How are military submarines designed to be quiet?

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How are military submarines designed to be quiet?

In: Engineering

Not an expert, so sorry if I miss some important details.

Modern military subs use electric motors powered off of a nuclear reactor, which is pretty quiet, so they naturally won’t tend to make a lot of noise. The biggest obstacle they face afaik is with the screw (propeller thingy that pushes the sub). One of the biggest giveaways sound wise comes from cavitation. Cavitation is when you run the screws fast enough that it creates turbulent water with little pockets of low pressure, hot water, that basically turns into little steam bubbles. This makes lots of noise. To avoid cavitation, they carefully design screws to be able to push as much water as they can without creating these conditions. They also figure out at what speeds a sub will start to cavitate, so if a sub wants to remain hidden, the crew will stay below that speed.

A lot of the other stuff that keeps a sub hidden revolve around how it is operated, rather than just not making noise. They can hide in the “blindspots” of other vessels, or by sitting in an opposite sound channel from vessels they want to hide from, which involves using layers of water that are different temperatures in the ocean which bend sound in a way that makes it difficult to hear what’s on the other side.