How a Submarine Sinks on Purpose

The story

Ships are built to do one job above all others: stay UP. Keep the water out, keep the people dry, float, float, float.

So a submarine sounds like a terrible mistake. It is a ship that sinks. On purpose. Then comes back up, smiling, and does it again. How can sinking possibly be safe? The answer is one beautiful trick, and you will see it everywhere, from submarines to the fish in a pond.

Start in the bath. Push an empty plastic bottle, capped and full of air, down under the water. Feel that? The water pushes it right back up. Now open the cap and let the air bubble out while water floods in. The more water the bottle drinks, the lazier its floating becomes, until at last it barely floats, and may even quietly sink. Same bottle. The only thing that changed is what was inside it: air, or water.

That is a submarine. A submarine is a strong steel bottle with people inside, and built into its body are big hollow spaces called ballast tanks. Those tanks are the answer. When the captain wants to dive, valves open, and seawater floods into the tanks with a great whoosh. The submarine drinks the sea and loses its floatiness, and then its propeller and tilting fins, called diving planes, steer it gently down under the waves. The sea fills the tanks and other outside spaces built for it, while the crew stays dry in their sealed rooms.

And when it is time to come home to the sunshine? It swims upward with propeller and planes, then spits the water back out so it can float again. It cannot open a window and pour, of course, so it carries air, squashed and squeezed into strong containers until it is packed tight with push. Let that squashed air rush into the tanks, and it shoves the seawater out through the openings, the tanks fill with air again, and the submarine rides high on the surface. Drink water to lose your float. Push water out to get it back. That is the heart of the trick, with propeller and fins doing the steering.

Now here is my favourite part. Long before submarines, fish were already solving the floating problem their own way. Inside many fish is a stretchy bag of gas called a swim bladder, and by slowly adjusting its gas, each kind in its own way, a fish can tune itself until it neither rises nor sinks. That is why a fish can hover in one spot like magic, resting in the water as if it weighed nothing. People used steel and squashed air to win the prize the fish were born chasing: command of your own floatiness.

So the next time you are in the bath with a plastic bottle, run the experiment yourself. Hold it under full of air and feel it fight to rise, then tilt it, let the air bubble away, and feel the fight fade. Your hands will be learning the same floating trick that lets real submarines dive and rise through the deep, dark sea.