speed and heading of the ship and is of primary importance in the determination of the ship 

 seaworthiness and for adequacy of the design of subassemblies such as antennas, masts, 

 etc., under heavy weather operations. 



For simplicity we may say that the "smoothness" of a ship under normal operating 

 conditions in a State 3 sea or less, is determined by the forces generated and the structural 

 response of the ship and is associated with forced vibration. In like manner we may say 

 that the ship seaworthiness, or the adequacy of the design of its subassemblies, is deter- 

 mined by the forces generated and the structural response of the ship under the influence 

 of hea\y seas. 



These two areas of vibration are distinct, the first having a criterion of comfort, and 

 the second of stress level. The evaluation of these two types of vibration would therefore 

 require widely different considerations in the determination of levels of acceptability. This 

 program deals with the establishment of "Norms of Vibration for Various Classes of Ships" 

 when stimulated by type I — Forced Vibration. 



B. PURPOSE 



A long term program of study, under the joint sponsorship of the H-8 and the original 

 S-6 Panels, has as its objective the complete understanding and control of the forces and 

 structural response of a ship as influenced by forced vibration. It is the purpose of this pro- 

 gram to define the "Norms" of vibration, for the forced vibration observed under trial condi- 

 tions, by empirical methods. By such an approach it is expected that the following benefits 

 will accrue in a much shorter period of time: 



1. By the association of design details to the performance of a ship, improvements may 

 be made in new construction; 



2. A "yardstick" for the e\aluation of ship \'ibration characteristics would be made 

 available: 



3. A basis for a hull vibration specification would be formed, for the mutual ad\antage 

 of both builder and operator: 



4. A set of industry-approved standards could ultimately be developed and periodically 

 impro\ed when appropriate, to pro\ide ships of consistently improved characteristics. 



C. APPROACH 



The basis for the establishment of such a vibration criterion or set of norms would 

 fundamentally be the response of the ship girder as a free-free beam. As a secondary basis, 

 the amplification of the hull motion in any local structure would be considered. It would be 

 the ultimate objective to keep the vibration of the hull girder to a minimum as well as the 

 magnification of this motion in the local structures. 



18 



