Simplex is a very common arrangement for oil lubricated stern tube bearings. Is is fitted to both inner and outer ends. The replaceable chrome liner prevents damage to the prop shaft which would be expensive to repair. A rope guard (not shown) bolted to the hull. It prevents material from being "wound" into the gap and damaging the seal. Rope cutters may be fitted with a fixed blade attached to the hull and a moving blade to the propeller. Oil pressure is fed to the area between the two opposite facing seals. This pressure is governed by the draught of the vessel and is often supplied via tanks situated at set heights. This pressure balances the sea water pressure on the seal and prevents sea water ingress, by opening the correct tank the pressure exerted by the oil is insufficient to cause oil to leakage out.
Stern tube seals with oil lubrication have tended to use rubber rings increasingly. Fluoric rubber (Viton) with additives has been shown to be more effective than nitrile butadiene rubber for seal rings
