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In the article Ship Traffic Increases Dramatically, to Oceans' Detriment , there is a quote (emphasis mine) :

"I was surprised to see that in 20 years, the growth is almost fourfold, or almost four times larger," Tournadre said. "We are putting much more pressure on the ocean."

The speaker does not intend to use the phrase literally, but this makes me wonder if this is true. Does putting more and more ships on the ocean increase the pressure on the ocean bed? If it's true, it will imply that the pressure on a scuba diver (or any object under water) would increase because of the ship!

On the one hand, adding more weight on top of anything must increase the force it exerts on the ground, so the pressure must also increase. But on the other hand, since the ocean area is not strictly bounded, the ocean spreads out, so the pressure must decrease; hence the dilemma.

Though the effect may be negligibly small, I would still like to know if a ship can increase the ocean's pressure. If yes, by roughly how much?

This question explains the case for bounded fluids, it does not explain unbounded fluids like the ocean.

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You raise the question for an unbounded fluid.

The idea of an unbounded fluid is a thought experiment. Let there be a horizontal surface, no curvature of any kind, that stretches to infinity. Let this surface be covered with a body of water, which will consequently be an infinitely large body of water. When you add a ship to that water, will the level of that infinite body of water rise?


Our physical reality is different.

Even if you have a planet that is entirely covered with a layer of water you have an ocean that is not unbounded.

That planet will have a finite amount of ocean water, hence that ocean is not unbounded.

Our oceans, here on Earth, contain a finite amount of water. It follows that the answer for bounded fluids that you link to is applicable in the case that you ask about: the oceans of the Earth.

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Do a simple experiment.

Take a glass of water, fill it up to a level. Float something smaller but heavy in it, I used a metal eggcup. The level goes up, but there is no motion of water away or under the extra weight, when the float comes to rest, which means the pressure is uniform away or below the eggcup.

Extrapolating: the extra floating weight of ships will raise the level of water of the oceans a tiny bit, but a scuba diver under a ship will not find a different pressure below the ship than the one at that level everywhere close by.

(Of course one should be careful not to go under a ship as there are water movement dangers from the motion, reaction to tides etc.)

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  • $\begingroup$ Maybe I wasn't clear enough... I understand that the fluid pressure is uniform, and that a diver will not experience a "column of pressure" when under the ship. What I mean by the "increase in pressure", is the pressure increase that a diver at a particular depth would measure using a guage, at the moment when when a ship is placed on the ocean (say, during a ship launch). This will of course, be independent of the position of the diver. $\endgroup$ – AlphaLife Apr 26 at 11:15
  • $\begingroup$ Well, it does not have to be a ship , does it? One could add the equivalent weight in water to the ocean, and yes, the pressure would infinitesimally increase (considering the volume of ocean to the added volume. $\endgroup$ – anna v Apr 26 at 12:50
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The academic way to answer the question is:

According to the laws of hydrostatic (Archimedes), the pressure existing a depth D is only a function of the height of the fluid (and other parameters like the value of the density of the fluid and the value of the gravitational attraction of the earth, both constants in the example of your question), so:

$ Pd = constant . Depth $

So, even if the oceans were full to the brim with boats, a diver swimming a a depth of 10 meters (for instance) will always feel the same pressure, regardless of how many boats float on top of him.

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