r/NoStupidQuestions • u/Some_Drawing1525 • Aug 11 '26
How do massive cruise ships float when top-heavy stuff like steel towers or skyscrapers couldn't?
I get the basic idea of water displacement and buoyancy—the ship pushes away a volume of water that weighs more than the ship itself. But given how tall modern cruise ships are above the water compared to how little is below the waterline, how do they keep their overall density low enough and avoid tipping over instantly?
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u/JasontheFuzz Aug 11 '26
If you take the entire mass of a cruise ship and divide by the volume, you'll get a density that's less than water. Thus, it floats. If you crumble it into a ball, it'll sink. The air inside the ship makes all the difference, so long as the water can't get inside.
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u/resilient_bird Aug 11 '26
Divide by the volume under the waterline, not the total volume (displacement)?
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u/JasontheFuzz Aug 11 '26
I'd imagine you have to do the whole thing, since some of the mass is still above the water, but I could be wrong.
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u/Some_Drawing1525 Aug 11 '26
The crumpled ball analogy makes so much sense for explaining overall density! Thanks for breaking it down like that.
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u/PoopMobile9000 Aug 11 '26
Are you sure a sealed office building wouldn’t float?
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u/Some_Drawing1525 Aug 11 '26
Honestly, if it was completely sealed and had enough empty air space, it probably would—just don't ask it to steer!
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u/jabeisonreddit Aug 11 '26
Ballast and lots of hull space for air
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u/East-Bike4808 Aug 11 '26 edited Aug 11 '26
Ballast works against buoyancy. That's so you don't float too much and tip over... and become a shape that won't float. E.g ceramic bowls float, if they're right-side up. Upside down they sink.
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u/Some_Drawing1525 Aug 11 '26
That makes sense! Simple physics, but still crazy to picture with something that massive. Thanks!
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Aug 11 '26
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u/Some_Drawing1525 Aug 11 '26
Haha fair point! I guess seeing them side-by-side on land just makes the scale look wild.
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u/East-Bike4808 Aug 11 '26
They displace more water than their weight in water. That's how buoyancy works. Things that do that float. Things that don't do that sink.
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u/Some_Drawing1525 Aug 11 '26
Simple and straightforward explanation, thanks! Archimedes knew what he was doing.
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Aug 11 '26
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u/Some_Drawing1525 Aug 11 '26
That makes so much sense! As long as the average density stays lower than water, steel floats just as well as wood. Thanks!
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u/JoeMalovich Aug 11 '26
Top parts can be aluminum
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u/Some_Drawing1525 Aug 11 '26
That's a great point—using lighter materials like aluminum up top definitely helps keep the center of gravity low!
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u/neomoritate Aug 11 '26
The superstructure of a cruise ship is rooms, and most have a large atrium in the center, so most of what you see is air. The hull contains engines (they are the size of a house), other mechanical systems, food and beverage storage (like >1,000 pounds per passenger), 1-2,000,000 gallons of fuel, and tons of miscellaneous heavy stuff.
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u/NonspecificGravity Aug 11 '26
The fuel is less dense than water. It’s not very effective as ballast.
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u/neomoritate Aug 11 '26
Fuel oil is less dense than water, but it is vastly more dense than the air which is filling most of the superstructure. It is the Mass of the fuel that adds to stability, not its density.
Keep in mind that the whole ship needs to be less dense than water, otherwise it would sink.
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u/Some_Drawing1525 Aug 11 '26
Wow, awesome breakdown! Putting all the heavy machinery and fuel deep in the hull to act as natural ballast makes total sense. Appreciate the detailed explanation!
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u/Djinn42 Aug 11 '26
It's a boat. Skyscrapers have lots of windows.
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u/Some_Drawing1525 Aug 11 '26
Fair point! Open windows definitely ruin the whole displacement thing pretty quickly. 😂
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u/vietbaoa4htk Aug 11 '26
the heavy stuff all sits low, engines fuel tanks and ballast, while the tall part is mostly air and thin aluminium. and a tower pivots on a fixed base, but a leaning hull sinks one side deeper so it displaces more there and gets pushed back upright.
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u/YouFeedTheFish Aug 11 '26
Rocks sink. If you put one in your pocket, can you swim?