I. CODE FOR SHIPBOARD HULL VIBRATION MEASUREMENTS 



OBJECTIVE 



The objective of this code is to establish standard procedures 

 for gathering and interpreting data on hull vibration in single screw 

 commercial ships. These data are needed for comparison of the 

 vibration characteristics of different ships, for the establishment of 

 vibration reference levels for this type of ship, and to provide a basis 

 for the improvement of individual ships . 



SCOPE 



This procedure is concerned only with the forced vibration 

 excited by the ship's propulsion system, and is therefore restricted 

 to vibration associated with shaft frequency, propeller blade frequency, 

 harmonics of propeller blade frequency, and with frequencies associ- 

 ated with the major components of machinery, such as engine rpm. 



FACTORS AFFECTING TEST REQUIREMENTS 



In general, comparative data can best be obtained under the 

 more uniform conditions prevailing during ship trials with known ballast 

 loading. 



Since the relatively uniform vibrations resulting from propul- 

 sion machinery excitation can be masked or distorted by transient 

 vibrations resulting from wave impact or slamming, it is important 

 that vibration testing be conducted in fairly quiet water. Changes in 

 wake distribution due to rudder angle and yaw can produce large in- 

 creases in exciting forces. Their effect must be determined during 

 controlled maneuvers and minimized during free -route measurements. 

 Propeller einergence, whether periodic, as a result of wave action, or 

 continuous, causes large increases in exciting force and should be 

 avoided during test. 



Operation in shallow water has a significant effect on hull 

 vibration. Experience indicates that a depth of at least 5 times ship 

 draft should be maintained during trials, to insure that reasonably 

 accurate data are obtained. 



The principal response of the ship's hull to the periodic forces 

 of its propulsion system is similar to that of a free -free beam. Since 

 the extreme stern is an antinode for all bending modes of vibration for 

 this beam, and since the mode shapes for bending vibration are similar 

 for commercial single -screw ships, the stern is an appropriate refer- 

 ence point for measurement of beam-like vibration. 



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