Stern tube bearing

The stern-tube bearing is the aft bearing of the propeller shaft, carried in the stern tube where the shaft passes through the hull, taking the weight of the overhung propeller and the bending it imposes. Two types dominate: oil-lubricated whitemetal bearings sealed by lip seals at each end, and water-lubricated bearings lined with composite polymer or, historically, lignum vitae, open to seawater. The aft bearing carries the highest load because the propeller hangs outboard of it, so its lower half wears most and alignment is checked by measuring shaft drop. A failed seal lets seawater into an oil bearing, raising corrosion-fatigue risk in the tail shaft, or oil pollution outboard.