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

The German type 212 is a submarine that uses hydrogen fuel cells to spend long stretches of time under water.

This type of submarine along with nuclear powered submarines have a hull optimized for traveling fast under water.

Older types of submarines that mostly use diesel and batteries don’t spend as much time under water, because they can’t and therefore have hulls more optimized for traveling while surfaced.

If you optimize a submarine for going as fast as possible under water it will be slower above the surface. They often create huge waves along their side if they try to go fast while surfaced.

Hydrodynamics are weird and counterintuitive and hard to compute, but generally something optimized for one sort of situation won’t work as well in another type.

Submarines like the 212 are designed for raveling underwater and this is how their hulls are optimized.