THE DRIFTING FORCE ON A FLOATING BODY 

 IN IRREGULAR WAVES 



J. H, G. Verhagen 



Netherlands Ship Model Basin 

 The Netherlands 



I. INTRODUCTION 



A floating body In waves experiences a hydrodynamlc pressure 

 force which is exerted by the surrounding fluid. Several factors 

 contribute to this wave pressure. One of them is undoubtedly the 

 conventional unsteady exciting force, which makes the body oscillate 

 at frequencies in the region comprising the bulk of the energy con- 

 taining waves* 



Another factor originates from higher order forces due to 

 various non-linear effects. In general these non-linear effects are 

 too small to influence the high-frequency motions of the body. They 

 can, however, be of importance in that part of the frequency domain 

 in which the wave energy is very small, i.e. in the low- frequency 

 range, in particular if one of the naturzil frequencies of oscillation 

 of the body lies in that range. 



In the limiting case -- zero frequency -- one arrives at the 

 well-known drifting force. 



It will be obvious , to assume that the force on a floating body 

 in irregular waves comprises not only a steady part but also a slowly 

 varying part, slow in comparison with the mean period of the wave 

 spectrum. The steady as well as the slowly varying part of the wave 

 force, both of which are proportional to the square of the wave 

 height, are denoted by "drifting force." The present paper is con- 

 cerned with the slow drift oscillation of a moored vessel In Irregular 

 waves. It is based on general observations revealed by an extensive 

 test program on the behavior of moored bodies In a seaway. 



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