be relied upon at the extreme ranges of the func- 

 tion." This forces us back, in merchant ship de- 

 sign at least, to the basis of long-range service 

 experience. 



This is not to deny or even belittle the other ap- 

 plications of this sort of study suggested by the 

 author. 



Mr. John Vasta, Member: The author has 

 demonstrated in a very able and convincing pres- 

 entation that ocean waves and ship response, 

 both as to motion and hull-girder stresses, fit into 

 the framework of a Rayleigh frequency distribu- 

 tion. This is a novel and worth-while contribu- 

 tion to our knowledge of the behavior of ships at 

 sea. One might logically ask now : Of what prac- 

 tical significance is this new information to the de- 

 signer? With today's emphasis on special pur- 

 pose ships, which impose exacting environmental 

 requirements, the determination of the motions 

 and structural deformations a ship is likely to ex- 

 perience in a particular seaway is of extreme im- 

 portance. The question however still remains: 

 Can we predict the ship response from known 

 theories during the design stage or do we have to 

 build the ship first and then send it out at sea to 

 observe its behavior? The author states that the 

 actual Rayleigh distribution applicable to a par- 

 ticular ship can best be obtained by appropriate 

 model tests. He does not consider that this can be 

 done by theory alone in its present form. Since 

 model tests are time-consuming this means that, 

 for planning purposes, adequate lead time must 

 be allotted in the early design stage to obtain the 

 optimum hull shape for ship motion without hav- 

 ing compromised the speed characteristics ad- 

 versely. 



From the hull-girder strength standpoint, Fig. 

 18 is particularly significant and most illuminat- 

 ing. It shows for example that for a destroyer 

 hull type a 10,000-psi stress range or less occurs 

 99.99 per cent of the time that the ship is at sea, 

 and conversely, a stress range of this magnitude 

 is exceeded but 0.01 per cent of the time. It is 

 realized that this stress range considers sea loads 

 alone. However, since the design stress range 

 for this type of hull is well over 30,000 psi does it 

 follow that the destroyer's structure reflects a 

 disproportionate conservatism, or does it mean 

 that the margin between 30,000 psi and 10,000 psi 

 stress range constitutes a judicious insurance 

 against those rare and extreme sea loads that 

 occur but 1/100 per cent of the time? Obviously 

 economies in structural design can only follow 

 after a realistic evaluation of the sea loads is made. 

 Does the author feel that our present hull struc- 

 tures are designed too conservatively? 



An important feature in the paper is Fig. 22 

 which illustrates a rule for estimating the "cumu- 

 lative damage" factor for any typical structural 

 component once the fluctuating stress pattern for 

 the component is known. The life expectancy of 

 5.4 years has been worked out for a hypothetical 

 case. It would have been much more significant 

 if the example could have reflected a realistic case. 

 The choice of the numbers for the purpose of this 

 illustration is somewhat unfortunate for it creates 

 the feeling that the useful life of ships at sea is 

 relatively short, whereas the records indicate 

 there have been many successful ships that have 

 plied the oceans for 25 and 30 years without hav- 

 ing had to worry about their endurance limits. 



The author's contention regarding life expect- 

 ancy of a ship with endurance limit as the con- 

 trolling factor is misleading. It is general knowl- 

 edge that ships have broken in two under stiU- 

 water conditions where the question of fatigue of 

 metal was not involved. Moreover it has been 

 established that by using a steel with a lower 

 transition temperature a major improvement in 

 life expectancy results even though endurance 

 limit and stress levels are not affected. Thus, al- 

 though the author is careful to acknowledge the 

 "variability of the endurance strength of nomi- 

 nally indentical structures," it is the writer's 

 opinion that the variability is of much more prac- 

 tical significance than the endurance-limit calcula- 

 tion. 



In discussing prediction of life expectancy of 

 structures, the author suggests that the quasi- 

 steady stresses, such as dead-load stresses, tem- 

 perature-induced stresses, and built-in stresses 

 must be estimated, and added to the sea-load 

 stresses in order to reach a more realistic approach 

 to the ship structural design. This suggested 

 course should be approached with considerable 

 caution. The writer contends that before new 

 design procedures are instituted, one must balance 

 the additional expenditure of efforts dictated by 

 the new approach with the returns that he expects 

 to obtain. In the final analysis the most effective 

 design procedure is that one which is simple and 

 direct to apply, and which in addition has had the 

 benefits of accumulated satisfactory sea-service 

 experience. 



One can speak with ardor about combining 

 several load factors such as thermal stresses, weld- 

 ing residual stresses, and so on, to the sea-induced 

 stresses. The truth of the matter, however, is 

 that if these factors were considered in today's de- 

 sign procedures the resulting hull structure would 

 be much too heavy, and most likely economically 

 unacceptable. The meticulous designer might 

 welcome an elegant rational design approach, but 



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