Nib asked
How does a submarine go up and down?
A submarine does not dive by pushing itself down with fins. It dives by getting heavier: it swaps air for seawater in its ballast tanks, and swaps it back to rise.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the myth
Most people picture a submarine diving the way a plane does, by tilting its fins. The fins matter, but they are not the main trick. A submarine dives by getting heavier.
A submarine dives by getting heavier, not by pushing down with fins.
a common mix-upFins do not make a submarine dive on their own. The main change is in weight, made by flooding ballast tanks.
2tug of war
A boat floats or sinks depending on a tug of war. Its weight pulls it down, and the water pushes it up. Seawater weighs about 1.025 kilograms per litre.
Weight against the push of the water decides up or down.
3ballast tanks
Inside the hull are ballast tanks, big chambers that can hold either air or seawater. Full of air they keep the boat afloat. Full of water they make it heavier.
Ballast tanks hold air to float and water to sink.
4dive
To dive, the crew opens vents at the top of the tanks, so the air escapes, and valves at the bottom, so seawater floods in. The boat becomes heavier than the water it pushes aside, and it sinks.
Open the vents, flood the tanks, get heavier, sink.
5surface
To come back up, the crew shuts the vents and lets compressed air from storage flasks into the tanks. The air pushes the water out through the bottom, the boat becomes lighter, and it rises.
Blow the water out with compressed air and the boat floats up.
6trim tanks
Smaller trim tanks near the front (the bow) and the back (the stern) hold a little water. Pumping water between them keeps the boat level. When weight and the push of the water balance, the boat can hover at one depth.
Trim tanks level the boat; balanced weight lets it hover.
7hydroplanes
Near the stern are movable fins called hydroplanes. They act like wings: they only make a force when water streams past, so the boat has to be moving. Tilted, a fin pushes the tail up, which angles the nose down, and the moving boat glides deeper.
Hydroplanes steer the dive angle, but only while moving.
Can a submarine that is sitting still use its hydroplanes to dive?
Show the answer
No. Hydroplanes need water flowing past them. A stopped boat changes depth with its tanks.
8the whole idea
So: flood the tanks to sink, blow them with air to rise, trim to stay level, and use the fins to steer while moving.
A submarine never fights the sea to go down or up. It changes its weight, and the water does the rest.