Definition
A stern gland is the assembly that seals around a propeller shaft where it passes through the hull, either as a traditional stuffing box compressing packing material around the shaft, or as a dripless mechanical or lip seal.
Every shaft-drive boat has one, and it is the point where a rotating shaft passes through the hull — which makes it worth understanding regardless of how much of the work you do yourself.
Two designs, two habits
Traditional stuffing box: packing material compressed around the shaft. It is designed to drip slowly when running, and that drip is lubrication rather than a fault. Over-tightening to stop the drip generates heat and damages both packing and shaft.
Dripless seal: a mechanical face or lip seal that does not drip. These need their own attention — water supply where fitted, bellows condition and age, and a service life after which the bellows must be replaced regardless of appearance.
What to check
Drip rate when running and when stopped, heat after a run, hose clamps, bellows age on dripless types, and shaft condition at the seal. Alignment affects all of it — see shaft seals, bearings and alignment.