purposes, is that, aside from the fact that they are a particular class of seagoing 

 vehicles of importance, they are relatively simpler to analyze theoretically than a dis- 

 placement ship. The forces are relatively localized and simple experiments can also 

 be used to determine various coefficients in the motion equation. The techniques of 

 aircraft motion analyses are easily applied to this case, and these may now be extended, 

 in the broad sense, to the displacement ship problem. 



The field of ship mechanics, as Dr. Weinblum refers to it, should develop 

 along the lines of aircraft studies. The development of aircraft dynamics, from simple 

 stability analyses to response to arbitrary gusts, even those of random nature, should 

 be followed by the natural extension of ship maneuvering and directional stability 

 studies in smooth water to studies of similar problems in waves. The general equations 

 of ship motion can be easily constructed (in a rough, not too rigorous manner) and the 

 evaluation of the pertinent coefficients is then the main problem. One of the techniques 

 that may be used for this purpose is a relatively recent development in the aero- 

 nautical literature known as slender-body theory. This theory allows a greater fullness 

 to the cross-sectional area of the body and may be more realistic than the classical 

 Michell ship. Some work along these lines has been done by Dr. Cummins of DTMB 

 for the case of wave resistance in smooth water, and has been indicated in a general 

 way recently by Professor Fay of M.I.T., for the case of symmetric motions of a ship 

 in waves, i.e. heave and pitch in pure head and following seas. The main difference 

 between the aeronautical application of slender-body theory and application to the ship 

 motion problem lies in the boundary condition for the potential, i.e., the free surface 

 boundary condition, but alas, that is the core of the problem. These ideas should be 

 taken up soon, in order to extend our meager knowledge of ship motions in the 

 horizontal plane in oblique waves. More rigorous techniques may be developed in the 

 future, but a working tool is now necessary and may be obtained in this manner. 



I may have departed somewhat from my original discussion on hydrofoils, but 

 I think I have maintained my general philosophy of applying the ideas of aircraft motion 

 analysis to the seaworthiness problem. 



R. W. L. Gawn 



Prof. Weinblum's survey is admirable and comprehensive and his proposals 

 for further research are in my view essential in all respects. 



One or two aspects may be usefully emphasized. The simple theory of rolling, 

 now nearly a century old, has pointed the way in association with model experiments 

 to the best arrangement of bilge keels to damp the motion. Much greater damping 

 has been effected in recent years by activated fins. Theory played a full part in the 

 development of this device although the engineering aspects are a major feature. 



Increasing attention is being given to the development of the linear theory of 

 pitch and of heave and in association with model experiments this has confirmed that 

 the scope for increasing damping by passive fins or change of hull shape is very much 

 greater than formerly supposed. This is therefore a real contribution from theory. 



So far research both by theory and experiment has been largely pre-occupied 

 with regular seas but there is a natural urge to extend the development to complex 

 seas. The additional variables and complications involved are many and the resourses 

 of staff and facilities will be overstrained and useful results unduly protracted unless 

 the work is confined to essential channels. It is suggested to the Author that theory 

 could help by defining a wave system of a standard complexity which would serve for 

 most tests and so avoid, or at least considerably curtail, the multiplicity of experi- 

 ments that would otherwise be necessary in different seas of random complexity. 



The important contribution of adequate freeboard to the sea-keeping qualities 

 of a ship cannot be too strongly stressed and it was a pleasure to hear Prof. Wein- 

 blum's insistence on this. This is a matter on which theory has drawn a blank and 

 the designer must perforce rely on previous experience and model tests in order to 



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