How does buoyancy work?


I’ve always wondered and never understood how buoyancy works, especially with huge metal ships that I think should surely sink.

In: Physics

Have to displace (move) more weight in water than an object weighs for it to float. So if a 10 tonne ship displaces more than 10 tonne of water it floats. If it displaces less it sinks. Is the most simple way to put it 🙂

It’s all about density. Density is mass divided by volume. Denser objects move to the bottom, less dense objects move to the top. Buoyancy takes the *entire* object; that is, the whole metal hull plus all the cargo and people and everything else, AND the hundreds of cubic meters of air inside the ship, into account.

Plus, water is actually really, really heavy. It’s about ten pounds per gallon. Air is obviously extremely light by comparison.

So, while the hull and cargo and everything else are denser than water, when you take into account the very large percentage of the volume of the ship that’s just air, overall, it’s less dense than the water, so it floats.

its all about water pressure essentially.

as you move water out of the way, that same weight of water is going to push upwards on you.

so think of an air filled ball. As you push it further and further into the water, that ball is moving more and more water out of the way.

All that water that is moved away is pushing upwards on the ball because the ball is in the way and gravity is pulling all that water down.

Remember that large metal ships are mostly hollow. all that water they push out of the way is pushing upwards on the boat.

so while a piece of metal on its own will sink, a large metal ball will float because it displaces water.

When you put something in a fluid, the fluid presses up on it. The amount it pushes up depends only on the volume of water that the object displaces, and it’s essentially the weight of this volume of water.

You can make ships out of steel, only if the inside of the ship consists of a mix of steel (= heavier than water) and air (= lighter than water). This also explains making ships out of concrete and other materials that are heavier than water. You can’t float a solid steel ship, but that’s not a super useful way to make steel ships (no inside is sub-optimal).

An object will float as long as it displaces an equal amount of water than it weighs. When a ship floats, a large portion of it is underwater. That underwater volume of the ship displaces the water. Let’s pretend we have a large tub of fresh water filled right to the top; if we add anything to the tub, the water will overflow. If we put a object that weighs one tonne in the tub, the water will overflow from the tub. If we collected the water that overflowed, it would weight exactly one tonne, the same as the object. One tonne of water was displaced.

For the object to float, the underwater volume has to equal the displaced water. One tonne of fresh water is one cubic metre, this means the object needs an underwater volume of one cubic metre. If the object is 1 metre long, and 1 metre wide, the object will sink 1 metre deep before it floats. Salt water is more buoyant, with a relative density of 1.025. The same vessel floating in salt water would only be 0.976 metres underwater (1/1.025).

So, in a sense, ships sink until they are deep enough to float, if that makes any sense. Ships are usually very deep; a loaded ship will have more of the hull in the water than out of the water. My ship has a loaded draft of 8 metres, while a tanker could go down to 25 metres or so. A box shaped vessel in fresh water that is that 225 metres long, 30 metres wide, and a draft of 25 metres would weight 168,750 tonnes and still float (225*30*25).

There’s some good answers here, but I would say the ELI5 version is that metal ships are made of metal & air. Metal on its own will sink, and air on its own will float, but a metal shell around air will float if there’s enough air. That’s why ships and boats will sink if they take on too much water.

I can’t believe everybody is getting so complicated and still missing the point.

Imagine a glass of water, half filled.

Add a small rock to the glass of water, and the water level rises. The water level rises in a manner directly related to the volume of the rock. For instance, if you could calculate the the volume of the rock to be 5 cubic centimeters, then the water level will have risen the same as if you had simply poured 5ml more water in.* Anyway, the rock sank. But, it had to move that water upward, and in response, the water is exerting an opposing force downward, with a net change equal to the weight of the water that was lifted — that is, the volume of the rock multiplied by the density of the water.

So, how does something float? It floats when there is an equilibrium between the weight of water getting pushed upward, and the weight of the object doing the pushing. The equilibrium is only reached when the object continues to have excess volume as it ‘tries’ to press into the water — that is, it must be overall less dense than water (although not necessarily homogenous in density, for example a boat that is essentially filled with air). This happens in any container of water, even if it is as big as the ocean.

* In fact this is a nice way of imperically determining the volumes of things.


So that was the simple version. Here’s something for a six year old, when you’re ready.

You wonder why the rock is pushing down *and* the water that was elevated is also pushing down? You wonder where is the balancing upward push? Well, the water actually is pushing in all directions, upon itself and upon every surface. So beneath the sunken rock water is pushing upward, just as it pushes upward from below anything buoyant. But, since the rock is now below the waterline, water is also pushing downward on it — very nearly negating the upward push.

But it doesn’t exactly negate the upward push. Remember the water pushes upon itself as well. This we call pressure. The pressure of the water may as well be identical to the weight of the water above itself at any given depth. So for example, if you examined a 1″*1″ slice of water that was 128″ below the surface, that slice has roughly 8 lbs total extra pressure than exists at the surface. (Because 1″*1″*128″ is a gallon, and weighs 8 lbs.) Or, as we say, any sized slice of water at 128″ depth has roughly 8 lbs *per* square inch more than that water at the top surface.

So what does that do? It actually compresses the water, increasing its density slightly but measurably at increasing depths. So that as an object pushes through deeper and deeper water, heavier volumes are being lifted. Objects that normally have the same density as water (water near its own surface that is), but which are more resistant to compression than water is, can sink below the surface, and yet find equilibrium before contacting the bottom. This is important to how fish and submarines control their depth, and it relates to those cool thermometers that have labeled weights on glass balls of air, and is exactly how those cheap antifreeze concentration detectors function.

It’s also why a rock slows down as it approaches the bottom of a pool — and why you can’t say that the pressure from below the rock is exactly the same as the pressure from above.