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

There are a few factors slowing down submarines on the surface. Water does have surface tension which makes it harder to push through it on the surface then under water. You also end up pushing water upwards into the air creating a bow wave which will push on your hull. This bow wave will not be the same under water. You also get waves hitting the ship slowing them down. A trick to traveling faster on the surface is to cruise in semi-submerged mode. This is when they trim the boyancy so the submarine have almost no boyancy. This menas that most of the submarine is under water and only the top of the sail or even just the periscope is above the water. Something comparable is done with big container ships in that they have a huge bulb on the bow just bellow the waterline looking similar to a submarine bow. This will then break up the bow wave making it easier for the rest of the hull to follow.