LOADS ACTING ON A SHIP AND THE ELASTIC RESPONSE OF A SHIP 



285 



Fig. 35 Sample oscillograph record Model 1723, test lK,run 10, K,„ = 6.84 fps, Z,„/Z. = 1.25, >5i = L/48 



(from Lewis and Dalzell, 1958) 



Ochi (1956a, 6, 1957) made a very extensive series of 

 slamming experiments. However, in reporting on these, 

 he has neglected to show the dependence of various 

 events on time. This makes the data not suitable for 

 the detailed investigation of slamming phenomena. 

 Only over-all results, such as the maxima of forces and 

 the en\-elopes of pressures can be used for empirical 

 or semi-empirical studies to be discus.sed later in Section 

 5.4. 



The full-scale data are equally meager. On the MS 

 San Francisco and SS Ocean Vulcan the instrumentation 

 was not suitable for recording slamming and slamming 

 accelerations. The bottom pressures occurring in slam- 

 ming will be discussed in Section 5.4. 



5.2 Observations of USCGC Unimak. Greenspon 

 (3-1956) and Greenspon, Jasper, and Birmingham (3- 

 1956) have reported on the observations of slamming on 

 the USCGC Unimak which was operated at two weather 

 stations in the North Atlantic. Quoting from the 

 original papers: "The .ship operated in very heavy .seas, 

 and the Captain allowed the ship to undergo severe 

 slamming .so that records could be taken." Neither the 

 sea description nor the speed of the ship is given in the.se 

 papers.-- The .subject is narrowly limited to the con- 

 ditions at the bottom plating and the general effects 

 of slams on the entire ship are not discussed. However, 



" The present author was advised by one of the authors of 

 these papers that the wave data are available at the David Tajdor 

 Model Basin. 



the data on the bottom pressures are instructive. They 

 are particularly valuable for the purpo.se of the present 

 exposition because they permit demonstration of the 

 contrast with the behavior of a destroyer which will be 

 discussed in the next section. 



A coast guard cutter has finer lines than the cargo ship 

 discussed in the preceding .section. Nevertheless a .small 

 part of the bottom near the keel is relatively flat and 

 demonstrates the nature of the slamming impact on a 

 small deadrise surface. The locations of the pressure 

 gages and strain gages used on USCGC Unimak are 

 shown in Fig. 38 and a sample of a slamming record is 

 given in Fig. 39. The magnitudes of pressures will be 

 discussed in Section 5.4. At present, attention will be 

 called only to the conspicuous characteristics of the 

 slamming impact. Tlie.se are: 



1 Shortness of the pressure pulse — about 0.015 sec. 



2 Simultaneous pressure pulse at several gages spaced 

 along a longitudinal line 8.25 in. from the centerline; 

 i.e., gages 2, 3, and 4. 



3 Very low pressure at gage 6 which is located farther 

 from the keel. At this location the deadrise angle was 

 probably much larger than at gages 2, 3, and 4. 



4 A very .short interval of time, about O.OI sec, 

 needed for the pressure pulse to travel in an outboard 

 direction from gage 4 to gage 6. 



The shortness of the pressure impulse on a surface of a 

 small deadrise is an agreement with Wagner's theory 

 outlined in Section 2-7.1. 



