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?



I would have thought that the opposite was the case as less mass is pushing against the water while surfaced.

In: Physics

Once you get down in depth even a short distance, the propeller gets a better bite on the water. On the surface, the top 10% of the propeller might still be underwater, but it is just making a lot of turbulence.

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.

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.

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.

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.

Boats slice through the water basically, pushing it out to sides, and they can be designed to lower the turbulence (which creates more drag). Subs are designed to slip though underwater with low drag, low turbulence. But they don’t have a bow designed to minimize turbulence when the hull is breaking the surface. The nose pushes water up over the front of the hull, and then it collapses back at the sides, creating a lot of drag.

Likewise, WWII subs were pretty quick above water, but very slow underwater.

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.

Submarines scare me. Is there a phobia for this ?

Hi, actual sailor here. Served as a rider on subs (not a true-blue submariner, sadly).

Modern submarines travel faster underwater simply because they’re designed to do so.

The more complicated answer is the difference between hydrodynamics and aerodynamics. Take a look at modern submarines like the *Virginia*-class. Now take a look at the much older WW2-era diesel boats like the *Gato*-class. The *Virginia*-class is a cigar shape, much more suited to spending and traveling most of the time beneath the surface. This is where modern subs thrive and perform their best. The *Gato*-class is shaped and built more like a traditional ship. This gradual change in hydrodynamic shape is due to advances in technology that contributed to submarines’ underwater endurance — basically, older subs like the *Gato* didn’t spend most of their time underwater, they spent it on the surface like a regular ship and only submerged in certain circumstances, which is why they’re shaped the way they are. Modern subs like the *Virginia* are the opposite: operationally spending most of their time submerged and for as long as possible, only coming to surface during certain circumstances.

I’m no engineer, but there are different hull shapes to accomplish different things, depending on how you want a warship to handle in the water: shapes that slice through the water and push it away from the bow, shapes that cause the ship to lift out of the water and “ride” the surface to get away from the drag of the water, and shapes that are more attuned to being completely submerged — that cigar shape of modern subs.