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 displacement vessel is mostly limited to hull speed* on the water. The general formula for hull speed (in knots) is the square root of 1.34 * waterline length in feet. Going any faster than hull speed requires a tremendous amount of power.

When operating at the surface some of the water must move out of the way by being lifted to either side of the submarine, that is the bow wave. The faster it goes, the bigger the bow wave gets. When the vessel moves faster than the water can get out of the way, the wave gets so big that it sinks the vessel.

There are stories of old sailing ships carrying too much sail going downwind that they sink themselves from going too fast.

A submarine trying to go too fast on the surface simply would not remain on the surface. Its bow wave would engulf it.

Once underwater a little ways the bow wave disappears because the water is able to move out of the way of the submarine more easily; it can move to either side, up, or down with equal ease.

* There are always exceptions, such as very long skinny hulls.