The proposed method of an early diagnostics of the real time technical condition of the vessels’ steel shafts is based on the magnetic fields study of mechanical stresses by means of electromagnetic approaches. The method is a non-destructing testing. A propeller shaft at operation is experiencing different kinds of mechanical stresses of different directions (loadings). Non-uniform water flow (in rough wave, impact of stern lines of hull and steering system) resulting in torsional vibrations, buckling, and other perturbations leads to stress concentrations in critical areas of the shaft section. It results, firstly, in the appearance of micro-cracks and their extension, and further to their growth and an emergency situation.