Eli5: How does water not get inside ships or submarines through the motor shaft?
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It usually does, a little bit. There are all sorts of sources of water in a ship, and ships just pump it back out.
To minimize the leakage, ships use some sort of seal around the prop shaft. Historically this would have been something fibrous like rope soaked in oil, wrapped around the shaft and compressed.
More modern systems may use finely ground surfaces pressed closely together to make the water gap microscopic.
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Modern large vessel propeller shaft seals are quite complicated and are done through a combination of bulkheads, special compounds known as elastomers, and oil. They are constructed in such a way that the seal is actually created by hydrostatic pressure from the seawater itself. The system is designed to provide pressurized oil into the seal structure, which eventually leaks out in to the water outside the vessel, and needs to be monitored during use. They also wear out every 3-5 years and are usually replaced in dry dock.
If water has a path, with pressure and time it will leak. Everything leaks. Solid metal impurities diffuses straight through solid metal, not quickly though. So we apply many different methods, sometimes in combination, to reduce leakage.
Looking up seals and packings for valve stems, large rotor shafts, turbine shafts, or just boat motor methods. Common ones are flanges and plates with complex paths, redirecting the pressure of the water back on itself, stuffing compressed material around the shaft so it squeezes the water back out, and using pressurized oil to create a seal against the water. Nothing will do the job for ever, maintenance is always needed, vibrations and wear will create an uneven surface over time and water will channel down those paths. It all depends on how long you can go before major repairs and what stresses you will be putting on the system. Sometimes just letting it leak and running a little pump and collection tank constantly is a fine solution.
In most cases it does leak a little bit.
Boats have pumps called bilge pumps that pup out the water from the lowest points of the boat. Its not unusual for the shafts to leak a little bit. When they leak a lot, they need repair.
On my USS Ustafish, we had a very smooth mechanical seal used to keep the water out of the people tank. The surfaces were lubricated by a slight flow of water past the seal. We would check it to ensure it was not too much. A backup seal was provided by a stuffing ring. If the mechanical seal fails, the stuffing ring is tightened until the flow is reduced but not stoped as the slight flow would provide lubrication to the packing seal. I’m sure the mechanical seal had a lifetime and would need eventual replacement.
There are several seals in line on the shaft. It leaks and there’s a catch-basin just below it, when it fills the water is pumped to a holding tank. Occasionally that water in the holding tank is pumped overboard.
The deeper the submarine is, the more the shaft leaks. Its not a bug, it’s a feature.
Its called a stuffing box. There are many different types, and all different kinds of applications. But the premise is: that the shaft goes through a (shaft sized hole) from inboard the boat, to the outside where the prop is. On the outboard side, theres a rubber bearing called the cutlass bearing that is usually rubber inside, and metal outside, and the shaft sits inside the rubber tube (imagine a condom, cut off at the end) and the rubber allows for vibration of the shaft from the propellar: dampening the vibration inboard that goes to the transmission, and the motor. Inboard: (as i said there are may types) but on the transom wall (where the shaft comes in the boat) there is usually a metal sleeve of some sort, that either bolts to the transom wall, or the shaft itself that covers the first part of where the shaft enters the boat (on its way to the transmission/engine) the types tat i have experience with, are the shaft sleeve type of stuffing box, where the shaft will come through the back of the boat, inboard: into this shaft sleeve, that has a coupler, a sleeve, and a locking nut… inside the sleeve, you put wax packing, which is just like it sounds. Its a piece of cloth that had been soaked in wax, and hardened into a square piece thats pliable: think like street chalk thats square… it comes in lots of different sizes… 1/4 inch 3/8 inch, 1/2 inch etc… so say that your shaft diameter is 1 1/2″ in diameter… your shaft sleeve/ coupler will be 1 5/8ths in diameter leaving an 1/8th of space around the outside of your shaft. This is where you stuff the packing into… then you tighten the coupler against the shaft tube, until it crushes this packing, and thus: stops the water… the reason they use wax though: is that you actually want a little bit of water to get theough the transom from outside the boat, to the inside. Because this is what lubricates the shaft, and reduces the friction of the thousands of revolutions per minute that the shaft spins… otherwise the shaft would heat up, and warp the metal, or ruin the engine through vibration. Then when you crunch the coupler to the sleeve, you take the locking nut, and snug it up to the coupler lip, so that it stays in place, and has the desired amount of lubrication (usually like a faucet with a single drip every 3 seconds or so!!)
Hope that helps!!
Cheers
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.
Nobody has mentioned this, I’m surprised? Anyway seal leakage is a good thing. Spinning metal surfaces on metal surfaces is called machining and not a good thing. So you want some leakage around the shaft for lubrication.
In pharma applications we’ll use a barrier fluid. Pressurize it and even sterilize it. Those seals last a long time… But a simple lip seal like on a through hull prop shaft? Yep its designed to leak.
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