The Influence of Freeboard on Wetness 



One question arises regarding ship speeds in the calculations. It would be 

 of interest to know what speeds were assumed for each ship and each sea spec- 

 trum, since the wetness certainly depends on speed. 



DISCUSSION 



R. F. Lofft 



Admiralty Experimental Works 

 Gasport, England 



As one who was concerned with Newton's original paper on wetness, I am 

 pleased to see this work being developed and extended in Goodrich's paper. 

 Both papers point to the need for more wave data, and the need for caution in 

 interpreting results based on present sparse data. 



In Newton's paper, the wave information was taken from Darbyshire's 

 tables of frequency of occurrence of waves of given length and height, published 

 in 1955. These were the dominant waves, and shorter or longer waves which 

 were present simultaneously were ignored. This may account for some empha- 

 sis on waves around 500-700 ft long, and so to an imderestimate of wetness of 

 smaller ships, in particular. 



On the other hand, the Darbyshire spectra on which Goodrich's work is 

 based, relates specifically to local wind-generated seas, and excludes swell 

 waves, which may affect larger ships. This paper therefore may give a some- 

 what optimistic picture of the wetness of the longer ships, as in Fig. 11. Clearly 

 we cannot obtain reliable estimates of wetness until more complete and reliable 

 data are available on sea spectra and their frequency of occurrence. 



It should be pointed out that the "wetness" derived by Goodrich corresponds 

 approximately to the very wet condition as defined by Newton. It is not uncom- 

 mon for ships to be under spray, i.e., Newton's wet condition, without the bow 

 becoming immersed. 



Finally, plottings of the form of Figs. 9-11 are piu-ely comparative. It 

 means nothing to the mariner, or to the designer, to be told that a particular 

 ship has a probability of wetness of 0.01%. Studies of this nature must be asso- 

 ciated closely with sea experience to be meaningful. If plottings of this type 

 were prepared for existing ships of known good or bad reputations for wetness, 

 as advocated by Newton, then perhaps an equivalence could be established be- 

 tween the estimated probability of wetness and a degree of wetness which could 

 be regarded as acceptable in practice. 



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