Eli5: How does water not get inside ships or submarines through the motor shaft?
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Edit: where were all these intelligent comments before I posted this? Stupid phone Reddit…
First, I served on a US Navy submarine. In the Engineering department. As a mechanic. Directly owning the primary propulsion shaft, including the “shaft seals” equipment, as it is called. Now that that’s out of the way, I can’t wait for all the Reddit experts to come explain how I’m wrong about how it works.
You’ve gotten some ok answers, but most of them didn’t properly explain how *modern submarines* accomplish this, only simple seals you’d find on a small craft. And it’s a little bit complicated, but I’ll try my best to keep it simple.
So there’s two things that make this system work, the mechanical seals themselves and the seawater that is used in them. These combine to form what’s called a shaft seal. First, to try and describe the mechanical seals, imaging a zipper that’s zipped up. Now imagine trying to pull something, even just a string, between the teeth all the way along the zipper. Difficult, right? This is the concept of the mechanical seal, except imagine this same path wraps all around the drive shaft. This is referred to as a “torturous path” because to get through this path requires constantly changing directions.
Second, the shaft seal water. Submarines go hundreds of feet beneath the water’s surface. The pressure increases at a rate of roughly 44 psi every hundred feet of water. So there’s hundreds of pounds of water pressure pushing through this “torturous path” and still able to get through into the inside of the boat. How do we stop this? We don’t exactly, but we can minimize it further by pumping water at it at an even higher pressure than the seawater outside the hull. Seems difficult, but it’s actually pretty easy. There’s already seawater piping inside the submarine that’s used for cooling equipment. We take a little pipe and route a little bit of this seawater (that’s at the exact same pressure, remember) and connect it to the inside of this shaft seal assembly, except we’re ALSO going to add a small water pump right before the shaft seal connection. Now, this pump can add just a little extra pressure to the water making it a slightly higher pressure than the seawater, enough so that it actually pushes *against* the water trying to sneak through those mechanical seals (the zipper, remember?) and keeps the outside water from getting in through the shaft opening. A much smaller amount of water leaks through the inside end of the mechanical seal than otherwise would, and this leakage can just drain into a tank that gets pumped overboard when it gets full.
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