A "screen door" on a submarine?

That would be silly, right? Hence the joke. I don’t really mean to ask about that, it just seemed a more inviting title than “What kind of seal is there where the propeller shaft exits a submarine ?”

The way I understand WWII submarines, there is a shaft that exits the submarine and connects to the propellers in the stern. Since the other end of the shaft has to be connected to either an electric motor or a diesel engine, that part of the shaft can’t be immersed in water all the time. Thus there has to be a seal that allows the shaft to rotate freely, but keeps water out.


How is it done on deep diving Subs like Alvin? My guess is that the prop is connected only to an electric motor. The motor can be immersed in water. There is a cable that goes from a dry part of the sub to the (wet) motor, but the cable does not move, so the seal can withstand more pressure.


You do realize that the same problem occurs for the screws and shafts (Down pervs!) of any ship? The engine is pretty much always below the water line, both to provide ballast and to keep the shaft level.

(Submarines need to have seals that will withstand greater pressure, but the principle is pretty much the same.)

In older ships, at the point where the shaft passed through the hull, a “box” was built that had the dual purpose of providing a final support for the shaft as well as providing a place for “packing” material. I am drawing a blank on what that packing material used to be, but basically, it was a soft material that would let the shaft turn while providing a combination of sponge-like water absorption and water resistance. New seals tend to be high grade carbon designed to fit directly against the shaft, again, allowing it to turn while preventing any water from sliding along the shaft into the ship.

Other methods have included O-rings (much like piston rings) that were recessed into the shaft and the housing, spinning with the shaft, but preventing water from passing.

Specific details of the current submarine technology may have to wai until some of our resident squids (of whom we have several) happen by.

Checking the Encyclopædia Britannica, I notice that they say that the seal itself, (rubber, carbon, or whatever), generally rides on a film of oil.

Here is an article that discusses several types of seals.

It’s called a “Gland Seal” or a “Packing Gland.” Similar to the stem of a globe valve assembly.
There are also high tech versions for extreme pressures and conditions.

You do realize they actually do have a screen door for a sub?

Actually, it is an expanded metal insert for the hatch, mainly to keep things out when in a foreign port [like grenades or anything that can be tossed up and into the hatch well]

Those hatches are incredible to look at - very inticate. They have to be extremely well made. My husband actually changed out the weapons shipping hatch on the USS Spadefish [SSN 638] when it was docked in La Mad on one of its last med runs. He and the guys that helped him said it was a bitch of a job but they couldnt have transited back without doing it.

On a DIY note, when making boats on one episode of Junkyard Wars, they packed the box Tom mentions with grease-covored strips of leather. It worked quite well.

To some degree the waterpump in a car has the same function - prevent water from coming through a rotating shaft, with the water under pressure.

From looking at a picture of Alvin, it seems my guess is plausible. Since Alvin dives to 450 Atmospheres, a seal as described above may not work. How much pressure can seals like these hold?