Mobility of the diesel engine base of a RO-RO ship

The mobilities of the engine base of a RO-RO ship were calculated with two FE models in frequency ranges 0 - 100 Hz and 100 - 200 Hz. The mobilities of the same engine base with the engines mounted have been measured on the ship and the diesel engine has been measured alone. Also the suitability of FE programs and the traditional calculation method was of interest in this study. In principle the mode superposition method should have worked well and with minimum manual work. However, at every step during the calculation starting from the modelling of the geometry the automatic functions failed due to the large size of the model causing much more manual work than expected. This makes it in practice impossible to use FE programs with the kind of limitations as I-Deas to calculate the mobilities up to required high frequencies. On the other hand the high natural mode density at high frequencies makes the use of the traditional mode superposition method questionable. It can be estimated that in the CPU time, which was used to calculate the natural modes, about 100 direct solutions could have been calculated. Thus for this kind of calculations the feasibility of the direct solution method should be studied.