuration.
So when totally filled I carry almost 400 gallons and burn about 3 gallons an hour at 1500 rpms.

I’m having trouble posting a picture of some tankage. If you give me your email I’ll try to send it that way

I know all the volumes of my tanks because I have replaced, measured or eliminated all of mine...but it would have been nice to know before all that.

The sad part was the manufacturers data, brochures, advertisements, etc....were al, off a little to quite a bit.

Do you have access to measure tank. Basically if you calculate the volumns of the tank in inches i.e. height x width x depth. Break the tank sections into rectangle, and triangular sections. Remember that a the area of triangle is 1/2 of the rectangle. Add to gather together total volume. One gallon US = 232 cubic inches. If you step up the height from the bottom at 1 inch intervals you can create a look up table with the gallons at various fuel depth. Key is to get good measurements.
If you start with an empty could and have access to a sight gauge you could add say at 50 gallon intervals until full and record the fuel height and then plot the data on excel. The 50 gallons don't have to be exact, record the actual quantities from the fuel meter on the pump.

Just takes time at the pump station.

Ball park figuring...
Length and height should be pretty constant. The width will vary from top to bottom. Measure both and use the average for the width.