A Vortex Theory for the Maneuvering Ship 



problem which has defeated many people in the past, so that the outcome of the 

 author's work is awaited with interest. 



I imagine that few will quarrel with the general principle expressed in the 

 paper, that the motion of a ship or submarine depends on the past history of its 

 motion. However, because of the insuperable difficulties involved in any other 

 approach, the use of quasi-static derivatives has been widely accepted as a 

 suitable approximation, since they were first introduced by G. H. Bryan in 1911, 

 Professor W. J. Duncan later attempted to justify the use of quasi -static deriva- 

 tives, and concluded that for the kinds of motion occurring in stability investiga- 

 tions of aircraft flight, the use of constant derivatives was justified. However, 

 he admitted that the influence of the frequency parameter had been neglected, 

 apart from its consideration in the studies of flutter of control surfaces. 



It is not therefore surprising that in the submarine field, for which the the- 

 ories from the aircraft world were adapted, a quasi-static approach has been 

 used to consider motions well beyond the range of the small deviations for 

 which it was derived. However, one cannot ignore the not inconsiderable argu- 

 ment that in submerged body work good correlation has been achieved between 

 theory and practice. To this extent one can reasonably claim that the end has 

 justified the means. 



From this point of view, which is all-important to the practising naval 

 architect, the introduction of a considerably more complex representation of the 

 problem seems unnecessary. However, the case of a surface ship in a disturbed 

 sea is entirely different and there may here be greater justification for the 

 author's approach. 



Comparison of data obtained by rotating arm and planar motion mechanism 

 will undoubtedly help to throw light on this problem, and the I.T.T.C. Maneu- 

 verability Committee by sponsoring a series of international cooperative tests 

 using the "Mariner" Class form, will eventually obtain data which may help to 

 answer Admiral Brard's question. 



Finally, I must admit to some lingering doubts about the basic concept of 

 the planar motion mechanism. If Admiral Brard will permit, I would like to re- 

 phrase Question 1 on page 29: "Is it possible to deduce from the forces and 

 moments measured in a harmonic forced motion, the true forces and moments 

 experienced by a ship or submarine in a motion which is rarely harmonic?" 



REFERENCES 



1. G. H. Bryan, "Stability in Aviation," MacMillan, 1911. 



2. W. J. Duncan, "Some Notes on Aerodynamic Derivatives," R. &M. 2115 (1945). 



3. W. J. Duncan, "Control and Stability of Aircraft," C.U.P. (1952). 



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