The Bottom Line Trumps Security
A key technology necessary to create acoustically silent submarines is the production of the precisely machined massive propellers that provide the motive force. Cavitation, a phenomenon that generates air bubbles in regions around the propeller where the local pressure is lower than the vaporization pressure of water, generates noises that sound like fingers snapping, and is a major betraying sonar signature of submarine models. Cruising at great depths where pressure is higher helps mask the effect, but the time submarines are most vulnerable are when nearer to the surface. The good news it that finite element computer modeling has produced shapes capable of eliminating the cavitation, but extremely precise machinery is required to craft the 20-foot-plus diameter, 40-ton-plus behemoths. Such technology would provide significant advantage to the Navy. The bad news is that a few years back Toshiba, who was contracted by the Navy to build the milling machines to do the job, illegally sold the technology to Russia, thereby undermining the effort.