RECENT PROGRESS TOWARD THE 



UNDERSTANDING AND PREDICTION 



OF SHIP MOTIONS 



T. Francis Ogilvie 



David Taylor Model Basin 



Washington, D.C. 



ABSTRACT 



Since the Symposium on the Behavior of Ships in a Seaway (Wageningen, 

 1957), many papers have been published on the theory of ship motions. 

 The present paper is a survey, collation, and evaluation of those con- 

 tributions which have led toward a rational theory for predicting ship 

 motions. 



During this period, evidence has accumulated which demonstrates the 

 validity of the superposition principle for ship motions in a seaway. 

 This concept was stated as hypothesis eleven years ago by St. Denis 

 and Pierson (and also sixty years ago by R. E. Froude); its validity 

 may now be considered as proven, beyond the fondest hopes of earlier 

 investigators. 



With this principle established, attention once again returns to the pre- 

 diction of motions in small- amplitude regular waves. The best prac- 

 tical approach to making ship motions predictions is probably still 

 through use of strip theory. However, the two-dimensions assumptions 

 of strip theory are so pervasive that the validity of the resulting anal- 

 ysis is always questionable except in the most routine problems. 



In the past decade, the concept of the thin ship has been extensively 

 applied to ship motions problems. Many elements in the complete pic- 

 ture have been developed on this basis, and in addition, thin-ship theory 

 has been highly systematized. This latter effort, involving the estab- 

 lishment of a rigorous development of the theory on a set of carefully 

 stated assumptions, has pointed up some basic shortcomings in applying 

 thin ship ideas to motions problems. 



Very recently, much attention has been devoted to developing a slender 

 ship theory for predicting motions. The motivation and basic ideas are 

 discussed; more thorough consideration will be found in other papers 

 at this symposium. 



