“Now, this tunnel drive system avoids the cavitation problem. You still have cavitation, but the noise from it is mainly lost in the tunnels. That makes good sense. The problem is that you can’t generate much speed without making the tunnels too wide to be practical. While one team was working on this, another was working on improved screw designs. Your typical sub screw today is pretty large, so it can turn more slowly for a given speed. The slower the turning speed, the less cavitation you get. The problem is also mitigated by depth. A few hundred feet down, the higher water pressure retards
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