How do ships, big ones, keep the ocean water from following the drive shaft from prop.?

Question

Water is under tons of pressure, and the power train must breach the hull to supply torgue to prop. Just curious.

Answer 1

Sure enough, and as you run tens of thousands of horsepower through the shaft, it *will* have eccentric vibrations that make it difficult to seal mechanically.

The secret is to have several packing or bushing rings in series-- outside pressure steps down as you pass each ring. Finally between the innermost rings (closest to the vessel interior / shaft alley), you pressurize the annulus with air or water (or less commonly now, oil). This pressure is low enough to be resisted on the inside mechanical seal, but increases the effectiveness of the next seal outboard. That annulus operates at higher pressure, and further increases the effectiveness of the next seal outboard, and so on.

Under that set-up, you should see *very* little or no leakage into the ship, but whatever fluid you use to pressurize the first annulus will slowly leak out to the ocean... which is why operators are steering clear of oil in these days of stricter regulations. Modern ships should not have to run a bilge pump continuously to accomodate a leaky stern tube shaft seal.

Answer 2

the device is called a stern gland. it tries to stop sea water ingress along the propeller shaft.
the most simple simple ones have gland packings, usually hemp, PTFE coated etc
then you have deep sea seals, like mechanical seals etc
for exact details refer to a marine engineering/ship building site.

Answer 3

Good bushings, I suppose.

Also, they run their bilge pumps continuously.

Answer 4

seals