how do ships float?
In: Physics
An object with less density will float on a something with a greater density. Water is actually pretty dense. Boats are hollow. When you calculate the average density of a boat vs the density of the water, the boat is less dense. Therefore it floats.
If a boat starts taking on water, those hollow spaces fill with water, and the boat’s density increases. Eventually the boat sinks because its density becomes greater than that of the surrounding water.
Water wants to seek its own level and push inward. so when we push down on it, it pushes back up. So a boat pushing down on water is pushed up by water. Eventually the force of the water pushing up equals the force of the boat pushing down and it floats.
We call this upward force “buoyancy” and it’s simply measured by the weight of the thing in the water vs the weight of the water being pushed out of the way. If the weight of the water being pushed out of the way is greater than than the weight of the thing in the water than the thing floats. If the thing is heavier than the water it pushes out of the way, it sinks.
A 1ft x1ft x1ft cube of water weighs approximately 64lbs. A hollow cube of steel with equal dimensions as the cube of water (with each wall 1/4 inch thick) weighs about 61lbs. The hollow steel cube weighs less than the water it pushes out of the weigh so it will float.
A rock will sink, because it’s more dense than water.
A balloon will float, because it’s less dense than water.
So what happens when you attach a rock to a balloon??
Assuming the balloon is much bigger than the rock –
the rock will pull the balloon down a little bit. But not much. In the end, the balloon floats, a little lower than before, with the rock underwater.
If you calculate the density of the rock and *just the part of the balloon under water*, the average density will be exactly the same as that of water (if it was more, it would sink down more, if it was less it would float up more).
A ship is made of heavy stuff that you’d think would sink. Eg, steel. But it’s also hollow, and watertight underneath. The heavy steel pulls the ship down a bit, but since the ship is mostly hollow, it’s less dense than water overall, on average.
The ship will have special equipment and procedures to make sure it stays filled with air – pumps to pump out any water that does get in, careful regular inspections to make sure there’s no danger of leaks forming, and so on. Because if it did fill with water, the average density would be more than water’s density, and the whole thing would sink.
Lookup Archimedes principle. Basically if the weight of the water the ship displaces (pushed aside with the shape of its hull) is more than the weight of the ship itself, then it floats.
Ship is made of steel. Steel is heavy, thus it will sink
Now we make a steel box with open top.
If the weight of the empty space (when filled with water) is more than the steel box, it will sink. But it’s currently empty, so it’s full of air. And air is lighter than water. So, air, with the steel box, floats
Now you have your ship
When something enters the water, it displaces the water; pushes it out of the way. But water is heavy, and will resist this by pushing back on whatever the object is, to push it back out.
If the object is denser than water, the object’s pushing force is greater than the water’s, so it sinks.
If something is lighter than water, then the water’s pushing force will win, and the object will float. And when this is the case, the amount of water displaced is equal to how much the object weighs.
So take a small fishing boat, it weighs 100kg(220lb), and has 200kg(440lb) of people and stuff in it. In order for it to float, the water needs to exert 300kg of force upwards. The boat needs to displace 300kg of water in order to do that. Really, that’s not much water; 300L/80gal. So the boat’s only going to sink in a few cm/inches.
The problem is that if there’s a leak, and water starts flowing in, it’s counted in the “weight of the boat”.
Archimedes’ principle says that the buoyant force acting on a submerged object equals the weight of the fluid it displaces.
That is, for a ship to float, it has to weigh less than a ship-sized volume of water.
And a ship-sized volume of water is *very* heavy. Steel is also heavy, true, and a solid chunk of steel would be heavier than that volume of water, but ships aren’t solid chunks of steel. They have lots of empty space inside, which takes up space (-> more buoyant force) without adding much more weight.
If you push one pound of water out of place, it’ll push back with one pound of force trying to get back home. Multiply by many, many pounds of water pushed out of place, and the many many pounds of force it pushes back on you to return home, and that force can keep a boat afloat.
Displacement. If the item in the water weighs less than the water it pushes out of the way, it’ll float. If it weighs more than the water it pushes out of the way, it’ll sink.
Imaging a log that takes up 20 cubic feet of space.
Water weighs 62 lbs per cubic foot (if you fill up a box of water 1 foot by 1 foot by 1 foot.)
So that log will float on the water if it weighs less than 20×62 or 1240lbs. If the log weighs around the same weight or a bit more, one way to make it float is to carve out the middle. It’ll still take up about 20 cubic feet of space but it weighs just a fraction of what it did before. You can then use that freed up space to put other things in it like a dog or a person or a some supplies. Voila, you just created a dugout canoe, a basic boat.
Ships float by pushing more weight worth of water out of the way than the ship itself weights.
As long as you can keep the water out, you’ll float.
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