The reserve of compressed air is usually just enought to provide positive buoyancy...about 1/4-1/2 of the main ballast tanks- the rest is done by a lower pressure turboblower that clears the tanks after surfacing. SHIII shows this, if you notice, as after surfacing it takes a bit of time to some to full surface cruise depth.
edit- on diving, the sub simply opens vents at the top of the ballast tank- there might be some bubbling, but its very localised- watch your wake as you go under.
Last edited by Patboot; 06-24-06 at 09:11 AM.
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