Hey! Great question! Back when they were first building submarines,
they didn't know much about what shape would work the best, so they
tried lots of different ideas. What they found was that if they built
the submarine in the wrong shape, it would collapse when it went too
deep underwater. The reason for that is because the deeper you go
underwater, the more water pressure there is. That's because when
you're way down underwater, there's more water pushing down on you from
above than if you're right at the top.
The other thing they learned was that when you're way down deep
underwater, the water pressure is the same on all sides of the
submarine. (in other words, the water pushes just as hard on the top of
the sub as it does on the bottom or the sides.) So the best shape for
the sub to be is one that's the same everywhere, a circle. That's the
reason that the sub is round through the whole length.
At the ends of the sub, they build it in the shape of a hemisphere.
(That's like the shape you get if you cut a tennis ball in half.) That
sounds kind of weird, because when you see pictures of submarines, the
front end looks like it's pointed. Well, it's actually a bit of both.
At the front of the sub, the metal is shaped into a point so that the
sub can slide through the water more easily. But what they do is they
build an extra wall inside of the pointed wall that's shaped like a
hemisphere. Then they set it up so that the water can get in between
the two walls. That way, the sub can still cut through the water
smoothly, but the part that the water pushes against is circle-shaped.
Another example of the same thing happening is bubbles. The air
pushes in on the outside of the bubble the same way that the water
pushes in on the submarine. That's why bubbles are round instead of
One way you can check this out at home is if you blow up a balloon
and hold it underwater in your sink or the bathtub. You'll see that the
balloon won't change shape, because it's round. But if you were to put
something hollow that's shaped with flat sides (like a box made of tin
foil) underwater, it squishes in on itself pretty easily.
(republished on 07/30/06)