c. Similar remarks apply to the Japanese report on the "Nissei Maru" [54]. 

 Although interesting data on power, oscillations and stresses have been collected, the 

 piece de resistance are model experiments designed to elucidate the full-scale work. 



Since statistical methods have not been used in all three cases so far discussed, 

 the latter must be considered obsolete from a methodological standpoint. 



d. Modern investigations in this sense are being conducted by the David Taylor 

 Model Basin and other agencies in this country.* 



e. A prototype for future full-scale research has been supplied by Cartwright 

 and Rydill by their recent paper "The Rolling and Pitching of a Ship at Sea" [40]: 

 For the first time a satisfactory correlation between seaway and ship motions, which 

 earlier investigators have aimed at, has been reached. The statistical aspect used is 

 somewhat different from that proposed by St. Denis and Pierson and by Woznessensky 

 insofar as it is based from the beginning on Fourier series — an approach, the validity 

 of which has been questioned by the authors quoted. 



Cartwright and Rydill show how some modes of ship motion can be related 

 to the seaway. Waves have been measured by a ship-borne wave recorded [56]; the 

 analysis is primarily based on an automatic Fourier analyser. To check theory, linear 

 equations of motion are used in a "simple" and "advanced" form. Both are based 

 on the Froude-Krylov hypothesis. Only roll and the coupled pitch and heave motions 

 are treated, for arbitrary heading angles. 



Numerous graphs show the comparison of results obtained by experiment and 

 calculation. The agreement is from reasonable to good, especially considering the fact 

 that basic parameters had to be estimated. 



The statistical results are thought by the authors to be just as important as the 

 spectral analysis. Almost any aspect of the motion of the ship can be dealt with in 

 statistical terms once the connection between the energy spectrum and the statistics 

 has been established. 



The determination of damping by auto-correlation following Tukey deserves 

 attention, and a proof is given by the authors that the auto-correlogram under special 

 conditions coincides with the extinction curve. 



6. Seaworthiness of the Hydrodynamic Class of Vessels 



So far we have considered displacement ships only, because of the overwhelming 

 importance of this class of vessels. However, from the viewpoint of future development, 

 this attitude is not correct. A consistent theory should be able to furnish information 

 on the inherent seaworthiness qualities of all existing classes of ships. We are still far 

 from this goal. 



While in the field of aircraft building there is a strong correlation between 

 research and development, the hydrodynamic class of vessels — planing and hydrofoil 

 boats — has for a long time been developed widely by a trial and error procedure. This 

 applies especially to gliding craft; but even in the case of the hydrofoil boat, the suc- 

 cess of which depends to a large extent on its seaworthiness qualities, the rate of 

 investment of scientific work did not correspond to the importance of the task. 



We review briefly the status of our knowledge of the behavior of hydrofoil 

 systems in a seaway. 



Even accepting the most drastic simplifications as to hydrodynamic effects 

 involved (quasi-steady conditions) the problem is tedious because of the large number 

 of significant variations: wholly submerged and piercing hydrofoils, number of foils 

 and distribution of lift amongst them, the air gap between the bottom of the hull and 

 the free surface in planing conditions, elastic cushioning etc. Model experiments yielded 

 a decisive superiority of hydrofoil craft in a head sea over high-speed displacement and 



* After completion of this report an important paper has been presented by Jasper 

 before the SNAME [55]. 



78 



