How Do Submarines Dive and Surface? Ballast, Buoyancy and Staying Hidden
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A submarine is a ship that can choose its own weight. On the surface it floats like any vessel, displacing its weight in water; to dive it takes water into tanks until it weighs a little more than the water it displaces, and sinks; to surface it forces the water out with compressed air and floats again. Between the two, at depth, it balances so exactly that a few tonnes either way, the crew moving aft or a torpedo leaving the tube, must be corrected at once. The principle is Archimedes', the practice is an engineering discipline, and the point of it all is that a submerged submarine is the hardest thing in the world to find.
Archimedes underwater
Anything in water is pushed up by a force equal to the weight of the water it displaces. A submarine's hull displaces a fixed volume, so its buoyancy is fixed, and what changes is its weight. The main ballast tanks, which sit between the pressure hull the crew lives in and the outer hull, are open to the sea at the bottom and vented at the top; on the surface they are full of air and the boat floats high. To dive, the vents are opened, air escapes, water floods in from below, and the submarine becomes heavier than the water it displaces and goes down. To surface, high-pressure air from storage bottles is blown into the tanks, forcing the water out through the bottom, and the boat rises. An emergency blow, with every bottle at once, brings a submarine from depth to the surface in seconds, breaching like a whale.
Trim
The main tanks are all or nothing; the fine control is done with trim tanks inside the pressure hull, from which water is pumped in or out by the tonne to make the boat exactly neutral, neither rising nor sinking, and between a forward and an after tank to level it. Sea water is not uniform, since it is denser where it is colder or saltier, and a submarine that is neutral in one layer will sink or rise on crossing into another, so the trim is adjusted continuously. Once neutral and moving, the boat steers in depth with hydroplanes, small wings on the bow or sail and on the stern, like an aircraft, and can hold a depth to within a metre. A submarine at rest with no way on is at the mercy of its trim, and a boat that is a few tonnes heavy and stops moving will sink until something is done. The main parts:
- •Pressure hull: a cylinder of thick steel or titanium that holds the crew at atmospheric pressure against the sea
- •Main ballast tanks: outside the pressure hull, flooded to dive and blown to surface
- •Trim and compensating tanks: inside, for fine adjustment of weight and balance
- •Hydroplanes and rudder: for controlling depth and heading under way
- •Sail: the tower on top, holding the periscopes, masts and, on the surface, the bridge
How deep
Pressure rises by one atmosphere for every ten metres, and at 300 metres the sea presses on the hull with 30 times the force of the air inside, about 300 tonnes on every square metre. The pressure hull resists it as a cylinder, the strongest shape for the purpose after a sphere, and its thickness sets the limit: military submarines are believed to operate at 250 to 500 metres and to have crush depths somewhat deeper, the figures being secret. A hull that fails does so instantly, as the Thresher's did in 1963 and the Titan submersible's did in 2023, in a fraction of a second. The deepest-diving crewed vehicles are not submarines but bathyscaphes and submersibles with small spherical cabins, and the Trieste reached the bottom of the Mariana Trench, 10,900 metres, in 1960, and the Limiting Factor has since made the dive routinely.
Air, power and time
A diesel-electric submarine runs its diesels on the surface or at snorkel depth to charge batteries, and runs on the batteries submerged, which limits it to a day or two underwater at slow speed before it must come up and become findable; air-independent systems using fuel cells or Stirling engines stretch that to weeks. A nuclear submarine's reactor needs no air, drives the boat at over 30 knots indefinitely, and makes oxygen from sea water by electrolysis and scrubs carbon dioxide from the air, so that the only limit on submergence is the food, typically 90 days, and the crew's sanity. The first nuclear boat, Nautilus, went under the North Pole in 1958, and the ballistic-missile submarines that have patrolled since the 1960s spend months at a time where nobody can find them, which is the whole reason they exist.
Hiding and finding
Water blocks radio and light, so a submerged submarine is invisible to radar and cannot be seen, and it is hunted by sound. Its own noise, from machinery, propeller and the flow over the hull, is what gives it away, and every design choice from rubber-mounted engines to the shape of the propeller blades, which is classified, is about quietness; a modern boat at slow speed is quieter than the sea around it. Hunters listen with passive sonar, hydrophones on ships, aircraft-dropped buoys and cables laid on the seabed, and ping with active sonar when they must, at the cost of revealing themselves. The submarine listens back, and the contest between hiding and finding, run for a century with no clear winner, is why the world's navies still build a machine whose main achievement is to make itself disappear.
The takeaway
A submarine dives by flooding ballast tanks so that it weighs more than the water it displaces and surfaces by blowing them empty with compressed air; at depth it is trimmed with internal tanks to be exactly neutral and steered up and down with hydroplanes. Its pressure hull resists an atmosphere of pressure for every ten metres, nuclear power removes the need to surface for air, and because water blocks radar and light it is hunted and hides by sound alone.