ABSTRACT

From this last affirmation we can deduce that this conversion coefficient, which is actually the Conversion Factor, is used with means to describe vibrational phenomenon that are not purely sinusoidal, that is signals whose amplitude is variable in time regardless their frequency remains constant like for example signals modulated in time. On board a ship, this modulation phenomenon can occur due to the fact that the propeller, being submersed in a sea condition with waves, perceives the variations of orbital speed that the

1 INTRODUCTION

1.1 Problem definition

The procedure for the validation of the global vibration levels of merchant ships proposed within the ISO 6954:1984 normative is done with reference to the Maximum Repetitive Amplitude (MRA) that can be obtained during the measurement procedures and not with the Root Mean Square (RMS) value obtainable from an analysis in the frequency domain as it is the normal practice in the mechanical vibration analysis. Within the standard no definition is given concerning the Maximum Repetitive Value (MRV) other than the self-explaining name, and no further information is given in the ISO 4867 or ISO 4868 standards where, as severity of vibration, reference is made to the peak value during stationary vibrations. This last affirmations might mislead the reader into the conclusion that a stationary phenomenon can be treated as a sum of purely sinusoidal signals, obtaining the peak value from an effective value multiplied by a factor of 2 . This last definition seems scarcely compatible with what

fluid experiments during the wave motion, and also because of the turbulences present in the wake flow, generated by the ship’s own surge motion, in which the propeller works. The consequence is that we have a continuous variation of the thrust and the amplitude of the pressure oscillations in the stern area on above the propeller.