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

A ship on the surface produces “gravity waves” in the surface of the water as it moves. These are basically what we normally think of as waves – bits of water going up and down. Mariners will talk about a “bow wave” and a “wake”, also sometimes of “wash” which is water disturbed by the engine. The energy required to generate these waves is a significant factor in the drag on a surface ship. Also to be considered are frontal area drag, skin friction drag, and pressure wave generation.

When a submarine is submerged it is not generating gravity waves. Being fully submerged changes the effects of the other factors but not enough to outweigh the saving made on the gravity waves. Therefore the boat moves more efficiently and the same motive power can move the boat faster.