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Does a capsized boat of iron float?

  1. Dec 9, 2016 #1
    Hello Forum,

    I know that a block of iron would sink in water no matter its shape, unless we shape it like a boat in which case it will float. What if we turn that iron boat upside down? Will it still float? I don't think so since the concave area of the boat will slowly fill with water (which does not happen when the boat is upright).
    However, I have seen capsized boats float at the lake and people sitting on them. Maybe those boats are made of materials that have density less than water so they would float no matter their shape or position?

  2. jcsd
  3. Dec 9, 2016 #2
    How about the trapped air once it flips? that should help it float
  4. Dec 9, 2016 #3
    So as we flip the boat, some air gets trapped between the hull and the water and that air prevents the boat to float, like a pillow?

    It seems strange that the small amount of trapped air can have an effect. I would think that the inside of the hull would quickly touch the free surface of the water and at that point the space of the hull is filled with water and the boat should sink.
  5. Dec 9, 2016 #4


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    It all depends on how much air is trapped in the hull after capsizing. If the entire volume fills with water, the boat sinks. If the volume of trapped air displaces more mass in water than the weight of the boat, it will float upside down.
  6. Dec 9, 2016 #5
    Thanks Mech_Engineering

    I would image that trapped air to slowly leak out but I guess it does not. People trapped under the boat could breath the trapped air and exhale it depleted in oxygen so they could only survive for a limited amount of time.
  7. Dec 9, 2016 #6
    Well, then they'd fill it up with co2

    Eat some beans and asparagus, and that boat might flip back upright.
  8. Dec 9, 2016 #7
    Keep in mind that ship hulls are not completely solid metal. You have two layers and a bunch of air trapped in between. This helps keep the average density of the ship lower than that of water which is why they float. If the ship capsizes, unless the hull is cracked and fills with water, it should still float. It's not dependent on the air in the work/living quarters, though that doesn't hurt it.
  9. Dec 9, 2016 #8


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    See for yourself: http://abcnews.go.com/blogs/headlines/2013/06/man-survives-nearly-3-days-submerged-in-capsized-boat/

  10. Dec 11, 2016 #9
    Thanks everyone.

    Let's now consider the same simple boat but with the space above the hull sealed at the top instead of being open to the atmospheric air. Air is trapped in that space. When the boat is upright, would the boat float differently (get more or less submerged) compared to when the boat has no a seal? I think the floating would be exactly the same...
  11. Dec 12, 2016 #10


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    It depends on the shape of the displaced volume.
  12. Dec 19, 2016 #11


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    Boats usually have sealed buoyancy tanks built into them as it's the law in many countries. I once helped someone test a dingy for buoyancy. As I remember it involved deliberately capsizing and then sitting on the upturned hull for 15 mins. Then after removing it from the water we had to demonstrate that there was less than a litre of water in each buoyancy tank. The last bit was that hard part because his boat was quite old.
  13. Dec 21, 2016 #12
    I think you have to take a glass an do some experiments with it in water so you can see how it works for yourself. As long as there is sufficient air volume trapped to make up for the weight of the vessel, regardless of whether it's upright or capsized, it'll float... The reason the air stays trapped when capsized is the same reason the boat floats when upright... the hull is sealed. The inverse is also true, the reason the boat sinks when it has a hole in the hull is the same reason it will sink when it's capsized.. it's just air leaking out (thus being displaced by water), while when upright it's water leaking in (displacing air)

    Try some experiments with a paper cup and a few pinholes in it.
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