On modern ships, particularly small coastal tourist ships around 50 meters in length, ship owners often prefer stainless steel propeller shafts, lubricated by an emulsion of grease and seawater, with no aft sealing. The IACS unified requirement UR M68 prescribes the expression for evaluating shaft diameter based on service loading and tensile strength of the shaft material. However, it should be noted that the formula is only applicable to shafts made of carbon, carbon manganese, and alloyed steels. Stainless steel is excluded. The aim of the paper is to propose the extension of the UR M68 formula for propeller shaft dimensions to cover stainless steel shafts. The chosen dimensions are dependent upon the shaft line concept, especially the type of lubricant, material and predetermined shaft loading (torque or power at the relevant speed). These are currently the only available initial quantities for dimensioning the propeller shaft. Dimensioning propeller shafts, including stainless steel, is a crucial aspect in the development of marine propulsion shafting systems. The presented approach is based upon the fact that the expected failure mechanism of the shaft is fatigue due to cycling loading. Additionally, it highlights and implements the relationship between fatigue strength and the tensile strength of the shaft material.