Why is Compressed Air a Necessity to Surface?

[Fennec E Fox]

I'm no submarine expert or even much of a submarine amateur so I might be missing something obvious.

That said, I know compressed air is vital to surfacing and diving quickly. But if you somehow manage to completely expend the tanks could you not still use speed and dive planes to crawl to the surface? The tooltip speaks as if that's an impossibility, and I'm wondering if it's patently impossible or just excruciatingly difficult outside of perfectly ideal circumstances to surface on empty air tanks.

I'm tempted to make an expendable save state and test that out, but I'm curious about the real life details a game simply can't fit regardless of what the game rules are.

[Grumpy Old Dude]

The sub submerges, by taking water into the ballast tanks. Compressed air, is used to push that water out. Try it...float on your back. Inhale deeply. FILL your lungs with air and you will float higher in the water. Now exhale...

[Fennec E Fox]

So basically when the tanks are full of water the sub is too heavy to breach the surface on dive planes alone?

[Grumpy Old Dude]

way to heavy...which is technically the wrong term since an aircraft carrier or battleship is heavier...it has negative bouyancy when the ballast tanks are full of water. Regardless of weight, something with negative bouyancy...sinks

[QuailLover]

Almost. It's more of an issue with buoyancy than weight, though both are dealing with gravity. There are special tanks that can be filled with water or air depending on what you want the sub to do. If you need to be on the surface, it's filled with air. The air makes it more bouyant and float.

Once it's filled with water, the sub is less buoyant and will go under the water. It will "hover" in place once it is neutrally buoyant.

If the sub needs to surface, it needs to replace the water with air. Thus the compressed air is needed. Especially when conducting an emergency surfacing.

[Fennec E Fox]

Makes sense. Thanks.

[wolf310ii]

Submerged u-boats have neutral buoyancy (slightly positive) and change of depth was done with the depth planes, not with compressed air or ballast tanks (they had to be fully flooded, otherwise a stable controlled dive was impossible).
The slighty positve buoyancy was compensated with a slightly nose down trim (the flat deck acted as a depth plane too) and the change of volume, because of water pressure compressing the hull, was compensated by pumping water in and out of the regulating tanks.
Compressed air to surface was only used for the last few meters, or in an emergency blow.
So yes, u-boats could surface without compressed air, just with speed and depth planes and once the conning tower is out of the water, they could use the diesel to blow out the ballast tanks.
And even if the boat didnt move, it would slowly go to the surface, as long as there is no flodding.

The negative buoyancy tanks were only used to disapear from the surface quicker and were blown out as soon as the u-boat was submerged

[Amsterdamaged]

Diving has something similar. You have a BCD (a vest with air) And during a dive, you want to be neutral buoyant in the water..

For example.. if my vest is full with air.. try to swim down.. it takes a lot of energy, it's nearly impossible, it's very hard.. And when you are finally down, you just shoot up anyway, because you are positively buoyant.

A sub is the same.. sure it can use it's dive planes.. but you have to expend energy by going forward, and aiming the dive planes down, because if you stop doing that, you will go back to the surface..

What you want.. (just like with diving) is to be neutral buoyant.. This way.. you can be 40 meters down.. and you don't need to do anything.. You can switch your engine off and still be at the same depth. Same goes for going to the surface, sure you can your dive planes.. but it will cost more energy and you won't be floating when you finally are on the surface. You will just sink down again, because now, you are negatively buoyant.

[ramjbjb]

Is worth adding that water is virtually incompressible. That means that as pressure goes up, water volume remains constant.

I.E. if you're neutrally buoyant at 10m depth, you're equally neutral at 250m.

I point this out because I've seen in several places the idea that it takes more air pressure to blow tanks down at big depths because you need to shove more water out of the tanks. Not the case, the ammount of water is the same, but it's at a much higher pressure. Contrary to water, air is extremely compressible in almost an one on one basis. Air at 2 atmospheres takes half the volume as air at 1 atmosphere, so to completely expel the water out of a tank of a given dimension, at 10 atmospheres you'll need 10 times the ammount of air to expel the water out of the tank that you'd need at surface pressure levels.

BTW pressure increases at a rate of 1 atmosphere each 10m of depth. Down at 250m you're dealing with 26 atmospheres, so you need almost 13 times the ammount of air to empty a tank full of water than you'd do at periscope depth.

[wolf310ii]

Originally posted by Amsterdamaged:

Same goes for going to the surface, sure you can your dive planes.. but it will cost more energy and you won't be floating when you finally are on the surface. You will just sink down again, because now, you are negatively buoyant.

No, at neutral buoyancy i doesnt takes more energy to go up than going down.
Also when going up, the neutral buoyancy changes to positive, because the pressure decreases and the air in the vest expands (to preventig shooting up, the BCD releases air on the way up).

Originally posted by ramjbjb:

I.E. if you're neutrally buoyant at 10m depth, you're equally neutral at 250m.

No, while water is almost incrompressible, the u-boat is not.
The hull gets compressed, displace less volume and is therefor less buoyant the deeper the u-boat goes.

[Grumpy Old Dude]

@wolf....thank-yu for your explanation. Engineering and physics are not my forte, and I have a layman's grasp at best on the subj matter. Appreciate the correction.

[Gorlos]

Originally posted by wolf310ii:

less volume and is therefor less buoyant the deeper the u-boat goes.

https://www.khanacademy.org/test-prep/mcat/physical-processes/fluids-at-rest/a/the-buoyant-force-does-not-get-smaller-as-you-sink

[Colonel Sanders Lite]

Originally posted by Gorlos:

Originally posted by wolf310ii:

less volume and is therefor less buoyant the deeper the u-boat goes.

https://www.khanacademy.org/test-prep/mcat/physical-processes/fluids-at-rest/a/the-buoyant-force-does-not-get-smaller-as-you-sink

That article explicitly states that they assume that a rigid steel submarine does not change volume as it descends.

That being said, I don't think that this effect is all that significant myself. I'm not really aware of any publicly available scientific measurements regarding dimensional changes in pressure hulls with depth. They certainly do exist, but they can't exceed the plastic deformation point of steel so they're not *that* big.

If you do exceed that point were the pressure hull is undergoing plastic deformation, you're probably dead.


So yeah, this is the sort of fine balancing that's controlled with the dive planes.

[wolf310ii]

Originally posted by Gorlos:

Originally posted by wolf310ii:

less volume and is therefor less buoyant the deeper the u-boat goes.

https://www.khanacademy.org/test-prep/mcat/physical-processes/fluids-at-rest/a/the-buoyant-force-does-not-get-smaller-as-you-sink

Yeah, they simply wrong because they ignore that an normal u-boat gets compressed.
The articel is about a deep sea research submarine and even dont get this right.
This typ of submarines uses solid ballast to dive and drop ballast to gain neutral buoyancy when it reaches the desired depth and drop more ballast to surface.
Also that little air inside the submarine, doesnt gives it the needed buoyancy to surface, for that they have buoyancy tanks filled with oil or have solid floaters.

On an u-boat you can even visualize the compression of the hull, by simply attach a string from one side to the other on the frame. On the surface the string will be streched straight and when the boat dives the string will hang loose and will become more loose the deeper the boat dives.

[DECAFBAD]

Originally posted by ramjbjb:

Is worth adding that water is virtually incompressible. That means that as pressure goes up, water volume remains constant.

Water might be incompressible, but steel pressure hulls filled with air contract under extreme pressure. The Type VII would gain about 1 ton negative buoyancy per 100m due to this effect, which needed to be compensated for during the depth change otherwise the boat would keep falling.

[Gorlos]

Originally posted by wolf310ii:

Originally posted by Gorlos:

Yeah, they simply wrong because they ignore that an normal u-boat gets compressed.
The articel is about a deep sea research submarine and even dont get this right.

Did you read it?

quote:The stiff walls hold the pressure inside constant even while the pressure outside increases. The walls themselves actually get compressed due to the gradually-increasing difference in pressure across them. The reason that they don’t collapse onto you and Jacques is that they compress (like springs), which counter balances the force arising from the pressure difference. So as the submarine dives deeper, the internal pressure (thankfully) stays the same… but the walls actually get thinner!



...still the-buoyant-force-does-not-get-smaller-as-you-sink