Stern Tube Lubricants

Stern tube lubricants primarily fall into two main types: mineral oil-based and Environmentally Acceptable Lubricants (EALs), Specially formulated marine lubricants offering superior resistance to water ingress and corrosion. They ensure long-lasting protection for stern tube bearings and seals in all marine environments. Environmentally acceptable grades are also available.

Mineral Oil: Derived from crude petroleum, typically turbine or engine oils. They have been the conventional choice due to cost and availability
Environmentally Acceptable Lubricants (EALs): These are used in applications with oil-to-water interfaces to meet environmental regulations (e.g., US VGP). They are typically synthetic esters or bio-based oils.
Emulsifying Lubricants: Special formulations designed to readily mix with seawater to form a stable, adhesive emulsion. This is crucial for systems like the Cedervall type, which expect some water ingress.

Viscosity: Proper viscosity is essential for maintaining a sufficient hydrodynamic film between the shaft and the bearing, even at low propeller shaft speeds and high loads. Different ISO grades (e.g., ISO 68, 100, 150) are available depending on manufacturer specifications and operating conditions.
Water Separation/Emulsification: Depending on the seal system design, the lubricant must either separate water effectively (circulating systems) or emulsify with it to maintain lubrication and prevent metal-to-metal contact and corrosion (Cedervall type and some emergency grades).
Rust and Corrosion Protection: The lubricant must provide excellent rust and corrosion protection to critical bearing surfaces, especially when exposed to seawater.
Load-Carrying Capacity and Anti-Wear: The oil film must be durable to handle high bearing pressures and prevent scoring or wear.
Oxidation and Hydrolytic Stability: The oil needs to resist chemical breakdown due to heat, water contamination (hydrolysis), and exposure to air (oxidation) to ensure a long operational life. Saturated synthetic esters generally offer better stability than unsaturated vegetable oils.
Seal Compatibility: Lubricants must be compatible with the specific seal materials (e.g., FKM rubber) used in the stern tube system to prevent seal degradation and leakage.