I have no idea how it is possible that a boat floats. Some of them are made with concrete, or metal, and other heavy material that on their own would sink, regardless of shape. But somehow, when it’s used to build a boat, it floats just as fine even if it carries extremely heavy load.
So what is the science behind boat floating, but explained to me like I’m five? 🙂
In: Engineering
Imagine a glass of water. If you were to drop a big marble into it, it’d disappear straight to the bottom, but the water level would rise some amount. That’s because there’s been a marble’s worth of water pushed out of the way to allow the marble to be under the water, so the water level rises. The marble’s weight is heavy so it sinks to the bottom. Makes sense right?
A boat is clearly very heavy, and as it sinks into the water it pushes water out of the way, just the same as the marble. Here’s the thing though, water is also very heavy. Ever picked up a big jug of water for a water cooler? It ain’t light. And this is the boat’s secret. It’s built to have a shape that’s spread out and wide (compared to the solid lump of a marble), so as it sinks into the water, before the water reaches as high as the boat’s sides, the amount of water being pushed out of the way is *heavier* than the boat. At that point it stops sinking and floats.
If you imagine a solid block of water 1 meter by 1 meter by 1 meter, that would be 1 metric tonne. It’s heavy stuff is water.
This is like a tupperware tub in the sink. It floats there happily. WIthout actually turning it on its side and letting water flood into it, to shove it under the water requires you to push down on it on above. That’s because the tupperware tub on its own can only push a certain amount of water out the way because it isn’t heavy enough to push any more out the way.
A boat is the same, scaled up.
Displacement, in a word.
Try this: fill your kitchen sink with water. Put a bowl in it, in a way that the “U” shape of the bowl is going to stay dry, despite the outside & underside of the bowl being wet & submerged.
What’s happening is the weight of the bowl displaces a volume of water. If that volume of water would weigh more than the weight of the bowl, the bowl should float. Or at least, that is the general idea (you can probably google up the exact specifics – the weight & volume are both important factors for how much water is pushed out of the way by the boat).
Objects have buoyancy in different substances. This is caused by density (weight versus volume). Water has a density of about 1 kg/L ( 1L of water weighs 1 kg; you can fit 1 kg of substance in 1 L of space). Anything that is *more* dense than water (i.e. 1 L of it weighs more than 1 kg) will sink. Anything that is less dense will float.
Now, I’m sure you’re thinking “concrete, metal- all these things are *way* more dense than water” which is true. But boats use something very important: air.
The shape of a boat means that you end up having air underneath the waterline. Air has a density of 1.29 g/L (0.00129 kg/L). So although metal is more dense than water, you have to think about the density of the boat as a whole. That section of the boat that is at and under the waterline is *less* dense than water because its shape allows air between it.
If you’ve ever had a mug in a full sink, it’s the same principle.
When you throw a rock in the water, it pushes (displaces) some of the water out of the way. When you put a boat in the water, it’s not heavy enough to push enough water out of the way that it sinks.
As long as water doesn’t get into them, the average density of a boat’s underwater part is much lower than the density of the same size blob of “solid” water, because boats are mostly air inside. So they float. (Note that the parts of the boat above the waterline push down on the part that floats without themselves contributing to floating, so there is an equilibrium depending on how loaded down the boat is)
Displacement. The amount of water displaced by the boat is denser than the area the boat occupies, so it floats rather than sinks.
Latest Answers