Applying Results of Seakeeping Research 



to the North East Coast Institution of Engineers and Shipbuilders,* gave the re- 

 sults of observations on two trawlers in rough seas. It is evident from Fig. 8 of 

 this paper that in rough seas pitching is the same in beam seas and in following 

 seas. Rolling of these trawlers in extreme seas is roughly the same in bow seas 

 and in beam seas, as is evident from Fig. 11 of the same paper. Altogether the 

 short- crestedness of extreme seas has in a certain way a smoothening effect on 

 stresses and on motions. These irregularities — and others — of random seas al- 

 low in certain circumstances to turn a cargo ship of 12,000 tons even in a sea 

 Hl/io = 35 ft of which some waves are as high as 50 ft. 



Finally a question. A speed of 12 knots of the fine 600 ft ship ahead in an 

 extreme sea is somewhat surprising. Has Prof. Lewis some information as to 

 what extent this 600 ft ship was able to maintain this speed in such a severe 

 sea? 



DISCUSSION 



G. J. Goodrich 



National Physical Laboratory 

 Teddington, England 



Prof. Lewis has, as usual, produced an extremely practical paper. It is 

 obvious to us all that research in seakeeping, to be of worth, must ultimately 

 produce design data for the improvement of ship performance. 



I would question the practical use of pitch and heave response operators in 

 regular waves, for such waves never exist. Uni- directional long-crested seas 

 are rare and even one node two dimensional spectra are few and far between. 

 The sea in general consists of multi-nodal spectra and predictions should ulti- 

 mately be made for such conditions. However, full scale sea data on such spec- 

 tra are almost non-existent and it is probably sufficient for the present time to 

 consider the one node two dimensional spectrum. 



There is no doubt that ship operators look to those of us working in the field 

 of seakeeping, for help in assessing new designs and we must look extremely 

 closely at what we consider to be the important features of a design which should 

 influence the choice of such a design. Prof. Lewis has suggested in the closing 

 paragraph of his paper some of the important criteria which should be consid- 

 ered. 



'■'Aertssen, Ferdinande and De Lembre: Service Performance and Sea Keeping 

 Trials on Two Conventional Trawlers, Trans. North East Coast Institution of 

 Engineers and Shipbuilders, Novennber 1964. 



211 



