A submarine is built around a strong inner shell that keeps the crew dry and safe at depth. Look inside to see the two hulls, the tanks that flood or empty to dive and surface, and the fins and frames that hold the whole shape together.
A submarine is a vessel built to travel and work underwater. Its key feature is that it can change its depth at will — sinking below the surface, holding a chosen depth, and rising again. Everything in its design serves that ability and the need to keep the crew safe while the water presses in.
To do this, a submarine is built like a tube within a shell. The inner tube is the pressure hull, a strong rounded shell that holds the crew and machinery at normal air pressure. Around it is a lighter outer hull that gives the boat its smooth, streamlined shape. In the gaps between the two hulls sit the main ballast tanks, and along the length run fins, frames and small trim tanks.
From outside, a submarine is a smooth dark shape with a tower on top, and there is no clue that two hulls share the same body. Hiding the outer hull shows the layout: the strong pressure hull running through the middle, the ballast tanks at bow and stern, and the frames that hold the shape against the sea.
Two ideas hold the design together. The first is the difference between the two hulls: the outer shell only shapes the boat, while the inner shell takes the pressure. The second is control of weight: flooding ballast tanks makes the boat heavier so it dives, blowing them clear makes it lighter so it surfaces, and small trim tanks keep it level in between.
Most submarines have two hulls that do different jobs. The outer one is a light shell that gives the boat a smooth, streamlined shape and carries equipment such as the dive planes. It is not built to resist deep water. Inside it sits the pressure hull, the strong watertight shell that protects the crew.
The pressure hull is a thick, rounded shell, often shaped as a long cylinder with domed ends. Its curved walls spread the squeezing pressure of the surrounding water evenly around the ring, so no one spot is overloaded. Because it stays watertight, the crew inside live at ordinary air pressure even when the sea outside is pressing hard.
Around the pressure hull, ring-shaped frames are fixed at regular intervals. These ribs stiffen the shell and help it keep its round section against the water pressing in from every side, in much the same way the hoops of a barrel hold its staves together. They are part of what makes the rounded shell so strong.
The main ballast tanks sit between the two hulls, at the bow and the stern. To dive, valves are opened and the tanks fill with seawater, so the boat becomes heavier than the water it pushes aside and sinks. To surface, compressed air is pushed into the tanks to force the water back out, and the boat becomes light enough to float up again.
Trim tanks hold smaller, precisely controlled amounts of water. By moving this water forward, aft or across the boat, the crew set the exact angle they want to hold and keep the submarine level and steady. The main ballast tanks do the diving and surfacing; the trim tanks fine-tune the balance in between.
On the bow and stern are the dive planes, small fins like the tail of an aircraft. As the boat moves forward, angling these fins up or down pushes the nose deeper or shallower. Working together with the ballast tanks, they let the submarine change depth smoothly instead of just sinking and rising.
The sail is the tall tower rising from the top of the hull. Its masts and periscopes reach up through it, and it is the part that breaks the surface first when the boat comes up for air or to look around. The deck runs along the hull beside it, with watertight hatches through which the crew can pass from the sealed inside out onto the working deck.
At the stern, the propeller pushes the boat forward through the water while it is submerged. Behind the propeller is the rudder, a vertical fin; turning it redirects the water being pushed back, which swings the stern and steers the boat left or right. Together with the dive planes, these control where and how deep the boat travels.
The light outer hull gives the boat a smooth, streamlined shape and holds equipment, but it is not built to resist deep water. The strong pressure hull inside is the watertight shell that protects the crew, so the two hulls do different jobs.
A curved shell spreads the squeezing pressure of deep water evenly around itself, so no single area is overloaded. A flat-sided box would have sharp corners where the pressure concentrates, which is far harder to build strong.
Main ballast tanks between the two hulls are flooded with seawater so the boat becomes heavier than the water it pushes aside and sinks. To surface, compressed air is pumped into the tanks to force the water out, making the boat light enough to float again.
Dive planes are small fins on the bow and stern. When the boat is moving, angling them up or down pushes the hull deeper or shallower, so they act like the elevators on an aircraft and control depth.
Trim tanks hold smaller amounts of water that can be shifted between compartments. Moving this water fore, aft or sideways keeps the boat level and sets the angle it holds, separate from the main dive-and-surface tanks.
The frames are ribs fixed around the pressure hull. They stiffen the shell and help it keep its round shape as the surrounding water pushes in, much like the hoops that hold a barrel together.
The sail is the tall tower rising from the hull. Masts and periscopes are mounted on it, and it is the part that breaks the surface first when the boat comes up for air and observation.