Distribution of Hydrodynamic Forces on a Shipmodel 



DISCUSSION 



E. V. Lewis 



Webb Institute of Naval Architecture 



Glen Cove, Long Island, New York 



This is a noteworthy paper in an important series by Professor Gerritsma 

 and his colleagues that is of vital importance to ship motion theory. This con- 

 tinuing work has been characterized by unerring choice of the right research 

 subjects and by extraordinary experimental skill. The results have served to 

 clarify the so-called "strip theory" of ship motion calculations and to provide 

 step by step confirmation of the different elements of the theory. Thus the tre- 

 mendous power of this comparatively simple approach to the problems of ship 

 motions is being reinforced and the value of the pioneering insight of Korvin- 

 Kroukovsky and others confirmed. 



It may not be generally realized that this type of experiment, in which 

 forces on seven different sections are measured, is of unusual difficulty, not 

 only because of the many simultaneous readings to be taken, but in the need for 

 accurate determination of in-phase and out-of -phase force components in spite 

 of extraneous noise. The authors have mastered this difficult problem. 



The particular value of the resulting research is in showing that when the 

 ship velocity terms are included, excellent predictions of the longitudinal dis- 

 tribution of damping forces are obtained. Furthermore, the nature of the cross- 

 coupling coefficients, E and e, has been clarified by the demonstration that they 

 should be equal at zero speed and differ only by the term ±Vm at forward speeds. 

 (Incidentally, m is not defined, but is apparently equal to -a.) 



Incidental features of the paper are simplifications in the coefficients, which 

 are not immediately obvious. It is mentioned that 



fx^V ^ dx = -2v[m'xdx , 



which makes the B coefficient, Eq. (13), much simpler than given in (1). Also 



x — - dx = xdm' = m' dx = -m (= a) , 



and therefore the e coefficient is also simplified [Eq. (13)]. Hence, the simple 

 relationship between e and E emerges in Eq. (16) and Fig. 13. 



It is hoped that this important work strengthening the strip theory approach 

 will be continued, including oscillation tests at zero speed and restrained tests 

 in waves. My congratulations to the authors for a beautiful piece of research. 



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