How are boats that are incredibly heavy able to float on water and sail across ocean? I mean, the cruise ships these days are MASSIVE!! 🛳️
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Yeah, they’re really heavy, but they’re even bigger than they are heavy. On average, across their whole volume, ships are much lighter than water: that’s what buoyancy is.
The Principle of Flotation says that any object that displaces its own weight in water will float. A metal cube doesn’t displace very much water, so it sinks. Flatten out the metal into a canoe shape and it will displace more water and float.
Yes they are heavy, but they’re also buoyant.
The way a ship is designed, when it sits in the water, the amount of water it displaces (pushes on) actually weighs equal to or more than the ship itself. This means there is more water pushing up on the ship than ship pushing into the water.
It’s the same reason why an empty bowl will float in your sink, but a fork (which probably weighs less) will sink right to the bottom.
The average density of something determines if it will float or not. Ships have a lower average density than water despite their size, because all the air in the ship is included in that density, and the air is FAR less dense than the water; gravity pulls on the air and the ship both, but on average what it’s pulling is less than the density of the water below it. It’s mass vs mass, and the air-filled ship has less than the water.
When you have an object of equal mass that does not contain any air in it, then only its own mass will act against the mass of the water below it, and it will sink. You can see this if you swim in a pool; let all the air out of your lungs, and you’ll sink like a stone. Take a really, really deep breath and your torso, at least, will float easily (your legs might still sink, but that’s a whole other thing).
Consider a bucket.
It takes up a huge amount of volume, but only comprises a small amount of plastic.
Try to push it under water, and it’s really hard work: in order for the bucket to go down, an equivalent amount of water has to go up, in order to make room. If you’ve got a five-gallon bucket, that’s five gallons of water you’re pushing up in the air.
(its a thin wide layer that rises up, to you won’t be able to see it in a large body of water, but try doing it in something small, like a sink or bathtub, and it’ll make a big change in the water level)
Water is *really heavy*, and it wants to go down, please – it’s a lot heavier than an empty bucket, and it wants down [a lot harder than the bucket does](https://64.media.tumblr.com/401f2aaac806672928f1afaf7403032a/tumblr_pitl19tIbs1tlgqkgo4_540.gif)
And this only gets more pronounced, the bigger you get.
Make a million-gallon bucket, and that’s a million gallons of water you’re lifting up in order to sink it, which is an absolute *fuckton* more than the empty bucket weighs.
And that means the total weight the bucket can support – is one fuckton minus one bucket.
You can make that bucket out of iron or even concrete, and it’ll still have plenty of floatiness leftover.
Congratulations, you have just invented the ship. Just maybe make it pointy so it goes faster.
Gravity wants to pull everything down. The ocean, ships, you and me. The denser the object, the stronger gravity pulls it down.
When it comes to floating, density is everything. An object less dense than water will float because size-for-size, water is pulled down harder than that object, so the water will always be underneath the object and the object will always be above the water (aka floating).
Ships are WAY less dense than water because they’re mostly filled with air with a thin structure of metal. Another way to think of it is if you took a ship-sized blob of water, it will weigh many many times more than the ship, therefore, the water wins the tug of war.
It’s akin to a balloon floating on a lake, you wouldn’t expect it to sink unless you filled it with water instead. The Titanic sank because as the air inside was replaced with water its total density began to increase until it weighed more than the water it displaced.
Side note, this is also the same reason hot air balloons fly. Hot air in the balloon is less dense than colder outside air.
Take a cruise ship. Take an equivalent volume of water. The water will have more mass. So the boat will float.
Luckily, displaced water increases with volume at the same rate does weight.
This means that you can escalate a ship infinitely and it will always float. And will always float with a big part of it above water.
A friendly factor is that floatation force is spread under the entire ship, meaning that it’s structurally easy to make it work. Conversely, aircraft and road vehicles do rest their weight in few small spots and that complicates the scaling up a lot more.
The big factor in limiting ship size is handling and water channels like Panama or Suez.
Handling is a big issue because propulsion and steering do scale up square to dimension, while mass does scale cube to dimension. This means that the bigger you get the more awful handling becomes. Up to needing miles to slow down few knots, and steering few degrees per minute maximum becomes a reality.
Last issue are waves. You risk the ship snapping in two if the ship is very long and travels above a very long wave. This can be countered structurally, but it costs more the bigger you get.
they are also hollow inside. they can be heavy, but all they need to do is weigh less than the volume of water they displace, and they float. the air inside the ship ensures that.
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