1. How Can a Ship Made of Metal Float at All?
Ordinary iron and steel are several times denser than water, so a solid lump sinks. A ship floats because of its shape. Archimedes' principle says that the upward push (buoyancy) on an object in water equals the weight of the water it pushes aside [1]. A ship's hull is mostly empty space, so it has to sink only part of the way before the water it displaces weighs as much as the whole ship. Load more cargo and the hull sinks a little deeper until the two weights match again.
Floating, though, does not guarantee staying the right way up. That is a second question, and the rest of this page is about it. First, a look at how deep a ship is allowed to sit.
2. How Deep Is a Ship Allowed to Sink? The Load Line
If a ship floats lower with every ton of cargo, someone has to say where to stop. Load lines are marks on a ship's side that show how low it is allowed to sit in the water when loaded [2]. They limit loading; they do not guarantee that a ship cannot sink. The idea is old. Royal Museums Greenwich notes that ships from Venice were marked with a cross to protect them as long ago as the 12th century. In the 19th century overloading was a serious problem, and the British MP Samuel Plimsoll campaigned for load lines. Lloyd's Register, a ship classification society, had published freeboard tables for loading in 1835, but they applied only to ships it classed [2]. Today an international treaty, the International Convention on Load Lines (adopted 1966), covers the subject [3].
The load line stops a ship from sitting too deep. It does not say whether the ship will lean over, which depends on where the weight is.
3. Why Does a Tilted Ship Swing Back Upright?
Naval architects track two points. The centre of gravity (G) is the point where the whole ship's weight can be thought to act. The centre of buoyancy (B) is the centre of the water the hull pushes aside [5]. When the ship sits upright, G and B lie on the same vertical line. When it tilts, the shape of the underwater part changes: more hull goes under on the low side and less on the high side, so B slides toward the low side while G stays put, because the cargo has not moved [5]. The weight pulls down through G and the buoyancy pushes up through the shifted B, and the pair twists the ship back toward upright. That twisting effect is called the righting moment [4].
Engineers summarise it with one more point, the metacentre (M). For small tilts, M is where the line of the buoyancy force crosses the ship's centreline. The distance from G up to M is the metacentric height, written GM. A bigger GM means a stronger pull back to upright for small tilts [5]. A simple way to picture it is a weighted toy with a rounded base: its weight is low down, so when you push it over, it rocks back upright. The comparison is limited: a ship rights itself because its changing underwater shape shifts the buoyancy force, while the toy's rounded shape and low weight make it roll upright. And like that toy, a ship tipped far enough no longer comes back. The GM idea is a guide for small tilts only; for big tilts engineers use a full curve of righting arm against tilt angle [4].
4. Why Does Stacking Cargo High Make a Ship Less Stable, and Can Stability Be Too Much?
Raising weight raises G, and raising G shrinks GM, so the pull back to upright gets weaker. A ship with small GM is called "tender": it leans easily and rolls slowly. Too much stability is also a problem. A ship with a very large GM is "stiff": it snaps back quickly, so it rolls with a short period, which is uncomfortable for people and puts more strain on whatever holds the cargo [5]. The target is not the biggest GM possible but a sensible one for each voyage.
Partly filled tanks add another trap. When a ship leans, the liquid in a half-empty tank runs to the low side, which acts as if weight had moved upward and reduces GM. This is called the free surface effect [5], and it is one reason why a tank that must be used is normally kept either full or empty rather than half full.
5. Why Do Empty Ships Take On Seawater?
You might expect an empty ship to be the safest. But a ship with no cargo floats high, so the propeller and rudder may not be deep enough in the water to work well, and the weight is no longer where the designers wanted it. Ships therefore pump seawater into tanks called ballast tanks. According to the International Maritime Organization (IMO, the United Nations agency for shipping), ballast water has been used to stabilise ships since the introduction of steel-hulled ships. It reduces stress on the hull, gives transverse (side-to-side) stability, improves propulsion and steering, and makes up for the changing weight of cargo, fuel and water during a voyage [6].
Ballast water has a side effect. It carries bacteria, small animals, eggs and larvae from one port to another, and when it is released some of them can settle and become invasive species. IMO says scientists first recognised this with a mass appearance of an Asian algae, Odontella, in the North Sea in 1903 [6]. To tackle the problem, the Ballast Water Management Convention was adopted in 2004 and entered into force on 8 September 2017, requiring ships to manage their ballast water so that organisms are removed, made harmless, or not taken up or discharged [7]. A cargo ship's safety tool is also a way of moving living things around the world, and the rules try to deal with both.
6. So How Do Container Ships Stay Upright With Cargo Stacked So High?
The short answer from the top of the page can now be filled in. A wide hull can increase the distance from B to M, but GM also depends on the ship's weight distribution and hull shape, so designers balance these factors for each ship. Heavy cargo goes where it keeps G low enough. Ballast is added or left out to fine-tune GM. And because every voyage carries a different set of boxes, officers plan the placement for each trip rather than reusing one plan.
I could not find an English-language primary source that gives a precise rule for stack height or a required GM value, so this page does not quote one. What the sources do show is that staying upright is not the only danger. Gard, a marine insurer, explains that in head or following seas a ship can suffer parametric rolling, in which rolling grows suddenly. The roll angle can increase from comfortable motion to more than 30 degrees in only a few cycles, putting heavy forces on the container stacks [8]. A ship can stay afloat and upright and still lose boxes overboard. Understanding that gap between "does not capsize" and "keeps its cargo" is part of what ship designers and operators work on today.
7. How Can You See Stability for Yourself?
At home: float a small plastic tub in a sink or a large bowl of water. Fix a lump of modelling clay low inside the tub, gently push one side down and let go, and watch how quickly it returns. Now stick the same clay on top of a small stick taped upright in the tub and try again. Keep the water in the sink, wipe up any spills, and keep the clay away from small children and pets. Moving the clay higher raises the tub's centre of gravity and makes it less stable, so it should lean more easily and roll more slowly. If you visit a harbour, look at a ship's side for the load line mark. For further reading, NASA's short page on Archimedes' principle [1] and the Royal Museums Greenwich story about Plimsoll [2] are both written in plain English.