[paul.fr]

how do submarines sink, stay "sunk" and rise to the surface?

[another_someone]

Same as the swim bladder in many species of fish.

Whether you float or sink depends totally on your density relative to the water - i.e. the amount of volume to take up, and the mass they contain.

The volume of a submarine is fixed, but the amount of mass they contain depends on the amount of water they contain (same as with a ship, if you punch a hole in the ship, water comes into the ship, which then increases the mass of the ship, and generally this can cause the ship to sink).  The difference between a surface ship and submarine is that a surface ship generally tries to keep the water out in order to stay buoyant, while a submarine has ballast tanks that are designed to be flooded when required to sink, and to have the water pushed out of it when the submarine wishes to rise towards the surface.

[eric l]

Check this link.  Click on "Submerge" to see how the submarine submerges, then click on "Surface".
http://science.howstuffworks.com/submarine1.htm

[ukmicky]

Its a bit like a scuba diver, in order to control where they are n the water whilst moving over reefs etc they take bigger breaths from your tank in order to  rise and smaller breaths to go back down.

[pete_inthehills]

How do you sink an Irish submarine?


Swim down and knock on the hatch!


pete
inthehills

[chris]

Its a bit like a scuba diver, in order to control where they are n the water whilst moving over reefs etc they take bigger breaths from your tank in order to  rise and smaller breaths to go back down.

Certainly true Micky, but not the whole story. Divers use a BCD - bouyancy control device - which is a vest worn over the wetsuit that can be inflated using compressed air from the tank. By increasing or decreasing the amount of air in the BCD the diver can alter the amount of water he is displacing, thereby altering his effective density and therefore the buoyancy force acting to propel him upwards.

As you go deeper you need to add more air to the BCD to maintain neutral buoyancy because the increased water pressure reduces the air volume in the BCD so you displace less water. This increases your density, so you sink more. This is why if you swim down beyond about 5m below the surface you will continue to sink without further effort - because the water squashes you and increases your density.

We discussed some of these principles on a recent edition of the naked scientists radio show and podcast in which we looked at free diving:

This is a link to the interview I did with Mark Harris, free-diver extraordinaire!

http://www.thenakedscientists.com/HTML/content/interviews/interview/762/

Chris