How they make propeller shafts on large vessels like ocean liners or aircraft carriers waterproof considering the intense water and mechanical pressures that come into play?

Question

It seems like a grommet-type seal would fail in no time. Also, looking at an image of one of the propellers for the Queen Mary, it hit me that this is decades old, and would have to go to dry dock to be serviced, and there is no way they are doing that so… How is that even possible? I’d figure dry rot would set in somewhere along the way.

In: Engineering

Answer 1

Oh I can actually answer this one.

The seals used are not water tight. There are different designs for different applications, but basically there is a static portion mounted to to bulkhead where the shaft penitrates into the water. A second portion of the seal is fitted on the shaft itself and pressed up against the static side. These are primarily Watercooled by the seawater and sometimes has a flushing freshwater connection to help flush out the silt/salt/abrasives that go between the running surfaces.

They leak by design. The water layer between the running surfaces reduces wear, cools from heating and drips below. There are volumetric measurement takes from the leaking water to determine if its an allowable amount or if too much water is seen, its a sign of an improper fitment or the running surfaces have worn down to much requiring an adjustment or replacement.

There are also back up inflatable devices. Kind of like an intertube but wrapped around the shaft on the dry side. So if something goes wrong you can inflate it and stop the shafts rotation to seal it up temporarily until the proper fix can be made.