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Q & A: Submarines and Water Pressure

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Most recent answer: 10/22/2007
Q:
I am trying to find out more about how submarines are capable of operating. Specifically, how does their hull design help them withstand water pressure?
- R Murdoch
Trenton Elementary, Trenton , Maine, U.S.A.
A:
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 square-shaped.

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.

-Tamara

(published on 10/22/2007)

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