Read a book long ago (can't recall the title) about the basic design of submarines. Essentially the engineering consists of making a cylinder, add a bow and stern, fill with machinery. At this point the air inside the vessel is still enough to make it float - don't want it to always float, want it to submerge sometimes.
So fill the keel with lead ballast until the weight overcomes the air inside, too heavy to float, it sinks. Don't want it to always sink and stay sunk, need it to float sometimes - so add more air. Saddle tanks full of air on the outside, now it floats.
Open the vents, water comes in the valves/ports at the bottom, forces the air out the vents at the top, when the air in the tanks that kept it afloat is gone, down we go. To get back up vents are shut, compressed air blown into the tanks forces water out the bottom, we're floating again - held up by the air in the tanks.
With all that in mind, why call them "ballast" tanks? Technically what they really are is flotation tanks!
Change my mind!
