Submarines going deep

On the picture below, we have a classic submarine.

It dives well, and is fun to drive – that is because, when submerged, it has no weight. On the other word its average density is equal to that of sea water. Its hull is made of high-strength steel and the submarine can descend 300 meters deep. After that it gets crushed!

But we want to go deeper. What can we do?

We have to increase the strength of its hull to withstand the pressure. One approach is to increase the hull thickness. Great, but now we have another problem – the submarine gets too heavy. It is now more dense than surrounding water and may sink to bottom - it is a most serious problem. You may think that we can compensate making the submarine bigger either by prolonging it or by making its diameter larger.

Unfortunately this doesn’t work. In both cases you are forced to add more material to its hull and you don’t benefit from greater displacement – the average density of the submarine doesn’t change much.

But there is another way to compensate for greater weight of the thick hull. You can banish all the luxury stuff you don’t need out of the submarine: teaspoons, toasters, even your favorite sofa... still not enough. There are no luxury stuff in a submarine. Okay, you can implant a smaller and lighter engine into the submarine. It will contribute considerably but now the submarine will not be as fun to drive as it was before.

Are there ways to increase hull strength without increasing it mass? Actually yes – you can change its shape. Some shapes can better handle pressure stress than others. We can make a ball-shaped submarine.

Well, I understand your disappointment. Hardly a ball-shaped submarine can be competitive. In addition this is not an ideal ball (power shaft, entry bridge...) and will not do much better than our original submarine. But perfecting the shape of a submarine is still an important factor and submarines must be welded very precise.

Down, 1km

Up to now we didn’t find a good solution to make our submarine go deeper. Let’s try to build the submarine hull from material that has greater strength than high-strength steel alloy. And what has more strength than high-strength steel alloy? The answer is – almost nothing. The steel is a gift from heaven (from stars actually :)

But wait, we don’t need a material that has a higher strength than steel. We only need material that has a higher specific strength than steel (strength to density ratio). The titanium can be a bit weaker than steel because it is much lighter. We can make much thicker hull from titanium than from steel but yet both will have the same weight. And titanium hull, because is so thick, will be much stronger.

Using a titanium hull, our submarine could go as deep as 1km.

Titanium is expensive. Why don’t we use aluminum? Honestly, I have no idea. If anyone can tell me, please.

And even deeper

And what do we do if we want to go even deeper? No material has the specific strength needed to visit Mariana trench (some 11km deep).

To do so, you have to be ready for sacrifices. This is how it was done on the Trieste bathyscaphe.

First you build small tight hut for your crew and instruments. The hut has extremely thick high-strength steel walls capable to withstand more than 1000 atmospheres of pressure. The hut is very heavy and would sink to the bottom of the ocean as a rock (actually even faster than a rock).

Then you build a very large lightweight shell made form thin sheets of steel. You fill it with, say, gasoline. This way you made a container that is lighter than water.

You connect your small heavy crew hut to your large low-density container. Now you created a vessel that has the same average density as the sea water. Your are ready to go all the way to the bottom. It works on the same principle as a hot-air balloon, only instead of hot air you use incompressible fluid – gasoline.

Danijel Gorupec, 2006<