308 



THEORY OF SEAKEEPING 



Short-Crested Oblique Seas should be made in wide 

 tanks, a number of which is now becoming available. 

 The ship log studies of E. V. Lewis, Section 6, indicated 

 that high stresses occurred in light seas and in beam seas 

 although at a lower frequency than in high and head 

 seas. It is theoretically conceivable that critical 

 bending-moment conditions may occur in steep oblique 

 or beam short-crested wa^^es of too small a length to be 

 critical in head waves. 



The vertical and lateral bending moments should be 

 measured in these tests and/or strains should be re- 

 corded on both sides of the deck. In particular, the 

 models representing ships with lai-ge metacentric height 

 may develop severe rolling and in so doing generate 

 large unsymmetrical stresses (see end of Section 6). 

 These programs, therefore, must include variation of the 

 metacentric height. Realistic short-crested irregular 

 wave system should be used in these tests, and exaggera- 

 tion of natural sea conditions should be avoided. 



10 Model Experiments on Slamming of cargo-type 

 ships are needed. Previous experiments of Lum, E. V. 

 Lewis, Akita and Ochi, Section 5.1, clarified the general 

 pattern of events, but more complete and detailed in- 

 formation is needed for the correlation with theory, which 

 is necessary for the proper understanding of the slamming 

 process. In particular, it is important to record all 

 events simultaneously as functions of time, and to dis- 

 tinguish among approach conditions, the impact process 

 and subsequent vibration. It is suggested that experi- 

 ments be made in reasonably realistic irregular sea con- 

 ditions at practical values of a light ship draft. Most of 

 the experiments in the past were made in regular waves 

 and the models were made to slam by artificial means, 

 such as unusually steep regular waves or unusually 

 shallow draft and unusual ship forms. 



In the approach conditions, it is desired to evaluate the 

 relative ship-wave vertical velocity and the local wave 

 profile in the impact area. In the impact process, the 

 development of the wetted area and of the total hydro- 

 dynamic force with time should be recorded. This 

 process in cargo-type ships is extremely rapid and high- 

 speed recording is needed. The low-speed records, 

 such as are shown in Figs. 39 and 41, do not permit 

 sufficiently accurate evaluation of the force develop- 

 ment with time. The time at which a slam occurs can 

 be established by preliminary tests in reproducible ir- 

 regular wave pattern in a tf)wing tank. The short in- 

 terval of time of particular interest in a continuous wave 

 pattern can be covered in subsequent tests with high- 

 speed recording; i.e., open time scale. 



Open-scale recording also is needed for the history of 

 slam-caused free vibrations in order to evaluate the 

 effect of transients and to correlate with a theory. It is 

 emphasized that it is impossible to obtain the combined 

 hydrodynamic and structural response similarity for the 

 model to represent directly a full-size ship. Therefore 

 the history of model vibrations should be used for veri- 

 fication of the elastic-response theory and the latter 

 subsequently applied to full-size ships. 



In pressure measurements, a thorough investigation 

 must be made of the transducer and recording-system 

 frequency response. Also it should be remembered that, 

 according to Wagner's theory, the recorded pressure will 

 be a function of the pressure-gage area. 



11 Records of Slams of Ships at Sea should be 

 obtained. The observations reported by Warnsinck 

 and St. Denis, Section 5.3 and Fig. 40, can be taken as a 

 guide, but should be improved by instrumental recording 

 of wave profiles. The use of the more open time scale 

 is suggested in order to permit a more complete analysis 

 of the initial stages of the impact and of the subsequent 

 free vibrations. Strain-gage instrumentation should per- 

 mit tracing of the time history of shear and bending- 

 moment variation along the length of a ship. Tests in 

 waves at sea should be supplemented by investigation of 

 ship structural properties in calm water, including forced- 

 vibration tests and derivation of damping coefficients 

 in several vibration modes. These calibration tests .should 

 be made in a sufficiently deep water, since hydrodynamic 

 masses and damping are strongly affected by water 

 shallowness. 



12 Effort to Evaluate Water Pressure Experienced 

 in Slamming should be continued using theoretical 

 metlujds, model tests in towing tanks, and observations 

 at sea. The first two of these approaches may err in 

 stipulating unrealistically severe conditions. There- 

 fore, sea observations are particularlj^ valuable in 

 establishing pressures encountered in practical operating 

 conditions. 



It should be remembered that there is no uniciuely de- 

 fined figure for the magnitude of the water impact pres- 

 sure. It depends on ship-motion characteristics, details 

 of the bottom shape, and operating conditions of a ship, 

 Section 5.4. In model tests the size and frequency re- 

 sponse of a gage also affect recorded pressure. 



13 Efforts to Develop a Suitable Theory for Flat- 

 Bottom Impact are needed. Presently available Wag- 

 ner's theories fail in case of a flat bottom. 



14 Methods of Theoretical Analysis Should Be Sought 

 for the Impact of Surfaces of Very Small (Vanishing) 

 Deadrise. An approach different from Wagner's 

 expanding-plate theory should be sought in that the 

 evaluation of the hydrodj'namic mass may differ from 

 the one derived by analogy with a fully submerged 

 plate in a steady flow. The approach also may differ 

 from Wagner's spray-root theory in that the potential 

 theory is expected to fail locally as the result of large 

 velocity gradient in the spray-root region at a small 

 deadrise surface. This is expected to cause a reduction 

 of peak pressures given by potential theory. 



15 Impact of a Flat or Small Deadrise Surface on 

 the Rippled Water Surface Should be Investigated. 

 The impact of a body on smooth water surface hereto- 

 fore has been investigated in theoretical and model- 

 test approaches to the impact problem. It cori'esponds 

 to a ship slamming in a smooth-surfaced swell in the ab- 

 sence of any wind. While such a condition may occur 



