If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

How do ships float?

The technology that keeps huge cargo ships afloat is amazingly simple. PJ, a masters student in Naval Construction and Engineering at MIT, explains just how simple it really is.

Want to join the conversation?

  • leaf green style avatar for user colin
    How come the technology of ship-making has not advanced within the past centuries? Is this technology the most innovative and useful?
    (5 votes)
    Default Khan Academy avatar avatar for user
  • winston default style avatar for user Tai Roelke
    at did anyone think of the Titanic?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • primosaur ultimate style avatar for user Jessiman
    The Titanic used those same compartments and sank! Are we seriously still using that same technology? Can't we improve?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Henri Hyacinthe TAMO FODJO
    good how can i do if i want to learn it now , and make my own at the end of learning
    this is my mail henrihyacinthe86@gmail.com thanks for help
    (2 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Yug Patel(Spycakes)
    is there a nother way that a ship can sink
    (1 vote)
    Default Khan Academy avatar avatar for user
  • mr pants green style avatar for user lee dickey
    what is the smallest ship that sank
    (1 vote)
    Default Khan Academy avatar avatar for user
  • marcimus pink style avatar for user Kim
    Can't ships sink from a large wave / to much water on the boat? Also, Is there a larger chance that a smaller ship will sink? Is it the other way around? Is it equal? EDIT: Also, if the water fills all of the front "slot" and the front is heavy, will it sink then? Also, what kind of things can damage a boat? (sorry if any of these questions were answered in the video... I probably forgot or asked before they answered!)
    (1 vote)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user nathan smith
    can people make videos about the titanic
    because that's what i want to learn about.
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

Science Out Loud. So if I take this box and just obliterate it in the corner right here and then put it in the water, the weight of the water rushing in will cause the box to sink. If I take this box and do the same thing, it tips over a little bit, but it still floats. This box uses the same kind of design that naval architects use to prevent ships 1,000 times bigger from sinking. What's so special about this box that prevents it from sinking, even with the big hole in its side? Let's talk about why ships sink in the first place. You probably know that things will float in water if they're less dense than water and sink if they're more dense. The metal that this ship is made out of is way more dense than water, so you might think it would sink. But that metal is shaped so that it traps a lot of air, which is less dense than water, inside it. So the average density of the hull of the ship is actually much lower than that of water, so the ship floats. But if the hull springs a leak and fills up with water, there's no more air to lower the average density of the hull, so the ship sinks. The design of this box basically prevents too much water from getting into it. This box's hull was divided up into watertight compartments with these walls called bulkheads. So when I made a hole right here, only this compartment filled up with water. The rest stayed dry and were able to keep the box afloat. This box is subdivided, and so is this ship. Say this ship is divided into ten watertight compartments. If one area of this ship got damaged, only that compartment would flood, but not any of the others. The added weight of the water here would cause the ship to tip over a little, to be angled or trimmed in the water, but it wouldn't completely sink, and could still be taken to a port for repairs. So even with subdivision, why do ships still sink? Well, it's impractical and expensive to design an unsinkable ship, especially because most of the time, ships just don't see that much damage. What naval architects do now is try to predict what kind of damage is most likely to happen when designing a ship. Now, ships are more complicated than this box. Naval architects have to think about where to subdivide the ship, the shape of the hull, and equipment that goes into the compartments. We don't know when people first start subdividing ships, but the caps of Chinese trade ships as far back as the fifth century indicate that water would be able to enter the vessel without causing it to sink. It's pretty crazy that technology that existed so long ago is still being used today. Hello, I'm Paul. Thank you for watching Science Out Loud. For more information, please visit our website. [LAUGHTER] See what they had me do?