# Fuel tank volume calculations

Discussion in 'Boat Design' started by Zappi, Dec 30, 2009.

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### ZappiSenior Member

Hi. was wondering if there is a program or calculator I can punch in the dimensions of my trapezoidal fuel tanks and then punch in current depth and it will calculate gallons currently on board. I can do it longhand but it seems there would be something already out there and simple to use.
Thanks!

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### FanieFanie

Ok, which way are the angles slanted - downwards or sideways ?

If the scew sides are sideways but upright you can calculate it very easy.

If not vertical you will have to measure the level and use a table if your electronics cannot compensate for that.

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### ZappiSenior Member

If it posted.... Measurements are in inches. Total volume... 85 gal.

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### SamSamSenior Member

How do you measure the current depth?

5. ### Submarine TomPrevious Member

No there isn't.

Simply do the math, it's not difficult.

You already know how many gallons it holds when full (if indeed correct).

-Tom

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### LucklessSenior Member

Is the 18 line a vertical?

If so you can make a table fairly easy by pretending your tank is two sections. Pick how fine of a scale you want (such as every 1/8th of an inch if you're using a dip stick, or finer if you want) and calculate the volumes out long hand or use an excel script.

I just got home for a large turkey dinner, so double check the math, but should be easy enough to use Excel or Open Office Calc.

=(A1*15*59)+(A1*((7*A1)/36)*59)

7. ### mark775Guest

It would be easier for you to go inside and plug it into a program than to simply have already calculated "four inches is twenty gallons, five is twenty seven, six is 36, or whatever"? You don't even need to do that - just fill it and mark the stick you'll use every five gallons as you put it in. I'd do the calculation for you but I couldn't get the thumbnail to come up.

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### gunshipSenior Member

sigh, metric is much easier for this, but, as said, make an excel spreadsheet where you simply punch in the dimension, and it calculates automagically!

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### tom28571Senior Member

In case we have all forgotten our math, the answer is:

Gallons in tank = h(885 + 11.44h)/231
h = height of gas in tank in inches

Based on dimensions given. The actual answer will be a little less because of tank wall and joints thickness.

10. ### mark775Guest

I came up with a different formula. I'm rusty. Can you explain the variables in yours?

volume = L * (b1 + (b2 - b1) * h1 / h + b1) * h1 / 2

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### FanieFanie

Hi Zappi,

If you can live with the non-linearity of the tank (level drops slow when full and faster when empty) then you simply devide the slanted side in half and pretends you have a rectangular tank. Many cars are like that, I guess one gets used to it.

I don't know what setup you use for indicating the fuel level, but if you have an analog indicator for instance, you can redo the face plate where the scale is on. A bit of fine art work and it can indicate as accurate as can be expected.

If you need to know the exact amount of fuel, you can try to find someone that can write you a bit of code to compensate for the non linearity, it's quite easy with processors nowadays, and have a digital display on LCD.

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### FanieFanie

Oops sorry, your calculations - Also very easy.

In the tank imaginary cut the triangle off. You now have a rectangle and a triangle.

The rectangle is easy L B H, as is the triangle S A S, just measure the slant to get the angle and the sides are the height of your fuel level.

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### tom28571Senior Member

Mark,

You show a belly tank while the problem is a simple saddle tank.

Is the current reliance on computers and spreadsheets robbing us of the ability to analyze a problem based on basic principles We keep seeing people on forums ask for a spreadsheet to answer a simple problem that a little thinking would solve. A tug boat just fetched up on Bligh's reef. How were they navigating?

"h" shows up twice in the expression because we need it to derive the width of the top of the triangular section of the tank as well as the height of the fuel.

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### frank smithSenior Member

There are many online volume calculators

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### ZappiSenior Member

My method of measurement is using a wooden dipstick. It sounds like the easiest would be to take the time to calculate and mark in maybe one inch incriments on the stick. I'm no math whiz, did anyone double check my calc???
Thanks for all the replies!

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