DETERMINISTIC EVALUATION OF 



MOTIONS OF MARINE CRAFT IN 



IRREGULAR SEAS 



John P. Breslin, Daniel Savitsky, and Stavros Tsakonas 



Stevens Institute of Technology 



Hoboken, New Jersey 



ABSTRACT 



The concepts of linear system analysis are applied to the coupled mo- 

 tion of marine craft to illustrate in greater detail than previously pro- 

 vided the procedures for obtaining their instantaneous response in 

 arbitrary irregular long-crested waves. The solutions of the coupled 

 equations of motion for heave and pitch in long-crested regular seas 

 are exannined to show how the ship- sea transfer function can be identi- 

 fied when the wave is regarded as the input rather than the actual 

 forces and moments. The theoretical expressions for the response to 

 arbitrary forcing functions are next examined and shown to involve the 

 inverse Fourier transform of the ship-sea transfer function and this is 

 identified as the system impulsive response function. This function is 

 convoluted with the given surface wave record to provide the instanta- 

 neous response. The characteristics of these impulsive response func- 

 tions are discussed in some detail and means for their determination 

 from theory and experiments are outlined. 



Application of the procedures are made to exhibit the high accuracy of 

 deterministically calculated motions derived from models of a de- 

 stroyer, underwater body, and hydrofoil craft. Results of calculation 

 of the bending moment of a surface ship model are also exhibited. It is 

 concluded that the method can be applied to all features of marine craft 

 responses attending irregular wave motion which satisfy the require- 

 ments of linear systems. 



INTRODUCTION 



Operation of marine vehicles in irregular seas is a problem of serious 

 concern to the naval architect. It is important that reliable analytical methods 

 be available to predict the motions, acceleration, degree of deck wetting, etc., 

 of these craft before they are constructed and put to sea. 



In 1953, two important papers on ship motions in irregular seas were pub- 

 lished. St. Denis and Pierson [1] considered the statistical aspects of ship 



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