ABSTRACT 



This report gives the frequency distributions of wave heights and wave 

 periods obtained from weather ships stationed in the North Atlantic, together 

 with an evaluation of the reliability of the visual wave-height estimates com- 

 prising the basic data from which the distributions are derived. Visual esti- 

 mates are compared with values determined from stereophotographs. An addi- 

 tional check is provided by wave-meter measurements. It is shown that a log- 

 normal distribution is applicable to the frequency distribution of wave heights 

 experienced over a typical year and that this distribution is a useful guide to 

 the determination of the incidence of a particular sea state at a given location. 



INTRODUCTION 



The David Taylor Model Basin is conducting a long-range research project 1 to evaluate 

 present methods of ship structural design and to suggest improvements based on a realistic 

 knowledge of the loads, stresses, and motions which ships experience in service. Instrumen- 

 tation has been developed which measures the responses of ships to wave action in terms of 

 stress, roll and pitch angle, and the corresponding accelerations. A large amount of data has 

 been collected during voyages in the North Atlantic of aircraft carriers, destroyers, destroyer 

 escorts, seaplane tenders, tankers, and dry-cargo ships. Typical of these studies is that con- 

 ducted on the USCGC UNIMAK. 2 ' 3 



Since the stresses and motions of ships are induced by wave action, these studies have 

 included, as an important component, the problem of defining the surface variation of the sea, 

 i.e., the waves. 



It is the purpose of this report to present the frequency distributions of wave heights 

 and wave periods obtained from weather ships stationed in the North Atlantic together with 

 an evaluation of the reliability of the visual wave-height estimates which comprise the basic 

 data. Observations of wave heights and wave periods over a period of about six years have 

 been made available by the U.S. Weather Bureau. In order to gain some idea of the reliability 

 of the visual estimates made by observers, such as the Weather Bureau personnel, a special 

 effort was made by the Taylor Model Basin to obtain stereophotographs of the sea surface at 

 the same time that visual estimates were made. Such comparative data were gathered during 

 extensive sea operations of the USS VALLEY FORGE (CVS 45) and the USCGC UNIMAK 

 (WAVP379) in 1955. A comparison of the visual estimates with values determined from the 

 stereograms is given in this report. An additional check on the validity of the Weather Bureau 

 data on wave heights is made by comparing them with wave measurements obtained by 

 Darbyshire 4,5 at Ocean Stations I and J. 



References are listed on page 51. 



