Water can enter through even the smallest cracks, but how do you avoid water entering ships at the propeller shafts. I mean…seals in general, must fit tight, but here you still have to allow for movement. How do they do it?
In: Engineering
There is not much magic to it. You use a sealing material which is pretty smooth and does not create much friction and then you make sure to compress it just enough to stop the water but not enough to cause excessive wear. You do not expect it to stop all of the water comming through, as you say water comes in through even small cracks. However if you can reduce it to a small dribble of water this is enough that your bilge pumps will have no issues pumping it back over board or preferably store the water on board until you reach shore where you can dispose of it properly as it is often contaminated. Some boats do have systems that allow you to adjust the tension of the water seal around the propeller shaft. This means that if it wears you can tighten it down again. When you stop the shaft for longer periods you can tighten it extra tight to reduce the amount of water leaking through and then losen it later on to allow the shaft to spin freely.
They use a “Garloc” seal. It’s a lubricated seal that encircles a polished part of the shaft. The inner lip of the seal has a spring providing constant tension and the seal material is very hard and long-wearing. Some are even made of a graphite impregnated material to improve lubricious and wear properties. They are also packed on either side, often with a wax/grease impregnated fibrous material.
I’m sure some clever solutions exist that I don’t know about. Stuff like magnetic drive shafts. However, these solutions are not necessary for a functional boat.
First, boats leak. Boats take on water, and that water must be removed. That’s what bilge pumps are for.
Second, a good seal doesn’t have to prevent motion. Rubber can slide over a surface, especially a lubricated surface, while preventing any significant amount of water from seeping past it.
Seals.
The pressure isn’t very high, so a thin film of pressurized grease pushes the water back. Sometimes there are problems, even with [gazillion dollar destroyers](https://news.usni.org/2017/04/05/navy-found-fix-uss-zumwalt-engineering-problem ).
A lip seal of some sort. They usually sport a sort of “cup” where the pressure of something trying to get in or out actually makes them seal tighter. The same way a crankshaft doesn’t leak oil out of your car.
Imagine the wiper blade on a car, except formed into a donut shape, at its absolute most basic form. Google for lip seal or radial shaft seal for pics and more info.
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