I learned this today. The Trieste, a Swiss and Italian submarine reached a depth of 10,911m. This is the deepest point on Earth and is the Challenger Deep of the Mariana Trench.
Different submarines can reach different depths, depending on what they are needed for. A submarine that is built to go deep has to have several modifications that a military submarine that is only meant to cruise wouldn’t have.
The pressure at the bottom of the Challenger Deep is about 1.25 metric tons per square cm. That is about 1000 times more pressure than at sea level. In order to survive that, the walls of the Trieste were 12.7cm thick.
One of the problems with designing a submarine to go to great depths is its weight. The walls must be thick and made of a metal strong enough to withstand the pressure of the water. There must also not be any weak points or welds, so the walls of the submarine have to be a consistent shape.
The thickness of the walls leads to the major problem with submarines designed to go deep. Because they are so heavy, they are not naturally buoyant. A regular submarine is basically a boat. It displaces more weight in water than its mass and it floats. In order for it to go underwater, it must fill its tanks with water to increase its weight and sink. In order to come back to the surface, it empties the ballast tanks of water and it floats back to the surface. Deep sea submarines don’t float because their weight is greater than the amount of water they displace. To make them float, they have to have tanks of gasoline, which is less dense than water.
Another problem that the Trieste faced was the ballast tanks. In a regular military submarine, the ballast tanks are emptied using compressed air. The pressure at the bottom of the Mariana trench is too great for compressed air to be released. Compressed air is kept at a pressure that is less than half the pressure at the bottom of the Mariana trench. To get around this, the Trieste carried nine tons of magnetic iron pellets. The pellets were held in place by an electromagnet that could be switched off when the submarine was ready to resurface.
Military submarines can’t afford to be as heavy as the Trieste because it would cut down on their speed, maneuverability, and longevity. They have to balance the depth they will need and their weight. Modern metals are becoming stronger and lighter, so modern submarines can dive much deeper than their ancestors could without sacrificing weight. The Russian Typhoon class of submarines, can dive to 900m and they weigh 48,000 tons. They can carry 160 people and they are the largest submarines ever built.
Interestingly, and unconnected to this topic, nuclear powered military submarines can stay underwater for months at a time because they can produce their own oxygen and water. They use electrolysis to separate the oxygen from seawater using an electric current. They have carbon dioxide scrubbers to keep the air breathable. They also have reverse osmosis units which remove the salt from seawater. The record for a submarine being submerged is 111 days by the submarine HMS Warspite.
So, how deep could a submarine go? It turns out that it is an impossible question to answer because there is nowhere deeper on Earth to go. The Challenger Deep is literally the deepest point. Theoretically, though, if the walls of the submarine were thicker, it could obviously go deeper. As to how deep, I cannot find an answer. At some point, the submarine will be too heavy to be able to surface.
Pressure in the sea increases by about one atmosphere for every ten meters of depth. That is why at the bottom of the Marina trench it is about 1000 times surface pressure. At some point, if the pressure is high enough, the submarine will melt. Iron melts at about 40,000 atmospheres, which would be about 400km. If the sea was 400km deep, an iron submarine would melt. Although, to reach that depth, the submarine would have to be a solid ball of metal. This is the reason we will never be able to use a probe to see if the planet Jupiter has a solid core.
So, the deepest a submarine can go is the bottom of the Mariana trench. However, these submarines are designed to go deep. They are not fast or maneuverable and would not be any use as a military submarine. And this is what I learned today.
Image By U.S. Navy photo by General Dynamics Electric Boat – https://commons.wikimedia.org/w/index.php?curid=8224242
Sources:
https://en.wikipedia.org/wiki/Submarine
https://www.thebalancecareers.com/facts-about-navy-submarines-4058060
https://divediscover.whoi.edu/underwater-vehicles/how-deep-can-we-go/
https://divediscover.whoi.edu/underwater-vehicles/how-much-pressure-can-we-take/
https://en.wikipedia.org/wiki/Submarine_depth_ratings
https://navalpost.com/how-deep-can-a-submarine-dive/
https://en.wikipedia.org/wiki/Trieste_(bathyscaphe)