On an inboard boat the propeller shaft runs from the gearbox out through the hull to the propeller, and the stern gland, or stuffing box, keeps the water out where it passes through. A traditional stern gland is a tube packed with rings of packing, squeezed around the shaft by a gland nut. It's meant to let a few drops through while the shaft turns, which keeps the packing cool and lubricated; done up too tight it runs hot and wears the shaft, and as the packing wears it leaks more and more into the bilge.

In this video I remove Renko's corroded stern gland from the stuffing box and add two more rings of Teflon packing to stop a leak around the propeller shaft, with the boat still in the water.