In general, the response of local structure can be evaluated 

 in terms of the ratio of its vibratory amplitude to the amplitude of 

 hull girder vibration at that point. 



Alternating thrust forces produced by the propeller may 

 cause dangerous thrust bearing or machinery vibration as a result 

 of longitudinal resonance in the propulsion system. This possibility 

 may be evaluated by determining amplitudes of longitudinal vibration 

 at the thrust bearing housing throughout the normal operating range 

 of RPM. 



The study of hull vibration is severely limited by the lack of 

 information on ship damping. The frequencies of the first few vertical 

 modes can usually be determined by an anchor -drop and snub test and 

 the damping constants can be readily obtained from the decay curves 

 of the oscillograph records. The mode number of the lowest frequency 

 measured in the free -route test can often be determined by comparison 

 with the anchor -drop and snub test. The oscillograph data can, if 

 forwarded to The Society of Naval Architects and Marine Engineers, 

 be used to supplement existing information on ship damping. 



TEST REQUIREMENTS 



a. The test should be conducted in a depth of water at least 

 five times the draft of the ship, and deeper if possible. 



b. The test should be conducted in sea state three or less. 



c. The ship shall be ballasted to a displacement within the 

 normal operating range, and to a draft aft which will insure at least 

 2 ft. immersion of the propeller in calm water. 



d. During the free -route portion of the test, rudder angle 

 shall be restricted to 2° port or starboard. 



e. Measurements should be taken at the following locations: 



(1) Stern - Vertical and athwartship measurements, as 

 close to the centerline and to the after perpendicular as possible. Use 

 of bitts, chocks or other fittings attached to deep frames or heavy 

 stiffeners will insure obtaining the motion of the hull girder. 



(2) Main thrust bearing foundation - Fore and aft meas- 

 urements, to determine the response of the shafting system to alter- 

 nating thrust forces. Vertical and athwartship measurements should 

 also be made to obtain local response of the thrust bearing foundation. 



(3) Other local sub -structures as desired - When evidence 

 of local resonance occurs, measurements should be made to form the 

 basis for determining the need for stiffening. 



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