<p>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. </p>

<p>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).  </p>

CC BY-SA 3.0
2014-10-01T15:24:45.253Z
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?
L.B.
CC BY-SA 3.0
2014-10-01T15:25:22.277Z
I guess, in trying to sum up my question, I don't understand! :)
L.B.
CC BY-SA 3.0
2014-10-01T09:33:43.293Z
2014-10-01T14:29:57.723Z
2014-10-01T14:29:57.723Z
casey
<p>It would go down. </p>

<p>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 <em>V</em>, so relative sea-level goes up (second panel, below). </p>

<p><img src="https://i.sstatic.net/bHskY.png" alt="A boat displacing water before and after sinking."></p>

<p>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).</p>

<p>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.</p>

CC BY-SA 3.0
2014-10-01T13:47:52.373Z
Great figure to go along with the text.
casey
CC BY-SA 3.0
2014-10-01T17:30:30.280Z
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.".
arkaia
CC BY-SA 3.0
2014-10-01T19:54:06.443Z
And if you really want to be accurate, consider the water consumed in rusting iron. Also, iron expand when rusting.
Gimelist
CC BY-SA 3.0
2014-10-01T12:28:10.690Z
2014-10-02T08:28:40.740Z
2014-10-02T08:28:40.740Z
Matt Hall
<p>Let's assume that a ship is traveling on the ocean and it sinks, what is the effect on sea level? Even if the effect is small, would it go up, down or remain the same?</p>

CC BY-SA 3.0
2014-10-02T07:59:11.690Z
This question is offtopic; it would fit better at a site like Physics.
congusbongus
CC BY-SA 3.0
2014-10-02T12:49:36.440Z
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.
Matt Hall
CC BY-SA 3.0
2014-10-02T14:04:23.397Z
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".
arkaia
CC BY-SA 4.0
2022-12-10T17:09:20.077Z
i am thinking of the bird that dropped pebbles into a vesel to get the water ti rise so to get a drink shorley if millions of items were to be placed into the oceon there would be a lot of rise in the oceon also with icebergs and shorelines dropping in add to the problem
mr john powell
CC BY-SA 4.0
2022-12-11T05:32:37.143Z
The items would come from somewhere... generally the Earth.  So it's not like you're adding to the overall level of things.  There's also still basically the same amount of water in play.  Sure, you can always redistribute Earth to raise/lower areas... worse and comes to worst, you could take all the elevated land, dump it in the ocean, make a level Earth, and then you'd have a ["shallow" world-wide sea a couple km deep](https://worldbuilding.stackexchange.com/questions/84625/how-deep-would-the-ocean-be-if-continents-all-eroded-away)
JeopardyTempest
<oceanography><sea-level>
What happens to sea level when a ship sinks?
arkaia
# What happens to sea level when a ship sinks?

(Asked by: username_0 on 2014-10-01)

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

**Tags:** oceanography, sea-level

> **Comments:**
> **username_1**: This question is offtopic; it would fit better at a site like Physics.  
> **username_2**: 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.  
> **username_0**: 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".  
> **username_3**: i am thinking of the bird that dropped pebbles into a vesel to get the water ti rise so to get a drink shorley if millions of items were to be placed into the oceon there would be a lot of rise in the oceon also with icebergs and shorelines dropping in add to the problem  
> **username_4**: The items would come from somewhere... generally the Earth.  So it's not like you're adding to the overall level of things.  There's also still basically the same amount of water in play.  Sure, you can always redistribute Earth to raise/lower areas... worse and comes to worst, you could take all the elevated land, dump it in the ocean, make a level Earth, and then you'd have a ["shallow" world-wide sea a couple km deep](https://worldbuilding.stackexchange.com/questions/84625/how-deep-would-the-ocean-be-if-continents-all-eroded-away)  

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## 2 answers

### Answer by username_2 on 2014-10-01. Score: 22 | Accepted answer

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.](https://i.sstatic.net/bHskY.png)

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.

> **Comments:**
> **username_5**: Great figure to go along with the text.  
> **username_0**: 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.".  
> **username_7**: And if you really want to be accurate, consider the water consumed in rusting iron. Also, iron expand when rusting.  

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### Answer by username_5 on 2014-10-01. Score: 7

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).

> **Comments:**
> **username_6**: 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?  
> **username_6**: I guess, in trying to sum up my question, I don't understand! :)  

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