288 



THEORY QF SEAKEEPING 



fundamen+ol period O"! plote|ob+ained 

 pla-ie and ol lowing it to vibrate freely* 



Fig. 39 Representative slamming record on USCGC Unimak (from Greenspon, Jasper, and Birming- 

 ham, 1957) 



a clear picture of the slamming process. The quantita- 

 tive evaluation of pressures is, however, uncertain. 

 Theoretically, the pressures over 1300 psi were com- 

 puted by Bledsoe (3-1956). Such pressures were not 

 experimentally observed. In Chapter 2, the present 

 author expressed an opinion that these high pressures 

 extending over a very narrow peak have little practical 

 significance, and only a mean pressure over an un- 

 supported bottom-plate area is significant. When 

 pressures are measured on ship models, the relatively 

 large size of a gage with respect to a model and the time- 

 response characteristics of the gage provide an averaging 

 action of an uncertain degree. The slamming pressures 

 were measured on two models of cargo ships by Ochi 

 (1956a, b, 1957). The body plan of the model u.sed is 

 shown in Fig. 2-37. For the U-form ship Ochi found 

 the slamming pressures to be six times the hydrostatic 

 pressure in smooth water. For the V-form ship the 

 slamming pressure was four times hydrostatic. Assum- 

 ing a draft of 28 ft, the pressures can be estimated, there- 

 fore, at 75 and 50 psi, respectively. Partly by theoretical 

 means and partly from consideration of damage to 

 ships' plating, Watanabe (1957) arrived at the pressure 7 

 times hydrostatic, or 87 psi in the foregoing example. 

 These rather mild pressures may be explained by the 

 fact that Ochi used gages of large diameter and experi- 



Table 9 Slamming Stresses. From Warnsinck and St. Denis. 

 1957 NSMB Symposium 



.Ship 



Q 



Note: It is pertinent to note that the highest values of slamming 

 stresses were not obtained with bow emergence. Both of the slams 

 giving rise to the highest slamming stresses occurred in bow, not 

 head, seas. 



mented only with regular wa\'es. It is well known that 

 slamming is more readily brought about by irregular 

 waves. Watanabe's work has reflected the conditions 

 actually met by cargo ships in service. It represents 

 therefore an appraisal of a "residual slamming" after 

 the more severe cases were eliminated by good seaman- 

 ship. 



