> Instead, the battle of submarine silence has mostly revolved around obscure technical problems of fluid dynamics, since one of the loudest noises made by submarines is the cavitation around the screw.
Cavitation is loud, but usually only happens if they're running full out. What they're really listening for now are reactor plant noises.
> I don't know if this is true today, but at least years ago the low-noise design of the screw on modern US submarines was classified, and so the screw was covered by a sheath whenever a submarine was out of the water.
Many US fast attack (Virginia, Seawolf) and the upcoming Columbia SSBN use some sort of external pump jet. I'm not sure if they cover those up out of water like they did with more 'traditional' screws.