The paper provides the modern techniques to analyze and evaluate the shipboard vibrations. International Guidelines for the evaluation of vibration in merchant ships, ISO6954–1984 was fully revised and replaced by a new ISO6954:2000 in 2000. Since the criteria for the severity of vibrations in the new guideline are substantially different from old ones, special attention must be paid to the selection of the appropriate methods of data analysis. The real dada measured on the navigation bridge deck of a container carrier besides typical time series including sinusoid, beating and random vibration are analyzed by several methods. The digital filtering in time domain and the narrow band Fast Fourier Transform (FFT) in frequency domain are both employed. The effects of FFT block size, frequency resolution, antileakage windows and averaging methods on the evaluated vibration levels are investigated. Based on the results of the investigations, an adequate procedure, which complies with new ISO6954, is offered.
The vibration is now considered to be an important feature in the overall assessment of a ship''s qualities. The excessive shipboard vibration in an accommodation area may interfere with comfort and working efficiency of ship crews. It is essential that given time histories of vibration measurement can lead to the same conclusions as to the severity of vibration no matter who or which institute may analyze the data. The old ISO6954–1984 had been widely used over a number of years as a guideline for the evaluation of vibration in merchant ships, however, there were some confusions with respect to severity of vibrations because the term of Maximum Repetitive Value (MRV) in the old standard was not clearly defined. It was especially difficult to evaluate MRV for the random vibrations. Also the instantaneous revolution speed of propeller shaft is not constant in a strict sense.
