eli5: I was just reading about modern submarines on Wikipedia such as the German type 212. Why are they capable of travelling faster submerged than surfaced?

Question

I would have thought that the opposite was the case as less mass is pushing against the water while surfaced.

In: Physics

Answer 1

Hi, actual sailor here. Served as a rider on subs (not a true-blue submariner, sadly).

Modern submarines travel faster underwater simply because they’re designed to do so.

The more complicated answer is the difference between hydrodynamics and aerodynamics. Take a look at modern submarines like the *Virginia*-class. Now take a look at the much older WW2-era diesel boats like the *Gato*-class. The *Virginia*-class is a cigar shape, much more suited to spending and traveling most of the time beneath the surface. This is where modern subs thrive and perform their best. The *Gato*-class is shaped and built more like a traditional ship. This gradual change in hydrodynamic shape is due to advances in technology that contributed to submarines’ underwater endurance — basically, older subs like the *Gato* didn’t spend most of their time underwater, they spent it on the surface like a regular ship and only submerged in certain circumstances, which is why they’re shaped the way they are. Modern subs like the *Virginia* are the opposite: operationally spending most of their time submerged and for as long as possible, only coming to surface during certain circumstances.

I’m no engineer, but there are different hull shapes to accomplish different things, depending on how you want a warship to handle in the water: shapes that slice through the water and push it away from the bow, shapes that cause the ship to lift out of the water and “ride” the surface to get away from the drag of the water, and shapes that are more attuned to being completely submerged — that cigar shape of modern subs.