Why Alu tank float?

[Jean eve]

Just take one ALu 80cft and steel 80cft.
Alu:weight (empty)about 35lb
empty weight buoyancy about 2lb

steel(Faber):weight(empty) about 32lb
empty weight buoyancy about -7lb

From above can see that Alu is more heavier than steel but when it's empty why it become buoyancy?Is't because of the density of the Alu??Can anyone explain about this?Thanks.

 

[Diver0001]

Just take one ALu 80cft and steel 80cft.
Alu:weight (empty)about 35lb
empty weight buoyancy about 2lb

steel(Faber):weight(empty) about 32lb
empty weight buoyancy about -7lb

From above can see that Alu is more heavier than steel but when it's empty why it become buoyancy?Is't because of the density of the Alu??Can anyone explain about this?Thanks.

alu isn't heavier than steel. There is just more of it in your tank and therefore an aluminium tank is physically larger than a steel tank with the same internal volume. The larger tank (the alu) displaces more water which creates more buoyant force and blah blah blah. follow?

R..

 

[Jean eve]

I think I get it.Thanks

 

[IslandHopper]

Well... stand those two tanks next to each other, and you will see that the Aluminum tank is physically a bit larger than the steel tank.
Large Enough that it displaces 37 pounds of water (where the smaller steel tank only displaces 25 lbs of water).

(meaning a volume of water the same as the EXTERNAL dimensions of the tank will wiegh that amount)

so the Aluminum tank that weighs only 35 lbs, but displaces 37 lbs will float (2 lbs worth of "volume" will be above the water-line).


To exagerate the scenario... A plastic shell, the same dimensions as the tank might wiegh only one pound, but would still displace 37 lbs worth of water ... and would therefore float with almost the entire "tank" above the water-line (only one pound worht would be below the water-line).

 

[yc92602]

Here's what I am thinking.

If both aluminum and steel tanks are rated at the same pressure and (uncompressed) air volume, their actual internal volume must be the same. This is pure physics and must be true.

Now, why the aluminum tank is usually heavier and more buoyant? The only explanation is that the wall of an aluminum tank is much thicker, and this is mainly because the AL is not as strong as steel.

The density of the aluminum is about 1/3 of the steel, therefore, the thickness of the aluminum tank wall is about (or more than) 3 times of the steel tank. And thus generates more bouyancy.

 

[Jean eve]

Originally posted by yc92602
If both aluminum and steel tanks are rated at the same pressure and (uncompressed) air volume, their actual internal volume must be the same. This is pure physics and must be true.

Now, why the aluminum tank is usually heavier and more buoyant? The only explanation is that the wall of an aluminum tank is much thicker, and this is mainly because the AL is not as strong as steel.

The density of the aluminum is about 1/3 of the steel, therefore, the thickness of the aluminum tank wall is about (or more than) 3 times of the steel tank. And thus generates more bouyancy.

Seems that this explanation is btter!

 

[scubatoys]

Yes, aluminum tanks have much thicker walls...

Then just to confuse things, which weighs more... a pound of feathers or a pound of gold?? Answer... pound of feathers. Gold is weighed in Troy pounds that are only about 80% of a normal pound.

Same with tanks... we have to make sure we are measing things the same for both cases. Which holds more an Aluminum 72 (if you could find one) or a steel 72.

Old steel 72's are not 72 unless over filled by 10%, other wise they are about 67's. This will throw off a lot of your calculations and thinking as well.

 

[IslandHopper]

The density of the aluminum is about 1/3 of the steel, therefore, the thickness of the aluminum tank wall is about (or more than) 3 times of the steel tank. And thus generates more bouyancy.

Bouyancy is simply a function of total weight VS total displacement.

 

[nyresq]

the same reason a 65,000 ton aircraft carrier floats while a 1 lb block of aluminum sinks. A solid block of steel weighing 65,000 tons would be the size of a house, whereas the carrier is esentially hollow and is the same weight but the size of a skyscraper. It (the carrier) displaces more water so it floats, the solid block weighs the same but displaces very little water and it sinks.

the steel tank is physically smaller and compact and displaces less water relative to its weight, so it sinks. the aluminum being physically larger displaces more water relative to its weight and floats.

its like the trick question dad used to ask you when you were a kid.... which weighs more: a pound of feathers or a pound of cement?

 

[miketsp]

Now just to see if you got it.
A man goes out on a lake in a boat carrying 20lbs of ice.
He throws the ice into the water.
The ice melts.
Describe what happens to the water level of the lake in relation to the bank throughout the process.

Tip.: the ice is made from water from the lake.