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?

250 viewsEngineeringOther

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

13 Answers

Anonymous 0 Comments

Ships generally do go into drydock for maintenance on a fairly regular basis.

Source: I served on the USS Carl Vinson during a drydock overhaul, 1990-1993

Anonymous 0 Comments

Traditionally propeller shafts were sealed with a stuffing box. These were crammed with oiled rope to form a tight, waterproof but relatively low-friction seal. Modern boxes use more modern fibers than ropes of old but the concept is much the same.

Anonymous 0 Comments

Museum ships do have to enter drydock or they’ll cease to float. When they’re converted to museum ships, the owner may decide to perform modifications that make the seal more permanent (removing/cutting the shafts and blocking off the port, or at least removing propellers to reduce radial load on the seal).

Most boats I’m familiar with use either a modern seal package that has several different seals and bearings in a single assembly, or they use packing, which is essentially a fancy rope impregnated with something like wax.

Very large ships would have a highly engineered seal, but might still suffer from a certain amount of normal leakage.

Anonymous 0 Comments

In the old days it was a stuffing box. I worked on pulley-driven pumps with gland seals, the same principle; stuffing box HAD to drip or the gland packing would burn out. There was a network of drainage pipes in the boiler rooms to funnel the drips into a drain.

On ships, if the gland packing was overtightened, it could glow red. I’d assume you’d just undo the nuts and put more packing into it in service. The QM is stationary, you could tighten the gland packing up as tight as you like. Does it still have propellers?

Pumps have now changed to carbon/graphite seals, they don’t need to leak. I’d assume ships use the same technology.

Anonymous 0 Comments

The same kind of seal they have on ship’s propeller shaft, a packing box is also used on gate valves. Here is a video of a man repacking one. [https://www.youtube.com/watch?v=hvfvNjSXzcY](https://www.youtube.com/watch?v=hvfvNjSXzcY)

Anonymous 0 Comments

Water cooled ceramic seals. Cotton ring seals are also used on some large ships, mostly much older. Ceramic is preferred because they don’t leak into the ship bilge. Cotton or other fibre seals require a drop of water every second per inch if shaft diameter, so for a 24” shaft it’s a lot of water that has to be processed through the oily water separator. Plus they are very time intensive to keep adjusted and lived properly.

Anonymous 0 Comments

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.

Anonymous 0 Comments

Some shafts have several seals in a row. Also, there is leakage all the time. There is a catch-basin under the spot where the water comes in, and every d=so often a level switch kicks on a pump and the basin gets pumped overboard.

Anonymous 0 Comments

At most, a ship’s propeller shaft might be 15’ (5m) below the surface of the water. The pressure at that depth is not intense. In fact, it’s around 7-8PSI.

Years ago, shafts were sealed with a packing gland, not unlike the one that keeps water from escaping around a valve stem under your kitchen sink, which is probably experiencing pressures exceeding 50 PSI. They were tightened until the water just dropped from the packing nut.

Nowadays, mechanical shaft seals are used and can operate leak-free for years

It’s important to remember that the pressure at a shaft penetration actually decreases as forward speed increases, since ship propellers are located aft and push the boat through the water (normally). The motion of the ship creates a negative pressure at the shaft penetration.

Source: me. Boat builder for 20+ years

Anonymous 0 Comments

Submarine use a mechanical seal, too. I met a guy and that’s all he did. He flew around the world installing mechanical seals. Despite extravagant pay he was going to give the job up to stay home with his family.