306 



THEORY OF SEAKEEPING 



mental difficulties in formulating a satisfactory ap- 

 proach, the difficulties are expected in the attainable 

 accuracy of evaluating characteristics of higher modes of 

 \-ibration. These may be important in the early stages 

 (jf the transient process. The.v also may be important 

 in defining stress upsurge at structural and mass discon- 

 tinuities. 



8.72 Experimental data on elastic response. The 

 experimental data on structural responses to impulses are 

 e\'idently needed. The organization of experiments and 

 the reporting of test data, should correspond to theoretical 

 outline given. The deflections and stresses must be re- 

 ported as functions of time as well as of the location 

 along the beam. Experiments with simple rods in a 

 stiuctural laboratory will serve to verify the simple beam 

 theory. Built-up ship-hke structures may be used in a 

 laboratory in order to verify the "artificial-beam" ap- 

 pioach suggested earlier. Observations on ships at sea 

 must include the installation of a series of strain gages 

 along a ship's length. Simultaneous records must be 

 taken so as to bring out the time history of all gage 

 readings. Care must be taken in locating the gages to 

 make them represent the general structure and to be free 

 from areas of structuial or mass discontinuities. The 

 effects of these latter must be investigated separately by 

 additional groups of gages. 



It has been mentioned freciuentiy that slamming has a 

 great effect on bending stresses in destroyers and only a 

 mild effect on cargo ships. It is suggested therefore that 

 sea research on elastic response to slamming be made on 

 destroyers or other ships designed for high speed. This 

 re.search will reciuire a more elaborate strain-gage in- 

 stallation than is common in the bending-moment re- 

 search. As much investigation as possible should be 

 done in harbor since the difficulties of mahitaining the 

 extensive equipment at sea were emphasized in the work 

 on the SS Ocean Vulcan. 



The strain-gage installation should be supplemented 

 by the optical deflection measurement similar to those 

 used by Kempf on the SS Hamburg and Schnadel on the 

 jMS San Francisco. However, a continuous optical 

 recording must be de\'ised to investigate the time de- 

 pendence of deflections and to permit the harmonic 

 analyses of these. 



It is conceivable that the entire research project on a 

 ship at sea can be based on the optical method of de- 

 flection measurement. The extreme difficulty and 

 tediousness of an extensive strain-gage installation 

 thereby can be avoided and onlj' a few strain gages m 

 accessible locations need be used. The optical eciuip- 

 ment can be of semi-portable type reriuiring a minimum 

 modification of a ship's structure. 



8.8 Statistical Studies. The statistical studies of 

 ship stresses in relation to the ambient weather were 

 illustrated in Section 6 by an abstract of data obtained 

 on two C-3 cargo ships. The work of this kind should 

 be continued but more attention should be given to 

 reporting weather conditions. It is suggested that a 

 complete copy of a ship's weather log be included in the 



reports on stresses. The author does not consider the 

 wave forecasts or hindcasts on the basis of meteorological 

 data to be sufficiently accurate for judging the relation- 

 ships to ship stresses. A weak part of the weather 

 descriptions, as it is used at present, is the lack of clear 

 tlistinction between swell and wind sea. It is very 

 difficult to make this distinction visually even in moder- 

 ate weather and impossible to do so in a wind of Beau- 

 fort 6 and above. The value of stress measiu'ements, 

 such as were made on C-3 ships, would be enhanced if 

 ships were ecjuipped with shipborne wa^'e recorders. 

 From the analj^sis of wave records the presence of a 

 swell can be detected. On large ships, however, the 

 a\'ailable wave recorders (Tucker's) do not give satis- 

 factory records of moderate wind sea. This is caused 

 bj^ the depth of the gage installation and the resultant 

 attenuation of the effects of small waves. 



E. V. Lewis' (19o7c) report on stresses on two C-3 

 type ships indicated a frequent occurrence of high 

 stresses in moderate wind conditions. It is almost cer- 

 tain that in such conditions the stresses are caused 

 prunarily by swells. A simple method of photographing 

 the radar screen (Roll, 3-1952) gives a valuable quanti- 

 tati\-e information regarding the direction and wave 

 lengths of the swells. It is recommended that the 

 simple equipment needed for this purpose and the routine 

 operating procedures were de\-eloped for making these 

 recordings in connection with the measurement of ship 

 stresses. The swell heights can be determined if a 

 shipborne wave recorder also is available. 



The research in evaluating the maximum bending 

 moments, expected at sea, must continue. It should 

 not be limited, however, to pure statistical extrapolation 

 of existing stress measurements. Instead a rational 

 method must be de\'eloped in which the physical sea 

 conditions and ship dynamics are taken into account. 

 This was discussed in greater detail in Section 7.1. 



8.9 Condensed Summary. In the research aimed at 

 defining the bending stresses acting on ships at sea, two 

 broad domains particularly must be emphasized. These 

 are : 



1 The evaluation of wave conditions at sea in terms of 

 directional spectra. 



2 Evaluation of the elastic response of a ship to a 

 slamming impact. This appears to be particularly im- 

 portant for ship forms designed for high .«peed. 



In these two domains the available knowledge is in- 

 sufficient and the imcertainties of stress estimates are 

 exce.ssively large. Immediately next in importance is: 



3 Evaluation of ship motions with seven degrees of 

 freedom (including rudder motions) and resultant ship 

 stresses. These motions and stresses occur in oblique 

 irregular waves. 



9 Condensed List of Suggested Research Topics 



1 Bending Moment Analysis of se\-eral ships is .sug- 

 gested using equations of motion with variable coef- 

 ficients, Section 2.2. The linearized theory (using con- 



