Stern shaft sealing devices are essential for safeguarding the marine environment.

Currently, stern shaft sealing devices are mainly available in two types: oil seals and air seals. The working principle of oil-type sealing devices is relatively simple. Here, I will focus on the more advanced air-type stern shaft sealing device, which is particularly important for reducing environmental impact on the marine environment. Another notable feature is its ability to automatically adjust air pressure according to the vessel’s draft. For example, when the draft increases, the seawater pressure acting on the outside of the aft seal rises, which in turn increases the pressure in the compressed air sealing chamber. This pressure is then fed back to the oil circuit, maintaining a balanced pressure differential at all times and ensuring effective sealing.

Regarding performance, the friction pairs of the stern shaft sealing device (dynamic ring 20A and stationary ring 2A) are made of copper. The friction surfaces between the dynamic and stationary rings are paired in a “hard-on-hard” configuration, requiring high machining precision. At the same time, the clearance of the sealing surfaces is relatively large, making some lubricating oil leakage unavoidable during operation. According to current international standards for mechanical face seals, the allowable lubricating oil leakage rate is 1–4 liters per 24 hours.