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I was in class learning about density and stuff. Our teacher told us that things that are denser than water sink in water, and less dense things float.

Then, our teacher asked us why metal boats float in water, even though they are denser than water. Is it because of the surface tension of water? Some other thing?

Any help would be appreciated.

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    $\begingroup$ The metal may be denser than water, but the entire boat (metal plus the internal space) is less dense than water. Read up on Archimedes' principle $\endgroup$ – lemon Feb 4 '15 at 19:13
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    $\begingroup$ Actually, I've seen concrete boats afloat. And doing quite well. $\endgroup$ – Hot Licks Feb 4 '15 at 19:48
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    $\begingroup$ Consider a hollow sphere (or cube or whatever). Even if made of metal, the majority of its volume is air. But, when placed in water, the sphere/cube displaces water, not air, and water is much heavier than air. $\endgroup$ – Hot Licks Feb 4 '15 at 19:50
  • $\begingroup$ When you grasp this principle of buoyancy, you'll also understand why airplanes stay aloft: they displace more air than they weigh! F*ing turbines, how do they work! $\endgroup$ – New Alexandria Feb 4 '15 at 20:55
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    $\begingroup$ "why metal boats float in water" a solid block of metal will not float in water but a metal boat is not a solid block of metal. $\endgroup$ – Alfred Centauri Feb 4 '15 at 22:52
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This is because the whole boat, along with the air in the boat, is lighter than the water it displaces.

For example, if a small boat will take up 1 cubic meter of water, then it has to be heavier than the weight of 1 cubic meter of water. This is explained in this post by What If here.

For the same reason that bowling balls float (because salt water the size of a bowling ball weighs more), boats float (because the overall weight of a boat is less than the overall weight of salt water the size of a boat.

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    $\begingroup$ Am I being stupid or is it meant to say "lighter than [...] water" in the second paragraph? $\endgroup$ – wizzwizz4 Aug 6 '16 at 18:26
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Two factors:

Overall density. A boat isn't only metal. It contains copious amounts of plastics and wood, which have lower densities than water. You have to calculate the overall density of the object, not just the metal.

Shape: If boats were made in a sink-friendly shape, they would certainly sink (regardless of wood and all). Engineers design boats to be as much float-friendly and sink-unfriendly as possible. Imagine a boat which has a steep V-shape with a very narrow angle. This boat is far more probable to sink than one which is also a V-shape but has a much wider internal angle.

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It seems like they ought to sink because we're used to seeing things fall. But for the ship to sink it has to push aside some water, which has nowhere to go but up. So it's a question: does the ship 'want' to sink more than the water 'wants' not to rise? It turns out that just depends on whether the ship weighs more or less than the amount of water that would fill the same space. Real ships have lots of air inside, so they weigh less than the same volume of water, so they float.

Here's a cool experiment you could try. Fill your sink with water, then put a bowl in it. Now see how much weight you can put in the bowl before it sinks. You could even pour in water as the weight. If you use water, look at how high the water comes up on the inside and on the outside of the bowl as you add more water. Is it the same or different?

And by the way, isn't it cool that it doesn't matter how deep or shallow the water is? If something floats in 2 feet of water, it will float in 2000 feet of water!

I hope this makes sense.

(Source)

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  • $\begingroup$ This is copied from a site. $\endgroup$ – Pyraminx Feb 4 '15 at 19:19
  • $\begingroup$ And I have credited the site. $\endgroup$ – ʇolɐǝz ǝɥʇ qoq Feb 4 '15 at 19:23
  • $\begingroup$ You forgot to mention the shape factor. Put a happily floating ship upside-down in water and watch it go down, regardless of the water up and ship down politics. $\endgroup$ – Youstay Igo Feb 4 '15 at 19:51
  • $\begingroup$ @YoustayIgo When the water "spills" into the inside of the ship, now the ship (including the water inside it) is heavier; this continues until the ship is now heavier than the water it displaces and it sinks. If there's enough trapped air in the upside down ship it might not sink. $\endgroup$ – user253751 Aug 8 '16 at 1:55
  • $\begingroup$ That is correct. I was referring to the practical scenario of a capsized boat, not the physics behind it. I agree to the physics part. $\endgroup$ – Youstay Igo Aug 9 '16 at 2:15
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Displacement. Until the Vessel weighs more than the volume of water it is displacing it will remain afloat. Poke a very large hole in it.. Iceberg, torpedo or in the case of the Costa Concordia Sight seeing, it will most likely sink. Think of a Hydrogen filled balloon. It weighs less than the volume of air it is displacing so it rises.. Poke a hole in a ship the area affected is no longer displacing water, but the weight of the vessel has not changed.

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