How does ballast in a submarine work?

If you let air out, there’d be no more air left to take you back up again!

Compressed air perhaps?

The same for hot air balloons. How useful is ballast if the sand bags are only one-time use items?

Submarines have used a system of compressing air in a chamber, and filling a chamber with outside water to change buoyancy. Early designs used a piston in a single chamber to compress the air, and water filled the void as the air was compressed. I’m not sure how modern submarines do it, but its basically the same process.

Hot air balloons have sand bags primarily for one time use. Heat control can take the balloon up and down under normal circumstances. Some ballast may be dropped at take off if the load is heavier than expected, or added for light loads to improve stability. But they are most important for cases where you lose your heat source, to prevent descending too rapidly.

Submarines have three different types of ballast: the main ballast tanks (MBTs), variable ballast tanks (VBTs), and the fixed ballast.

The main ballast tanks are designed to either be full of water or full of air. If the tanks are full of air, the submarine is surfaced; when allowed to fill with water, the submarine submerges. On the class of submarine I was on, the submarine had a displacement of about 6,900 tons when submerged–900 tons of that was water in the main ballast tanks. The MBTs have permanently open grates at the bottom and valves at the top. When full of air, the air is pressurized so keep water from coming up the grates in the bottom. When the submarine wants to submerge, the valves at the top of the tanks are opened, and water is allowed to flow into the tanks, causing the submarine to submerge. The valves at the top are then shut.

When you want to surface, there are a couple of options. The usual procedure is to drive to periscope depth (using your control planes and main propulsion), extend the ventilation mast, and run the low-pressure blower to gradually push the water out of the tanks through the grates at the bottom. Another option is to use compressed air to force the water out–but this compressed air must later be replenished using the high-pressure air compressors. Most submarines have two compressed air systems for this purpose–a so-called “normal blow system,” which forces the water out relatively slowly, and the “emergency blow system,” which uses all of the emergency high pressure air onboard to quickly surface the submarine. If you’ve ever seen a photo of a submarine rocketing out of the water, it was doing a test of the emergency blow system.

The variable ballast tanks, as the name implies, carry a variable amount of water ballast. They are used for fine control of buoyancy for a submerged submarine. You generally want a submarine to be neutrally buoyant when submerged. As supplies are used during a deployment or if weapons are expended, a submarine will bring on more variable ballast.

Finally, you have the fixed ballast. Like all other ships, submarines carry heavy (usually lead metal) weights on the bottom interior of the submarine along the keel, to help keep the submarine upright. (More precisely, the fixed ballast provides a righting moment to keep the sub from capsizing in a roll.) The heavy emergency submarine battery also makes up part of the fixed ballast.

–robby (former U.S. submarine officer)

Note that lighter-than-air craft capable of long flights (e.g. blimps) often use ballonets. These act in a way similar to a submarine’s ballast tanks.

Thank you for your post Robby - that was very interesting!

Does this imply that there is an angle of roll past which the air in the MBT will spontaneously bubble out? I imagine a submarine resting on its side on the seafloor, unable to surface because the MBTs can only be partially filled due to the position of the ‘permanently open grates at the bottom’.

Yes, it’s what’s known as a very bad thing.

The only time the MBT has air is on the surface. If rolled enough to let some air out it would partially submerge. If it rolled enough to let most of the air out, it would submerge and should right itself with the permanent ballast (assuming there’s enough water depth to do so) and then the tanks could be blown.

Still, not a good day. There’d be solid water in the steam piping, the batteries would break loose, sailors walking on the overhead, cats and dogs sleeping together…

[quote=“robby, post:3, topic:550715”]

Doesn’t sound right to me. Submarines and sailing ships sure need to bring the centre of gravity down, but other ships? Any examples?

Obvously a hijack…

Does this mean you dump your garbage at sea? If so, is this standard practice for navy and/or commercial ships?

I’ve seen footage of a submarine cook preparing garbage for disposal. They take a thin sheet of metal and form it into a cylinder with holes in the side to let air out. then it’s packed full of garbage and jetissoned through a special port. They sink rather than float due to the holes therefore not giving away the subs position.

Wiki says it’s to increase draught.

When I was on a submarine 15 years ago, yes, it was standard practice to dump garbage at sea. It was done in accordance with international law, meaning it was done far out at sea (>12 miles, IIRC), and no hazardous waste was dumped, particularly oil or oily waste. Submarines also took care that no garbage was capable of floating up to the surface.

Today, from the Navy’s official FAQs here:

Besides garbage, sanitary waste is also discharged at sea, again far out at sea in accordance with international law. Submarines have no sanitary treatment capability, just holding tanks.

Sure they do. Any vessel with any superstructure to speak of, including cruise ships, warships, power boats, sailboats, etc. have fixed ballast along the keel to help keep the vessel upright and not capsize.

About the only exceptions I can think of are tiny vessels with no superstructure, like canoes and rowboats.

The ability to blow the ballast tanks is one of the limitations on how deep a submarine can dive. The extreme was probably the deep sea submersible *Aluminaut, *which devoted two-thirds of it’s volume to ballast tanks. Most deep-sea submersibles today get around the limitation by designing the submersible to be neutrally buoyant and dropping weights when it’s time to surface again.

In general, the hull’s strength is the limiting factor. Storing enough compressed air to achieve positive bouancy isn’t that big a technical challenge. Some deep-diving reseach submarines may be an exception to this general rule.

To nitpick slightly, Alvin-type submersibles are either negatively- or positively-bouyant, depending of the phase of the dive. Alvin herself uses four 208-lb steel weight to submerge and the positive bouyancy of syntatic foam to surface.

One of the big scenes in Das Boot covered exactly this, if you know what you’re looking at. I’ll spoil it because it’s a big part of the picture.[spoiler]The boat is on the surface at night when it’s jumped by an aircraft. Of course they crash dive but soon discover the diving planes are stuck at dive (and possibly an MBT or two has been destroyed; I don’t remember). The motors on the screws are reversed and they repeatedly blow air into the MBTs to no avail; the planes and their forward motion are forcing them down. When the Chief of the Boat said in a quiet voice, “I can’t hold her,” I sucked through my teeth making my then girlfriend look at me quizzically. “They’re doomed,” I whispered.

The rivets start to break as the hull is crushed but they land hard on the bottom, much shallower than they had any right to expect. Even so, the crew gets only a few moments to wonder why they are still alive before more rivets start shearing and various gaskets and seals start to fail, letting more water into the boat. After getting on top of that they work for sixteen hours patching things up and moving the internal water to the bilge where it is manually pumped overboard.

When all is ready they have enough air left in the tanks for one last try to get back to the surface. It was an agonizingly long time before they ever so slowly started to rise, like some battered corpse.[/spoiler]

It’s just that I’ve been naval architect for 15 years and I’ve never heard of ship size structures having lead ballast. I can believe that war ships might have this because they are often narrow and all aluminium. The standard way to save a project in cruise ship business is to build the top floor out of alumimiun. This increases your metacentric height and reduces weight. Lead is heavy and everything heavy is shunned in commercial shipping. You add a ton to your structure and you can reduce that from your cargo. Less income.

Submarines and sailboats need a low center of gravity for different reasons. When a submarine is submerged, it has no form stability. Whence the need for fixed ballast. Also, they typically need fixed ballast for buoyancy, something a surface ship does not need. When they surface, they have too high stability leading to uncomfortable motions. The logical conclusion is that if you have a surface ship which has the characteristics of submarine (particularly the same width), you’ll leave the lead ballast out.

Sailboats have enormous wind area so they need stability to counter that. Also, sails stabilize roll. Sailboats only heel, not roll. This is why too large stability is not a problem for them. Increased stbility means more sails for them. Therefore more hull weight means more power to them.

Any other type of vessel would try to avoid too high stability. It makes motions too abrupt, is very uncomfortable and when bad enough, is dangerous when steering gets difficult.

All ships need ballast all the time, but they use water. You can’t get rid of lead during the voyage, so avoid it if you can. But surface ships don’t need a low centre of gravity like submarines because of form stability. QE II is 32m wide, has an enormous superstructure and a GM of 1m. Lead ballast, seriously?

I’m not a naval architect, so I defer to your expertise.

Submarines certainly have heavy fixed ballast, made up of a combination of the lead-acid storage battery (which weighs something on the order of 100 tons), and lead along the keel. Surface warships also have heavy fixed ballast, for reasons mentioned above (large superstructure, narrow beam). (Incidentally, warship design has moved away from the use of aluminum because of problems with cracking and survivability. Cite.)

I assumed that other large vessels with large superstructures (such as cruise ships) would also use fixed ballast in addition to water ballast, but it appears that I was mistaken. :smack: Ignorance fought, and all that.