Re: how do iron or steal ships float

If you take a large aluminum pie pan and set it carefully on the surface 
of a sink full of water, it won't sink, unless it has a hole in it.  If 
you really want it to sink, you have to turn it sideways.  Then you can 
make it go all the way to the bottom because there is no air in it.

But if you "float" that pie pan on the surface of the water, and try to 
push it straight down into the water, it pushes back, doesn't it?  It 
doesn't want to sink.

What you feel pushing back is called a "buoyant" force.  And that is what 
makes iron ships float.  What you are doing when you push the pie pan down 
is to try and displace the water - to move it out of the way.  It doesn't 
like to do that.  When you try to move water with something lighter or 
less dense that itself, it doesn't like that.  If you blow through a straw 
into a glass of water, the water immediately pushes the bubbles up to the 
surface, doesn't it?

That's the key.  All the iron ships float because they are filled with 
air, and the combination of metal and air is, overall, lighter than water.

It's the same principal used to make helium balloons float, and blimps and 
dirigibles fly.  Without the helium, the balloon just lies flat, and stays 
wherever you put it.  Filled with helium, the balloon-helium combination 
is lighter than air, and it floats to the ceiling, or if there's no 
ceiling, you lose it .

Blimps and dirigibles used to be called "Lighter-Than-Air" craft.  So 
ships can be called "Lighter-Than-Water" craft.  The Titanic sank, not 
because it was made of iron, but because it filled with enough water to 
make it a "Heavier-Than-Water" craft.  It took two hours and forty minutes 
for it to fill with enough water to make it sink.  If you put a hole in 
your aluminum pie pan, it will sink, too.  How fast depends on the size of 
the hole.