Shear Stability of Base Fluids for Environmentally Acceptable Lubricants for Stern Tube Application

Legislative restrictions exist on using mineral oils for loss lubrication of marine propellers. A key question is finding a functionally suitable and economically acceptable substitute for mineral lubricants. The answer will make it possible to fundamentally limit the impact of international shipping on the marine ecosystem. This work presents the results of an experimental study of four types of base fluids and the influence of seawater on their properties. The results show that base fluids based on vegetable oils have a high viscosity index, i.e., their viscosity changes slightly with temperature. However, at the same time, they contain a relatively large amount of free fatty acids, which promote corrosion (high total acid number). The viscosity of saturated and unsaturated synthetic esters is more temperature dependent. However, their total acid number is significantly lower, making them suitable candidates for these applications.