sibly this elastic influence may not be too large if ship bottoms have a reasonable rise 

 of floor. 



Apparently without knowing Wagner's work, Bagnold has developed a physically 

 consistent qualitative theory of wave impacts on hydraulic structures, especially moles. 

 He assumes that air cushions can be sealed off at vertical walls by breaking waves. 

 These cushions may collapse. Such effects suffice to explain qualitatively the occurrence 

 of large impacts as has been demonstrated by model experiments. There may be some 

 parallelism with Japanese work hinted at by Akita. 



By accepting such a mechanism it is possible to cope with boundary conditions 

 at a wall in a manner which is at least physically consistent, although the present 

 writer does not see unsurmountable difficulties in assuming the rise of large impacts 

 due to shallow water wave momentum by direct water action only. 



5. Full-Scale Experiments and Trials 



We distinguish between 1) collection of data on ship behavior on board a ship 

 without ample instrumentation, and 2) full-scale experimenting in the proper sense, 

 which is performed by competent staffs using adequate instruments, and which, at 

 least occasionally, can rely upon special trials. 



We list some purposes of full-scale experimenting : 



a. Collection of statistical data on the seaway, on ship motions, accelerations, 

 impacts, speed including propeller and engine performance, pressures, stresses etc. 



b. Determination of limiting values of some items mentioned which can be 

 admitted from the point of view of safety and seakindliness. For example, the estab- 

 lishment of upper limits of permissible acceleration has to rely almost exclusively on 

 full-scale experience at least at present where facilities similar to those used in flight 

 research are not available. 



c. Exploration of effects so far unknown caused by the seaway. 



d. Check of results obtained by analysis and model research under actual con- 

 ditions; investigation of scale effects. 



e. Acquaintance of scientific staffs with actual conditions and ship operation at 

 sea. 



As an example of the first approach we mention a rather wide enterprise organ- 

 ized by the Hamburg Tank before the last war. Results obtained contributed some 

 general information which to a certain extent was deduced earlier from the simple 

 linear differential equations. An attempt was made to classify ship behavior as a 

 function of a vertical prismatic coefficient which agrees with our present ideas on 

 damping properties of hulls. 



a. Earlier, full-scale experiments on the M. S. "San Francisco" became famous. 

 The ship was adequately staffed and instrumented. However, because of lack of 

 experience, no thorough theoretical and experimental preliminary work was accom- 

 plished in the field of hydrodynamics before starting the expedition. Nonetheless, the 

 trip was successful, mainly for two reasons: 



1. the extreme luck that, close to the end of the voyage, exceptionally heavy 

 seaways were encountered. 



2. the outstanding ability of one of the leaders of the expedition, in interpreting 

 experimental data obtained in the field of strength of the ship. 



b. Such luck was absent in the case of the "Ocean Vulcan". Although investi- 

 gations extended over a long time, the ship was lavishly instrumented, the scope 

 of data obtained especially with respect to pressures is ample, and its evaluation is 

 exemplary — no revolutionary information resulted from these thorough tests. Prob- 

 ably the comprehensive Report 8 [50] is being more frequently consulted because of 

 its beautiful diagrams based on rather exhaustive routine computations than because 

 of the experimental material collected. Further, the author's attempts to consider the 

 influence of non-linear effects deserve attention. 



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