Volume of a tank (alcohol)

harbin2 · #1 (**permalink**) 06-20-2010

I'm trying to figure out if I need to add alcohol. I have a tank that is 8" in diameter. I emtied it over the winter and "cleaned" it out with some fresh alcohol. I added a gallon. I just got back from a 2 week cruise on the (Chesapeake) Bay and used the stove fairly extensively. I want to know how much I used.
OK - the formula for the volume of a cylinder is Pi x r squared x the height. I measured the depth at 3.5" with a wooden dowel.
My problem is I'm not sure about the units.
To come close to what I'd expect the volume to be, I converted the radius and the height to feet. That results in a volume of .916 gallons.
However, it seems like I have had to use more fuel than .084 gallons.
I should probably have posted this to a math forum but I figure someone here has done this calculation before (or has some more practical method for determining how much fuel is left).
harbin2
Islander 30 Bahama

Allanbc · #2 (**permalink**) 06-20-2010

Why not just refill the tank and see how much alcohol it takes? Sounds like a simple solution to me.

huguley3 · #3 (**permalink**) 06-20-2010

If your tank is like mine and it probably is. Then the rounded bottom is going to throw off your calculation since it will seam like there is a higher level in the tank.

I go through about a gallon of alcohol/month. We generally only cook dinner on it. I have never tried it but I think you could get 2 gallons into the tank. I have been surprised at how long a gallon of alcohol will last so have not really tried to fill it up.

Whatever you do don't leave the cans of alcolhol in the cockpit. They will rust surprisingly quickly and stain the non-skid in a permanent way. Ditto for propane bottles. I have unremovable rust stains from both.

harbin2 · #4 (**permalink**) 06-20-2010

Don't we have any math majors out there?
I got the boat 2 years ago and when I poured out what was in the tank, it was pretty rusty. At between $15 and $40 a gallon, I don't want to put the 3 or 4 gallons in the tank that I think it will hold. I'd rather add it periodically and keep it "fresh". Or, maybe I SHOULD keep it full to prevent condensation like I do with my diesel tank over winter???
Yes, mine is also dished at the bottom and top so that will make my figure a little high.

mitiempo · #5 (**permalink**) 06-20-2010

Assuming your stove is a pressure stove you don't want to fill the tank. If it is full you will be pumping air into it regularly. Best to keep it between 1/2 and 3/4 full.

wind_magic · #6 (**permalink**) 06-20-2010

Quote:

Originally Posted by harbin2

I'm trying to figure out if I need to add alcohol. I have a tank that is 8" in diameter. I emtied it over the winter and "cleaned" it out with some fresh alcohol. I added a gallon. I just got back from a 2 week cruise on the (Chesapeake) Bay and used the stove fairly extensively. I want to know how much I used.
OK - the formula for the volume of a cylinder is Pi x r squared x the height. I measured the depth at 3.5" with a wooden dowel.
My problem is I'm not sure about the units.
To come close to what I'd expect the volume to be, I converted the radius and the height to feet. That results in a volume of .916 gallons.
However, it seems like I have had to use more fuel than .084 gallons.
I should probably have posted this to a math forum but I figure someone here has done this calculation before (or has some more practical method for determining how much fuel is left).
harbin2
Islander 30 Bahama

V=(pi)(r^2)h
V=(3.14)(4^2)(3.5)
V=(3.14)(16)(3.5)
V=175.84 cubic inches
175.84 / 231 (cubic inches per gallon) = 0.7612 gallons

Why did I get something different than you did ?

I'll try it in feet too and see if I get the same thing.

8 in diameter = 0.3333 ft
3.5 in = 0.2917 ft

V=(pi)(r^2)h
V=(3.14)(0.3333^2)(0.2917)
V=0.10175 cubic feet
0.10175 / 0.133680556 (cubic feet per gallon) = 0.7611 gallons

Same result either way.

Just holding up your hands and making an 8 inch diameter circle and imagining that it is 3.5 inches high, I don't see how that could hold as much as a gallon container of milk, so I think the 0.7612 gallons is probably right.

harbin2 · #7 (**permalink**) 06-20-2010

Thanks Magic
3/4 gallon sounds more reasonable. And thanks for the math. I should have thought more about the units. I hadn't really considered that last step of converting cubic inches or feet to gallons. That was exactly what I needed and you made it crystal clear.
And it makes perfect sense to keep a volume of air in the tank too since liquid doesn't pressurize too well. Thanks.