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

For boats its about how cleanly you can move through the water.

The hull of modern submarines is optimized for underwater operations because they’ll be spending weeks at a time submerged. Nice clean lines mean limited skin drag (which is going to be the big player here).

On the surface they create a bow wake which eats up a lot of energy, modern cargo ships have the big bulbous bow to form the wave at just the right spot the reduce its effects at cruising speed and make it *just* the right length for the ship.

Probably the most important part of the confusion here is power.

A German Type VIIC U-boat (one of the most common) had two big diesels that could produce about 3,000 HP on the surface, but underwater it was restricted to about 740 horsepower from its weaker electric motors and batteries. It was this reduction in power that limited its submerged speed, not an increase in drag.

Modern non-nuclear subs have far more powerful electric motors, and are often *always* electrically driven. On the Type 212 it seems that the propeller is *always* driven by the electric motor and that on the surface the diesel just serves as a generator so there is no huge drop in horsepower when they submerge so the mild reduction in drag results in an increase in speed, unlike the old U-boats where the massive drop in power outweighed the reduction in speed.