Submarines dive by allowing water to expel air from their … tanks? How do they regain buoyancy in order to surface?
In: Engineering
Before diving, they fill some other tanks with compressed air and use this to expell the water.
By filling their tanks with air again, coming from compressed air containers.
Buoyancy depends on weight versus volume. It’s basically the same as blowing up a balloons with helium (but the other way around here as you force heavy water out of the tanks and replace it with lightweight air without changing your volume)
I believe you have that backward. IIRC subs take in water (making them denser) to dive and expel the water (creating a larger air-filled space) to surface. This is the same thing that SCUBA divers do (suck in air from the tank to fill their lungs with air to surface).
They have compressed air tanks on board (and compressors to refill them when they’re at the surface) but these are generally for emergency use only. Subs have control surfaces similar to the wings on a plane and adjust these with forward propulsion in order to ascend in normal circumstances. When they approach the surface they then use their snorkel mast to suck in air in order to blow the water out of the ballast tanks allowing them remain buoyant even without forward propulsion.
All the replies about using air are right, but it’s not the highly compressed air from the ship’s air bank. That’s only used to surface in emergencies. For normal surfacing, it goes something like this:
1. Sub goes to periscope depth to make sure it’s safe to surface
2. Sub raises snorkel mast to bring in outside air
3. Sub fires up a device called the low pressure blower. This is provides air at low pressure but in tremendous volumes
4. The blower output is sent to the ballast tanks to push the water out of the bottom.
One thing that’s not been mentioned so far–in an ideal situation they’d only take enough water into the ballast tanks to make the submarine *neutrally buoyant* (e.g. doesn’t have any tendency to sink *or* to float)–they then control their depth primarily using diving planes as they move forward. This means they can also use the diving planes to rise up close to the surface before blowing the water out of the ballast tanks, which makes the process much easier due to less pressure acting against the compressed air.
I think submarines are generally pretty close to neutral buoyancy most of the time when submerged and because they don’t compress (much) that buoyancy doesn’t change with depth like a scuba divers BCD does.
At neutral buoyancy as long as they are moving forward, they fly along wherever their control surfaces dictate.
At the surface they would add extra buoyancy so they can stay surfaced without forward movement, and then expell that air to dive.
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Smarter every day has an entire series about US nuclear submarines and how they work. They can actually change depth without changing buoyancy if they angle their diving planes (fins) during movement.
As for changing buoyancy they have compressed gas aboard that can be pumped into ballast tanks which expels the water. Or they can allow water into the ballast tanks while expelling the air.
The diving planes in combination with forward movement are actually how more modern submarines control their depth and angle.
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