The traditional STUFFING BOX
Cons: needs periodic adjustment, needs smooth shaft surface to work well (no burrs, no galling or anyother shaft surface irregularities). Shaft should be removed and 'dressed' by a machinist when replacing packing.
Pros: can be packed (temporarily) with virtually any 'lubricated rope' that fits when in a dire emergency ... and still affect a 'seal' with the propshaft turning.
Inexpensive to operate. Technology is over 200 years old; therefore, quite reliable. Easy DIY changeout of packing.
Better pros: can be packed with braided PTFE (Gore-tex) for 'virtually dripless' operation.
Cheap, even when using GFO packing.
Virtually Dripless Packing by Gore GFO
SHAFT SEAL .....
Cons: CANNOT be fixed/adjusted while underway. If it fails while underway, you usually must stop the leak by 'draconian methods' ... and that all stop the shaft seal from working thus you cant use the engine. Those that have 'auxiliary' water cooling need to be 'occasionally burped' - installation of the high-speed seal requires careful 'precision'.
Pros: dry bilge.