The paper outlines the details concerning historical perspective and recent developments to meet a requirement that lube oil and seawater leakage must be prevented under any circumstances. Using a compressed air chamber, the lube oil in the stern tube is completely separated from seawater by providing a controlled "buffer zone" between lip-type sealing rings. A constant quantity of compressed air supplied from within the ship passes through the air chamber and is spouted into the sea. An air control unit automatically detects any change of draft level and adjusts the pressures to maintain the optimum pressure on each sealing ring. The key mechanism to detect the draft change correctly and to adjust the pressure balance is explained. Specific design and project applications for the stern tube air seal on ocean-going and other marine vessels using line shaft propulsion and pod propulsion are explained.

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