Traditional and presently most common design of propeller shaft stern tube bearings is based on the application of white metal and the system of gravitational or forced lubrication of the bearings. An alternative arrangement involves polymer stern tube bearings offering significant advantages over the white metal ones. The advantages include a considerable reduction of the effective power loss resulting from heat generation in the bearings due to hydrodynamic friction of lubricants under identical operating conditions (bearing load and shaft speed), which results in fuel saving. Furthermore, polymer materials have good self-lubricating properties and lower friction coefficient than white metal; therefore they display improved performance in the area of mixed and dry fiction which occurs when starting and stopping the marine propulsion machinery. The application of seawater-lubricated polymer stern tube bearings eliminates the possibility of marine environment pollution which is not the case when using oil-lubricated white metal stern tube bearings. Finally, the use of polymer stern tube results in considerable financial saving during the ship's exploitation period. A comparative analysis of the application of conventional white metal stern tube sliding bearings and polymer stern tube sliding bearings has been made, with regard to operating conditions that are defined by the results of the calculation of the elastic shaft line and by measuring the responses of stern tube bearings under the most adverse conditions (entirely de-coupled shafting and non-immersed propeller in warm conditions) in three types of real ships. The analysis includes the three most common types of vessels in the world seaborne trade – bulk carrier, container ship and tanker – propelled by a slow-speed marine diesel engine. The white metal bearing operates in the hydrodynamic mode of lubrication. The Reynolds differential equation has been used to determine the properties of the lubricant film and the load capacity of the bearing. Approximating the solution to the equation has been carried out by means of a finite difference method. As opposed to white metal, the polymer bearing operates in the elasto-hydrodynamic mode of lubrication where the elastic deformation of the opposing surfaces considerably affects the minimum thickness of the generated lubricant film. The Hamrock and Dowson equation has been used for determining the lubrication film minimum thickness under operating conditions of the bearing in the mode of elasto-hydrodynamic lubrication. Relying on the theories for modelling hydrodynamic and elasto-hydrodynamic lubrication, special software has been developed for calculating the HD and EHD bearing operation mode and the operating flow of lubricant through the bearing. The method and the calculation models have been tested with regard to the data gathered in the above mentioned three types of ships in operation. The results show that the application of the designed software provides a high level of approximation accuracy by using a simple structure of the numerical model. Finally, a model for choosing the stern tube bearings has been made with regard to lifelong costs. A comparative analysis of economic effects has been performed, taking into consideration the application of polymer and white metal stern tube bearings over a period of twenty years of ship operation. The research findings have clearly revealed that the application of polymer stern tube bearings results in the ship's energy efficiency and significant savings when compared to the application of white metal bearings.