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The need of fast ships with more powerful and efficient engines at lower speeds, coupled directly to the propeller, gives an impulse to develop long stroke, high m.i.p., low speed two stroke Diesel engines. This trend challenges the researchers to find optimum solutions regarding mechanical, thermodynamic and emmisions aspects. An important direction of research is the shafting and structures vibration. As the engine torque increased constantly due to higher bore, stroke and m.i.p., the excitation and the supplementary stresses due to vibrations showed the same trend. For a given mechanical configuration the best way to reduce the supplementary shaft stress is to reduce at acceptable limit the excitation, or the amplitude of the defined harmonic component responsible for the dangerous resonance.