Let's assume that a ship is traveling on the ocean and it sinks, what is the effect on the sea level? Even if the effect is small, would it go up, down or remain the same?

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    $\begingroup$ The effect would be negligible I would imagine, given the volume displaced by a ship, compared to the volume of any of the oceans. $\endgroup$
    – user889
    Oct 1 '14 at 4:22
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    $\begingroup$ This question is offtopic; it would fit better at a site like Physics. $\endgroup$ Oct 2 '14 at 7:59
  • $\begingroup$ Agreed. The basic principle comes up in thinking about displacement by sediment, melting ice, and so on, but it's rather general in this form. $\endgroup$
    – kwinkunks
    Oct 2 '14 at 12:49
  • $\begingroup$ In my mind, the question is relevant to the principles behind changes in sea level. If you substitute the ship with other floating things, like melting sea-ice, then it is definitely relevant. To me the question is appropriate in a Physical Oceanography framework, which I hope is sufficiently "Earth Science-y". $\endgroup$
    – arkaia
    Oct 2 '14 at 14:04

It would go down.

In order to float, an object must displace a volume of fluid that weighs the same as the boat. In the case of a ship, this volume is less than the gross volume of the boat, including the air inside it. That's why it floats with the gunwales comfortably above the water. So when the boat is floating, it displaces some volume V, so relative sea-level goes up (second panel, below).

A boat displacing water before and after sinking.

However, when the ship is submerged and full of seawater, it displaces a smaller volume than it originally did. If it didn't, it wouldn't have sunk (assuming it hasn't changed in weight).

Since it displaces a smaller volume after sinking, relative sea-level must go down. But it will still be higher than it was before the ship was launched.

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    $\begingroup$ Great figure to go along with the text. $\endgroup$
    – casey
    Oct 1 '14 at 13:47
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    $\begingroup$ Great answer! The question was asked to me a long time ago by my Physical Oceanography professor. While I already knew the answer, the explanation and graphics provided by @kwinkunks are excellent. Even my professor agrees saying "The answer with the figure is right on, even noting that the sea level would be higher with the sunken ship than before the ship was first launched into the water. I've always felt that this is a question that provides a view into how someone approaches a physical problem, and how clearly they can explain their thinking.". $\endgroup$
    – arkaia
    Oct 1 '14 at 17:30
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    $\begingroup$ And if you really want to be accurate, consider the water consumed in rusting iron. Also, iron expand when rusting. $\endgroup$
    – Gimelist
    Oct 1 '14 at 19:54

The sea level should decrease. Consider a boat before it sinks. It must displace a weight of water equal to the weight of the boat to have neutral buoyancy. The material comprising a boat is more dense than water, thus is less voluminous than the displaced ocean. This displaced ocean volume will be realized as a rise in ocean height. However, once the boat sinks this volume of water is no longer displaced and this will be realized as a sea level fall.

Note that the sea level should be higher with a sunken ship than with no ship at all, so this answer depends on a working boat on the ocean surface being replaced with a sunken boat. Also note that the magnitude of sea level rise and falls being distributed across enormous surface area is likely too minuscule to be able to measure (e.g. smaller than instrument error).

  • $\begingroup$ I am confused, how would the water level go down? What if, for some reason, enough ships were to sink that it would be measurable. Would this not cause an increase in sea level? $\endgroup$
    – L.B.
    Oct 1 '14 at 15:24
  • $\begingroup$ I guess, in trying to sum up my question, I don't understand! :) $\endgroup$
    – L.B.
    Oct 1 '14 at 15:25

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