Fuel Tank dimensions for purposes of calculating remaining fuel volume

[jpendoley]

Hi Group,
As I dig into my new boat, the question of how to measure fuel volume comes up. After climbing into the port cockpit locker and measure length and width I found the height concealed by the plywood framing surrounding the tank fixing it in place. It seems the depth may not be uniform so I'm not sure how to calibrate a fuel dipstick. does anybody know how to correlate inches of fuel to volume on our boats?  I'm an english major BTW..

[PeteW]

1 cu inch = .004329 gallons. I'm assuming you know how to calculate the volume in cubic inches, L"xW"xH", where height is the dipstick mark in inches from the bottom of the tank. It does not matter if the tank is tilted as long as the dipstick measurement is made near the center of the length dimension. You can prove this to yourself empirically using a clear Tupperware container filled with water. Make a mark in the center then tilt the container. The depth does not change.  Pete

[jpendoley]

 But Pete, what if the depth of the tank is not uniform? In other words what if the tank is 12 inches deep at the Stern and 16 inches deep forward, in other words it's not a cube.

[PeteW]

Ah grasshopper, you learned nothing from the Tupperware lesson in my last post.  Because of gravity the surface of the fuel in the tank is level. That represents the length and width of the cube.  The tank is a tilted cube and measuring the depth midway along the length gives you the mean depth or height as if the tank was 14" on each side instead of 12" and 16".  




Pete

[INCOMMUNICADO]

Intern, The next trip to the boat I will get the fuel log then post the information. Allen

[SVJourney]

On Journey, I waited till the tank was empty,then added 5 gals at a time and marked the dip stick as it went up.  Hope to GAWD I never lose that stick!

[PeteW]

What's so complicated about  gallons in tank = L"xW"x Dipstick" x .004329?   LxWx.004329 is a constant. So now you only need to multiply two numbers to determine what's in the tank.    Pete

[SVJourney]

Our dip stick hole (tank filler neck) is in the front of the tank, at the lowest part of the tank.  So the calcs wouldn't work unless you did an integration of the volume as it is an irregular volume. (no constant for the lower part of the tank).  My 5 gal method proved this as the marks for the lower volumes were farther apart than the ones in the cubic area near the top of the tank.

[PeteW]

Yes, if you have the same 4" of slope forward and aft, the marks on first 4" of you dipstick will be further apart/gallon. Once the depth at the front of the tank gets to 4" you can accurately calculate the amount of fuel in the tank by subtracting 2" from the dipstick reading and multiplying. A backup position incase you calibrated stick ever goes missing.

I didn't think this was important because I always believed that was ill-advised to run a diesel tank down that low. At 2" on the dipstick you are probably looking at maybe a 3 gallon reserve and sediment. Some wave action and your sucking air into the lift pump.

I figured the original 50 gallon tank to be 24" x 32"? Let me know and provide the slope and I'll generate an app that will calculate your fuel in gallons.

Pete

[PeteW]

Here is an Excel Spreadsheet that will calculate the volume of any rectangular fuel tank in gallons with a known slope front to back based on a dipstick measurement made at the deep end of the tank.



I tested this with an actual tank using water and its accurate. It calculates the volume of the right angle wedge when the dipstick measurement is below the slope of the tank. Note the second order expression (square) for you Math types.  No integration needed. Simple geometry.

If I can't attach the actual file (out of space on the site) there is enough info here to type it in if you have a few minutes and if you know Excel.

Sorry about the typo here is the correct excel equation for cut and paste
= IF(B4<B8,(0.0021645*(B4^2)*B7*B6)/B8,0.004329*B7*B6*(B4-(B8/2)))                  


Pete

[PeteW]

As promised here's the IPhone app for calculating fuel tank capacity and remaining fuel for any size of tilted rectangular tank using a dipstick calibrated in inches and measured at the deep end.



You need to download the free spreadsheet app at the Appstore from Luminant Software. The format for the equation changes a little to this:    = IF("B1<B7";(0.0021645*(B1^2)*B4*B5)/B7;0.004329*B4*B5*(B1-(B7/2)))  


Pete

[jpendoley]

OMG I love this group!  really appreciate the enthusiasm for all my foolish questions.  I'm with Pete on not wanting to wait till the tank is low to dump fuel in and measure-seems like 90% of diesel engine issues are dirty fuel related and that's just daring the pump to suck up sediment.  Pete, I can't measure the slope because the tank is framed in with timbers supporting the sides and holding it in place. Am I correct in assuming you have measured the slope and its 4"? Or is that just theoretical for the sake of demonstration?

[Della and Dave]

Pete:  You really went above and beyond the call of duty on this one.  What am puzzled about is it seems like you guys must be configured differently than we are.  We have a fuel gage that is supposed to be visible through a round clear window in the cockpit floor.  While I can't read it through the window because it is all scratched up, I can read it directly from inside the starboard locker.  Are you guys just checking the gage accuracy by a dipstick through the fill port, or do you not have a gage?  In out installation, snaking a dipstick down the fill port through the hose and into the tank seems like accuracy would be suspect, unless I could calibrate it, not because the math isn't right, but because I wouldn't know how deep it was, or where in the tank it ended up. 

[PeteW]

I'm going to assume that 50 gallon steel tanks that Pearson installed have all rusted and had to be cut-up and removed. In its place owners have had to install smaller tanks capable of fitting though the sail locker opening.  My 365 has a 30 gallon plastic tank with a fuel gauge sender. My plan is to turn the sender unit so the float is in the deep end. Then I will calibrate the gauge reading in inches. From there I can use the dipstick app to convert that to actual gallons.

I also think that many Pearsons have panels in the sail lockers that seal the engine compartment off. My Pearson has no such paneling at the moment. My problem with my current install is that I cannot carry enough fuel.

Pete

[slokat]

I have original tank, had it inspected about 6 months ago thinking from all that others have gone through that I would need to replace.

No rust, no leaks, no sludge, no algae...

Inspector said probably has always been kept full & stored good fuel?

I know my boat used to be in Florida, then in San Diego.
So it's not a location advantage.... maybe all the PO's left her in the dock?