ABSTRACT 



Available observations of wave heights have been assembled and evaluated 

 in terms of statistical methods in connection with the study of the service strains 

 and motions experienced by ships at sea. Curves have been fitted to the distri- 

 bution patterns, and confidence bands, averages, and standard deviations have 

 been computed. Distribution patterns for wave heights observed in different parts 

 of the world are all of the same type with a peak displaced toward the lower wave 

 heights. Pitching motions measured on a ship at sea also follow this same pattern. 



INTRODUCTION 



The David Taylor Model Basin is making a study of the motions and strains in ships at 

 sea for the purpose of evaluating and improving methods for the design of the ship girder and 

 its structural components. It is probable that the frequency-distribution patterns of strains 

 and motions of ships at sea will be similar to those of wave heights. It is also expected 

 that the year-to-year variability in the distribution patterns of wave heights will be of the same 

 order of magnitude as the year-to-year variability in the distribution patterns of ship motions 

 and dynamic hull-girder stresses inasmuch as the latter are, to a large degree, functions of 

 the wave heights and wave lengths. To verify these expectations, observed wave heights 

 have been obtained from the Weather Bureau and the U.S. Hydrographic Office. These data 

 and data measured by the Model Basin on the USCGC CASCO have been studied to determine 

 (1) the type of distribution pattern, (2) the variation in this pattern over a period of time, and 

 (3) the mathematical function which will best fit these data. The results of the third phase of 

 the study are presented in this report. 



PRESENTATION OF DATA 



Figures 1, 2, and 3 are frequency distributions of wave heights, that is, depth from 

 crest to trough, obtained from various sources. These distributions are presented in the form 

 of bar-type graphs or histograms. The ordinates of these histograms give the percent of total 

 observations or measured values that fell between given limits of wave height as indicated by 

 the abscissa. 



OBSERVED WAVE HEIGHTS 



Weather Bureau Data 



Figure 1 shows yearly and combined wave-height data which were furnished by the U.S. 

 Weather Bureau at the request of the Taylor Model Basin. ^ These data represent wave-height 



References are listed on page 13. 



