What is the stern tube and stern tube seal on a ship?

The stern tube is the one point on the ship where a rotating shaft passes continuously through the hull below the waterline, which makes its sealing arrangement safety-critical, since a serious failure there floods directly into the ship rather than simply leaking somewhere convenient.

Inside the tube, the aft stern-tube bearing supports the tail shaft right where it exits the hull, working submerged and hard to inspect without dry-docking. In an oil-lubricated system, the bearing is white-metal-lined and runs in a bath or forced-feed circuit of lubricating oil, sealed at the after end, facing the sea, and forward end, facing the engine room, typically by multiple lip seals arranged in series so that even if the outermost lip wears, the remaining seals still hold. Oil pressure at the aft seal is kept slightly above the surrounding sea pressure, so any leakage path tends to push oil outward rather than let seawater in, and this pressure balance, along with the level in the header or gravity tank, is checked routinely on watch.

Water-lubricated stern tubes use seawater itself as lubricant for a bearing surface made from a suitable low-friction, water-tolerant material such as rubber compound or resin-impregnated staves, with a simpler gland to control leakage inboard rather than an oil-tight seal, and need a controlled, often filtered, water supply to the bearing to prevent it running dry, particularly at low shaft speed or standstill.

Seal failure matters in both directions. On an oil-lubricated system, a failed seal can let seawater into the lubricating oil, degrading it and risking bearing damage, or let oil leak into the sea, a genuine pollution event with reporting obligations, which is why many ships now use certified biodegradable stern-tube oils to limit the impact of any leak that does occur, and increasingly fit improved seal designs and monitoring under regulatory pressure to cut leakage further. Condition is tracked through oil consumption trends, water content testing of the stern-tube oil, and visual inspection of the seals at dry-docking, since seal wear is progressive and a seal nearing the end of its life is far better replaced on a planned schedule than left to fail at sea.