What Is Buoyancy? Why a Steel Ship Floats and a Steel Bar Does Not
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Steel is nearly eight times denser than water, and ships are made of it. The resolution is that floating has nothing to do with what a thing is made of and everything to do with how much water it pushes aside. A hollow hull displaces a volume of water far heavier than itself, and the weight of that displaced water is exactly the upward force holding the ship up.
The principle
An object placed in a fluid experiences pressure on every surface, and because pressure in a fluid increases with depth, the pressure pushing up on the bottom of the object exceeds the pressure pushing down on the top. The difference is a net upward force, and Archimedes' principle states that it equals the weight of the fluid the object displaces. That single statement covers every case. If the object weighs less than the fluid it would displace when fully submerged, it rises until it floats with just enough of itself under the surface to displace its own weight. If it weighs more, it sinks. If the two are equal it is neutrally buoyant and stays where it is put. A steel bar displaces only its own small volume and therefore a small weight of water, far less than the bar weighs, so it sinks. A hull enclosing air displaces a volume enormously larger for the same mass of steel, and floats. The principle applies in gases too, which is why a balloon filled with a gas less dense than air rises.
Floating is not the same as being stable
A vessel can float perfectly and still capsize, because staying upright is a separate problem governed by where the forces act rather than how large they are. Weight acts downward through the centre of gravity, and buoyancy acts upward through the centre of buoyancy, which is the centre of the displaced volume. When a hull tilts, the underwater shape changes and the centre of buoyancy shifts sideways, which creates a turning effect. If that effect rights the vessel, it is stable; if it worsens the tilt, it capsizes. Naval architects describe this with the metacentre, a geometric point whose height above the centre of gravity determines stability: a large separation gives a stiff vessel that rights forcefully and rolls uncomfortably, and a small one gives a tender vessel that rolls gently and has less margin. Loading matters directly, since cargo placed high raises the centre of gravity, and free surface effect, in which liquid sloshing in a partly filled tank or on a flooded deck moves with the tilt, reduces stability sharply and has sunk many ships.
How living things use it
Buoyancy control is a problem every aquatic organism has solved somehow:
- •Swim bladders in most bony fish, gas-filled sacs whose volume is adjusted to match depth, which makes a fish neutrally buoyant and removes the cost of swimming to stay at a level
- •Sharks, which have no swim bladder and instead rely on a very large oily liver, since oil is less dense than water, plus lift generated by swimming continuously
- •Marine mammals, which adjust buoyancy by controlling how much air they carry in their lungs and by exhaling before deep dives, and whose blubber contributes
- •Cephalopods including the nautilus, which pumps fluid out of chambered shells, and deep-sea squid that accumulate ammonium ions in their tissues
- •Plankton and algae, some of which produce gas vesicles to stay in the lit surface layer
- •Divers, who use a weight belt to offset the buoyancy of their body and suit and an inflatable jacket to trim, and who must manage the fact that a wetsuit compresses with depth and becomes less buoyant
Where the principle is exploited
Submarines change their own weight rather than their volume, flooding ballast tanks with seawater to dive and blowing them with compressed air to surface, and trim tanks make fine adjustments. Hot air balloons change the density of the air inside rather than the volume of the envelope, since heating air makes it expand and spill out, leaving less mass in the same space. Hydrometers float at a depth that depends on the density of the liquid, which is how the sugar content of a fermenting brew and the charge of a lead-acid battery are measured. The Plimsoll line on a ship's hull, introduced by legislation in Britain in 1876 after a campaign against overloading, marks the maximum safe loading and includes several marks because water density varies, so a ship floats deeper in fresh water than in salt and deeper in warm tropical water than in cold. Salvage operations refloat sunken vessels by pumping out water or attaching buoyancy bags, and the same logic makes the Dead Sea, with its very high salt content and therefore high density, a place where a person floats unusually high.
The takeaway
The upward force on a submerged object equals the weight of the fluid it displaces, which is why what a thing is made of matters less than the volume it occupies, and why a hollow steel hull floats while a solid steel bar sinks. Floating and staying upright are separate problems, with stability decided by whether the centre of buoyancy shifts far enough sideways when the hull tilts to right it. Fish manage the same problem with swim bladders, sharks with an oily liver, and submarines by flooding tanks.