Ships and Submarines: How Ships Float

Understanding Ships and Submarines: How Ships Float

A ship is not a solid block of steel. Its hull encloses a large space filled mostly with air, machinery, fuel, cabins, or cargo. This gives the whole ship a much lower average density than solid steel.

The broad, hollow shape matters because it can move a large volume of water aside before the deck reaches the water. A solid steel block with the same mass would need far less volume, so it would go down much farther and sink. Engineers therefore think about the mass of every part of a ship together with the shape of the underwater hull.

A floating ship settles at a particular level called its draft. Adding passengers, containers, fuel, or supplies makes the ship heavier, so its draft increases. More of the hull enters the water until the support from the water matches the new load.

This is why ships have load lines painted near their sides. The marks help crews avoid overloading the vessel. Water conditions matter too.

Salt water is denser than fresh water, so it provides more support for the same submerged volume. A ship usually sits slightly higher in the ocean than it does in a river or lake. Temperature can make a smaller difference because water density changes with temperature.

Floating is only part of safe ship design. A ship must remain upright when waves, wind, or a turn make it lean. Designers keep heavy engines, fuel, and ballast low in the hull.

This lowers the centre of gravity and helps the ship recover after a small tilt. The shape of the submerged hull changes during a tilt, which shifts the upward support sideways. If this shift creates a restoring turning effect, the ship tends to right itself.

A top heavy ship can become unstable. Water moving freely inside a partly filled tank is dangerous because it shifts toward the lower side as the ship rolls. Crews use tank layouts and operating rules to reduce this free surface effect.

Submarines use the same ideas in a more controlled way. At the surface, ballast tanks contain enough air for the submarine to support its total mass. To dive, valves let water enter the tanks while air escapes.

The submarine becomes heavier for its size and sinks. At a chosen depth, it can reach neutral buoyancy, meaning it neither rises nor falls much. Small adjustments are made with pumps, tanks, and diving planes, which act like underwater wings when the submarine moves forward.

To surface, compressed air pushes water out of the ballast tanks. Students should separate buoyancy from water pressure on the hull. Buoyancy controls rising or sinking, while increasing pressure at depth means the hull must be strong enough to resist being crushed.