Pierson 



DISCUSSION 



A. Silver leaf 



National Physical Laboratory 



Teddington, England 



Professor Pierson' s application of mathematical technique to sea state 

 studies has long been of the greatest value to those of us in Britain concerned 

 with the performance of ships in waves. I am sure that most of us will agree 

 that the two-dimensional or unidirectional wave spectrum is a "fundamental 

 building block" which will aid further developments. However, in Britain we do 

 not all agree with Professor Pierson's suggestion that the formula in (5) is the 

 best for naval architecture purposes at the present time. An independent anal- 

 ysis by Scott (Ref . A) on behalf of the British Towing Tank Panel suggests that 

 it is not the best or even the most appropriate fit to the Moskowitz data. For 

 example, Professor Pierson's relation between the frequency of the spectrum 

 peak f^ and the average wave period Ty is 



I 

 f„ = 0.77/Tv 



while that recommended by the B.T.T.P. is 



f„ = 0.501/Tv + 1.43/T 



2 

 V ' ^--TO/ ly 



Consequently, an alternative formula has been proposed for use by the British 

 towing tanks which are now carrying out experiments on models in irregular 

 waves much more frequently than in the past, so that it has become urgently 

 necessary to formulate a standard of sea spectra for such experiments. 



In his introduction Professor Pierson mentions three possible types of pre- 

 diction of seakeeping performance. I suggest that only the first of these repre- 

 sents the purpose of seakeeping research from the point of view of the ship de- 

 signer and operator, who is primarily interested in knowing whether or not ship 

 A will perform better than ship B for a particular purpose on a specified route. 

 Professor Pierson suggests that the data necessary to make this type of predic- 

 tion can be obtained from historical records and some current work in Britain 

 is being devoted to just this approach. Statistical information about wind and 

 wave conditions in the principal areas where ships operate is being analysed 

 and processed using data collected from voluntary observing ships and recorded 

 on punched cards at the Meteorological Office. At present data from 125 Mars- 

 den squares have been grouped into 52 areas defining most of the principal 

 shipping routes to give a detailed account of the likely sea conditions during all 

 seasons of the year. A first report on this scheme has recently been issued 

 (Ref. B) and it is intended to publish a complete compendium on ocean wave 

 statistics within the next year or so. 



434 



