Water pressure: Do ships stop sinking?

Many seamen think that the ships wrecked at sea never sink to the bottom but hang suspended at a certain depth, where the water supposedly " reaches the appropriate density due to pressure of upper layers".

Jules Verne, the author of Twenty thousand leagues under the sea also subscribed this view. In one place, Jules Verne describes a wreck suspended immobile in the depths. In another chapter he reminds us of ships "rotting as they hang freely suspended in the water".

So, is this right?

One might think there is some reason for such statements, since the pressure that the water exerts deep down in the ocean is indeed tremendous. At ten meters down, the water exerts a pressure of 1 kg to every sq. cm of the submerged body. At 20 meters down, this is already 2 kg, at 100 meters- 10 kg, and at 1,000 meters –100kg.

We know that in many places the ocean bed lies several kilometers deep, reaching to more than 11 km down at the deepest spots- the Marianne depression in the Pacific. You will easily realize what colossal pressures both the water and everything in it should be subjected to at such tremendous depths. If we push a corked but empty bottle down to a great depth and then pull it out again, we will find it full of water with the cork inside- all because of the pressure that the water exerts deep down. In his ‘The Ocean’, the celebrated oceanographer John Murray describes the following experiment. Three glass tubes of different sizes, sealed at both ends, were wrapped in a cloth and placed in a copper cylinder, which had holes in it to let the water through. The cylinder was sent down to 5 km and then pulled out. When the cloth was unwrapped, a snow like mass of crushed glass was found. Pieces of wood sent down to similar depths sunk like bricks later, so heavily compressed were they.

Hence it seems only natural to expect that this terrific pressure should make the water at great depths so dense that even heavy objects would not sink any further- in the same way as an iron weight does not sink in mercury. But this is a totally erroneous notion. Experiments have shown that water, like all liquids in general, yields very less to compression. Under pressure of one kilogram to every square centimeter it will compress only by a twenty two-thousandth of its volume, and the rate of compression increases by the same degree with every extra kilogram of pressure. We must make water eight times denser than it is for iron to float in it. But to make it just twice as dense, or in other words, compress it to half its present volume, one must exert a pressure of 11,000 kg/cm square. Provided that in general it were possible, we would get a pressure of that order only at a depth of 110km.

Thus it is clear that any noticeable compression at great ocean depths is out of question, because even at the deepest spot water loses only five percent of its volume due to compression. (The British physicist Tate has reckoned that if gravity was suddenly to cease and water become weightless, the level of the water in the oceans would rise by an average of 35 meters as the gravity compresses water would regain its normal volume. Berger has noted that in this case "the ocean would flood 5 million square km of dry land which is the dry land only because the water in the oceans is compressed") This would scarcely have any affect on buoyancy – all the more so since all solid objects at these depths are subjected to the same pressure and are consequently compressed too.

There need be no doubt, therefore, that wrecked ships sink to the ocean bottom. " Anything that will sink to the bottom of a tumbler of water," Murray says, "will practically sink to the bottom of the deepest ocean."

But there is this following objection that is heard from time to time. If you carefully immerse a glass bottoms up, it may stay thus, as it will displace an amount of water weighing as much as the glass itself weighs. A heavier metal tumbler might stay in this position, even below the water level, without sinking to the bottom. So it is claimed that a capsized cruiser or any other ship might also stop halfway down. If the air in the ships compartments has no escape, the ship may sink to a certain depth and stay there. After all quite a few ships sink with the keel pointing upwards. Couldn’t it be possible that some of them might have not reached the bottom and are still suspended in the murky ocean depths? And though the slightest push would be enough to disturb their equilibrium, righten them, fill them with water and send them to the bottom, could one expect jolts in the ocean depths- that domain of eternal silence and tranquility, where even the worst of storms have no repercussions?

All these arguments are based on a physical error. An overturned glass does not submerge by itself. It must be made to do so by some external force- in the same way a wood or an empty corked bottle. So will an overturned ship remain afloat- never. It will always rest at the bottom of the ocean and not midway between the top and bottom.