Verhagen 



V. COMPARISON BETWEEN EXPERIMENTAL RESULTS AND 

 CALCULATIONS 



Some tentative calculations have been carried out amplifying 

 and illustrating the aforementioned suppositions. 



Mean Drifting Force 



The mean drifting force on the moored vessel in the long- 

 created irregular waves has been determined by a linear superposi- 

 tion of mean drifting forces in the regular wave components of the 

 known wave spectrum. As has been shown by Maruo [ l] a.o. the 

 principle of superposition can also be applied to determine the non- 

 linear mean drifting force. For head seas the mean longitudined 

 drifting force is 





In beam seas the nmean lateral drifting force is 



^ TPgC L ^ 



K = 2pgL \ ^'V- S^(w) dw (2) 



Formulas for the mean drifting force in regular waves on freely 

 floating, completely restrained or elastic moored bodies are 

 obtained by Maruo [2] and Newman [ 3] . Numerical calculations 

 are ilsually carried out on specific formulas for the drifting force 

 on a slender body. [ 3] As Is well known, the agreement of these 

 calculations with experiment Is on the whole not very satisfactory. 

 The slender body approximation results f. 1. In a vanishing nmean 

 lateral drifting force. Therefore an engineering approach Is pre- 

 ferred using available experimental data on a similar ship form. 

 The estimated curves of the longitudinal drifting force In regular 

 head waves at zero forward speed and the transverse force In beann 

 waves are given in Figs, 13 and 14, Using these data the expressions 

 (1) and (2) for the mean drifting force In Irregular waves can be 

 solved. The results compared with the experimental results are 

 shown In Table I, The agreemient Is reasonable. 



972 



