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Efforts to reduce fuel consumption and to facilitate maintenance of two-stroke marine diesel engines lead to engines with higher stroke to bore ratio, smaller number of cylinders and reduced number of revolutions. Due to high power installed per cylinder, the excitation forces increase, while their frequencies reach the band of natural frequencies of engine and ship structure. Therefore, installation of supporting stays or moment compensator, depending on the excitation, sometimes may be necessary. This paper deals with the vibration response analysis in the test bed condition for a long stroke slow speed engine, utilizing the FE modeling. Vibration response is determined for external vertical and guide force H-moment excitations. A case of supported and unsupported engine, respectively, is considered. Different aspects of FE modelling are discussed (mass modelling, element type variation, mesh density, etc.). The eigenvalue problem is solved by Lanczos method, and forced vibration by the mode superposition method. Velocity amplitudes are evaluated by ISO standards and useful conclusions are drawn. The results confirm that it is preferable to predict dynamic characteristics of the engine at the test bed condition and to reduce vibration levels in the design stage if required.