A response analysis was carried out by using modal superposition method. Modal damping factors determined through experimental modal analysis were used. Excitation and response points were the same which were used modal testing in dry dock. Fig. 12 shows the Frequency Response Function (FRF) comparison at point near the propeller bearing. Excitation force has been directed in the transversal direction and acceleration response has also measured and calculated in the transversal direction. The lowest peaks in the measured FRF indicate global natural modes of the ship hull. These peaks can not be seen in the calculated FRF, because the model contained only small part of the ship structure. Fig. 12 FRF comparison of measured (red line) and FE-analysis (green line) Azimuthing Thruster in Operation in Water Comparison of calculated and experimentally determined natural modes during operation in water is presented in Table 5. The 6th column presents differences between natural frequencies in air and in water calculated by FE-model. The corresponding differences for experimentally determined natural frequencies are shown in the last column. As can be seen, the calculated natural frequencies of lowest global modes in water corresponded reasonably well with the measured ones. Due to added mass of the surrounding water, natural frequencies of the lowest longitudinal and transversal modes were degreased in this case about 5 % and 14 %, respectively. The lowest rotational mode around steering axis is dominated during operation by stiffness of the hydraulic steering system. During modal testing in dry dock the steering mechanism was locked. Therefore the natural frequency of this mode in water is significantly smaller than in air. Calculated frequency of the lowest vertical bending of the rotor was about 6 % lower than experimentally determined one. Difference for transversal bending mode was even higher. Influence of the gyroscopic forces was not taken into account in this calculation. This can be one possible reason for the differences. It is possible that influence of the gyroscopic forces for the coupled shaft line bending and thruster translational modes is different than the previously presented studies with the shaft line model indicated. Because vertical bending modes of the shaft line are coupled with vertical translation of the thruster, there are also significant deformations of the ship bottom. Therefore it is important that the structural-acoustic coupling is used between ship bottom and water model. Natural modes were also calculated without the coupling between ship bottom and water. It was found that the frequency of the vertical bending mode of the shaft line was increased as much as 14 % compared to the analysis where the coupling between the ship hull bottom and water was taken into account. 15
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