Exam Question

Radial Lip Seals for Propulsion Shafting
(a) Outboard Seal Arrangement for an Oil-Lubricated Stern Tube
An outboard seal arrangement on an oil-lubricated stern tube serves two primary purposes: to prevent lubricating oil from leaking into the sea and to stop seawater from entering the stern tube. This arrangement typically consists of several elastomeric lip seals fitted in sequence. These seals are mounted within a casing attached to the aft end of the stern tube and press against a smooth, hardened metal liner fitted on the propeller shaft. A garter spring wrapped around the lip of each seal provides continuous radial force to maintain the seal's contact with the shaft liner. The seals are usually made from durable materials like nitrile rubber or fluoroelastomer (Viton) , chosen for their resistance to oil and temperature variations.
In a typical arrangement, multiple seal rings are used. The outermost seal acts as a seawater barrier , while the inner seals retain the lubricating oil. The outboard seals dissipate heat to the surrounding seawater, while the inner seals transfer heat to the lubricating oil through convection. The entire seal assembly includes key components such as a flange, cover rings, intermediate rings, and the shaft liner, all designed to work together to create a reliable barrier.