Ming-Shun Chang 
spectrum is considered to be a function of wave frequency only 
with its direction arbitrarily specified. A commonly used represen- 
tation of ocean waves is the Pierson-Moskowitz spectrum eee 
spectrum is a special case of a form suggested earlier by Bretschnei- 
der [2], The International Towing Tank Conference (ITTC) in 1969 
[3] recommended a two-parameter idealized spectrum of the 
Bretschneider form whenever statistical information on the charac- 
teristic wave period and height was available and recommended the 
Pierson-Moskowitz spectrum whenever such information was not 
available. Because of the lack of confidence in wave period and height 
data, the Pierson-Moskowitz spectrum has been widely employed as 
the basis for evaluating ship performance in a seaway and in the de- 
sign of marine structures. 
For the study of long-term ship performance, Cummins [4] 
suggested the use of the two-parameter idealized spectrum of the 
Bretschneider form, This spectrum recommended by the ITTC has 
been recently applied to North Atlantic hindcast wave data [5], and 
the results obtained were considered to be less than completely satis- 
factory. These results showed that in the idealized spectra the wave 
energy was not properly distributed with respect to frequency. This 
could result in serious errors in the prediction of ship motions. The 
same conclusion is substantiated by more recent measurements of 
ocean wave spectra 
In modern ocean engineering, the need for knowledge of di- 
rectional wave spectra is especially important. Several techniques 
have been developed to determine the wave directions in the ocean. 
Examples are the stereo-photographic method developed at New York 
University gt the floating buoy method developed by Longuet - 
Higgins at the National Institute of Oceanography [8] and the array 
method suggested by Barber and Pierson =. Despite these efforts 
only a few measured [7] ae y ] directional-wave spectra for the ocean 
are available. Moreover, the accuracies of the measurements are 
not known. 
In an attempt to better assess the problem of determining di- 
rectional wave spectra, it was decided to measure and analyse wave 
data under controlled conditions. An experiment was conducted in the 
Naval Ship Research and Development Center's seakeeping basin to 
study the angular resolution associated with measured wave spectra. 
* References are listed on page 1347 
1332 
