Today’s difficulties in ship vibration prediction and a possible solution 1

High installed power and increased R.P.M. of today’s ships are the very cause of an increased vibration excitation, an excitation larger than that of regular ships designed some ten years ago. Also the larger dimensions of today’s ships result in an increased vibration amplitude. At this very moment an international evaluation of ship vibrations is being prepared by the I.S.O. and a proposal for international adaptation has been issued recently (Figure 1). In this proposal the acceptable maximum vibration levels are indicated, and for ships to be built this proposed norm is already nowadays a basis for the contracts between the shipbuhder and shipowner. Under these conditions it is necessary to have a reliable prediction technique available in order to analyze the expected ship vibration level in the design stage. A state of the art [1] of vibration prediction techniques shows that a reliable method for the vibration analysis is not yet well possible. In the various checkpoints, available for the ship designer, to check his design against vibration acceptance, the mass-elastic response of the hull girder is not incorporated (Figure 2).