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



283 



40 



I \ I I \ t 



Wave Bending Konnent Minus Still Water Bending Moment 



L/48 Regular Waves 



15 20 25 30 



Ship Speed, Knots 



25 



_ 20 



10 o 



__ 5 o 



10-^ 



_ 15 



_ 20 



I 23456789 



Vm, Ft /Sec 



Fig. 3 3 Wave bending moment minus still-water bending moment; L/48 regular waves 



(from Lewis and Dalzell, 1957) 



conditions for a destroyer operation at the speed of 20 

 knots can be judged to consist of an a\'erage wave height 

 of ?65 of ship's length, average double amplitude of 

 pitching of 4 to 4^ 2 deg-" and the slamming at the rate 

 of 6 to 8 per 100 waves. 



In tests of Lewis and Dalzell, described in the preceding 

 section, more severe wave conditions were used. In 

 regular wave tests, shown in Fig. 32 the wave height was 

 3-48 of ship's length. In irregular waves, u.sed in Table 7, 

 the average wave height was ^39 of model's length; 

 i.e., the waves were twice as steep as limiting wa\-es in the 

 Szebehely-Warnsinck-St. Denis desti'oyer tests. There- 

 fore, the maximum practical speed under these condi- 

 tions must be well below 20 knots. 



The average pitching double-amplitude of 4 deg, 

 corresponding to the ship limiting condition, occurred in 

 model tests in regular /( = L/48 wa\-es at about a 10-knot 

 speed. The practically applicable range of Fig. 32 

 appears, therefore, to be limited to 10 knots (2 fps for the 

 model). The irregular sea used in model tests was 

 decidedly more severe. At the mean L/32 wave height, 

 the average pitching double amplitude of 5.22 deg was 

 already observed at zero speed. The practical appli- 

 cability of Fig. 36 and Table 7 appears, therefore, to be 

 limited to hove-to, or, say, the speed well l)elow 10 knots. 



™ Using LongueHliggins relationships for conversion from the 

 mean of 10 per cent highest waves on Figure 3-51 to the average 

 height. 



Several spectacular features, indicated in Sato's and in 

 Lewis' and Dalzell's tests at higher speeds, appear to 

 lie entirely outside the range of practical application. 

 None of the data of Figs. 34 and 36 in L/20 waves at 

 significant forward speeds is applicable to the practical 

 ship operations. Only the hove-to condition is practi- 

 cal. 



In order to represent the practical conditions of the 

 destroyer operation, much lighter sea conditions should 

 have been used at high speeds. On the other hand, the 

 most .severe storm conditions to be found at sea should 

 have been used in hove-to conditions and at very low 

 speed. 



5 Rapidly Applied Loads (Slamming, Pounding) 



The hydrodynamic loads to be considered in this 

 .section are applied with such a rapidity that static 

 methods of stress calculation are not applicable. It is 

 necessary to consider the elastic characteristics of a 

 ship. 



In the introduction to slamming loads in Section 2-7 

 Kent's definitions of slamming and pounding were 

 quoted. These definitions are, however, not univer- 

 sally accepted and more often both words are used 

 interchangeably to describe slamming. This was or- 

 dinarily understood to mean an impact of a ship's 

 bottom on water surface after previous emersion. ()c- 



