Submarines and submersibles
Take a hull under water and bring it back: trim it with ballast, build it for the pressure at its depth, and dive only a classed, tested design with a trained crew.
Everything for this step
- Field guide
- None yet. A guide goes deeper than a step: technique, identification and the hazards.
- Builds
- None yet.
- Reference sheets
- 1 sheet on this page ↓
- Sciences it rests on
- Physics · Engineering
- Track
- On water, level 5 of 6 · next: Modern ships and small craft
Why this matters
A submarine must weigh exactly what the water it displaces weighs, and its hull has no margin for guesswork: at 300 m every square metre carries about 300 tonnes. Practical boats came with the internal combustion engine and the storage battery around 1900; nuclear power, from 1955, let them stay down for months. Research submersibles now take scientists to 6,500 m.
The shape of it
- Trim to neutral buoyancy: ballast tanks to dive and surface, trim tanks to hang level.
- A pressure hull for the depth: a ring-framed cylinder or a sphere, tested beyond the deepest dive.
- Diesel on the surface, batteries under water, or a reactor.
- Scrub out the CO₂ and supply oxygen.
- Classed designs only, pressure-tested; crew trained.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 — How it dives and surfaces
A submarine goes down by swapping air for water in tanks, not by getting heavier all over.
- Main ballast tanks hold air on the surface. Open the vents at their tops and the sea floods in from below: the boat loses its spare buoyancy and submerges. To surface, blow compressed air into them.
- Trim tanks move smaller amounts of water in and out, and fore and aft, until the boat under water weighs exactly what it displaces (neutral buoyancy) and sits level.
- Hydroplanes, small wings at the bow or sail and the stern, steer it up and down while it moves. Stopped, only the trim holds its depth.
- Sea water is 1,025 kg/m³, fresh water 1,000: a boat trimmed for the sea is about 2.5 % too heavy in a river mouth.
2 — See it work: a Cartesian diver
The whole principle fits in a plastic bottle.
- Fill a 1.5–2 L plastic bottle to the brim with water.
- Draw water into a medicine dropper until it just floats, its top level with the surface. Test it in a glass first.
- Put it in the bottle and screw the cap on tight.
- Squeeze the bottle. The pressure shrinks the air bubble in the dropper, water flows in, and the diver sinks. Let go and it rises.
- Hold a light, steady squeeze and it hangs mid-bottle: neutral buoyancy, the state a submarine is trimmed to.
The same squeeze is what depth does to every air space in a hull.
3 — Pressure: why the hull is round and framed
Sea pressure rises by about 1 atmosphere (101 kPa) every 10 m. At 300 m every square metre of hull carries about 300 tonnes; at the depth of the Titanic, 3,800 m, about 3,900 tonnes.
- Shape: a hull under outside pressure is a cylinder stiffened by ring frames, or a sphere for the deepest craft. Flat panels and square corners fail first.
- It fails by buckling, not by bursting: an unstiffened cylinder crumples inward long before its metal reaches its strength, so the frames matter as much as the plate.
- Material: high-strength steel (the US Navy's HY-80 yields at 80,000 psi, 550 MPa) or titanium; deep research spheres are titanium.
- Depth is set by test: the hull is pressure-tested beyond its operating depth, and the boat never goes below the depth it was tested to.
4 — Power and air under water
- On the surface a diesel engine drives the boat and charges the batteries. Under water the batteries drive an electric motor: the engine needs air, so it stops. By the First World War this was the standard layout (The Story of the Submarine, chapter IX), and every non-nuclear submarine still uses it.
- Snorkel: a mast that lets the diesel run with the boat just under the surface.
- Nuclear: a reactor needs no air, so the boat can stay down for months. USS Nautilus signalled "Underway on nuclear power" on 17 January 1955.
- Air: each person breathes out about 1 kg of CO₂ a day. It is scrubbed out with lithium hydroxide, soda lime or amine, and oxygen is made or carried. In a sealed hull CO₂ becomes harmful well before the oxygen runs low.
5 — Classed designs only, tested before anyone dives
This site gives no construction procedure for a submarine or submersible that carries a person, for the same reason it gives none for a manned aircraft. The recognised route is a design classed by a classification society (for example ABS's Rules for Building and Classing Underwater Vehicles, Systems and Hyperbaric Facilities, or DNV's rules for manned submersibles), built under its survey, pressure-tested, and crewed by trained pilots.
Why: Titan, a carbon-fibre submersible that was never classed, imploded on 18 June 2023 on a dive to the Titanic, killing all five aboard. The NTSB found that the company's engineering failed to establish the actual strength and durability of the hull, which delaminated on earlier dives and failed by local buckling (NTSB). The Coast Guard's investigation board called the loss preventable.
The classed route works: Alvin, a three-person research submersible in service since 1964, is certified by the US Navy to 6,500 m (WHOI).
Reference sheets
One drawn sheet covering the whole subject at once — the breadth a photograph cannot give, bought at the cost of accuracy. Read them for range and vocabulary, never as the thing you identify from.