282 



THEORY OF SEAKEEPING 



Ship Speed, Kno+s 



20 25 30 



2 3 4 5 6 7 6 



V^, F+/Sec 



Fig. 32 Bending-moment amplitudes in regular waves (from Lewis and Dalzell, 1957) 



additional judgment as to the severity of conditions can 

 be formed on the basis of instrumentally recorded ship 

 motions. 



Two .sea conditions wei'e met. The first consisted of a 

 long swell of an apparent dominant length ranging 

 between 250 and 350 ft and of significant height between 

 5 and 6 ft. (^n this a moderate local sea was superposed 

 of shorter wave length. The combination resulted in 

 occasional wave heights of some 10 ft. Assuming this 

 10 ft height to correspond to 10 per cent highest waves 

 and using Longuet-Higgins relation.ships, the average 



wave height can be estimated as 4.9 ft 



of the 



length of the intermediate of three ships. 



The second weather condition is described as fully 

 developed state 5 or (5 sea superposed over a swell. 

 Maximiun wave lengths and wa\-e heights were of the 

 order of (300 ft and 12 ft, respectively. If the 12-ft 

 height again was assumed to correspond to the 10 per 

 cent highest waves, the average height would be 5.9 ft 

 or 1 eo of the ship's length. This latter description would, 



howe\'er, exaggerate the wave steepness, since the lengths 

 of the largest waves were indicated to Ijc 1.7 times the 

 .ship's length. 



A series of maneu\'ers was carried out imder sea con- 

 dition I at a series of speeds up to 28 knots. Under 

 sea condition II the maneiu'ers were carried out at the 

 17-knot speed. ( )nly two runs at higher speed — between 

 17 and 25 knots were attempted in condition II. At 

 25 knots the trials were suspended liecause of exce.ssive 

 motion. 



The data on pitching of these destroyers are found in 

 Fig. 3-51 on page 205. The average of ^ lo highest pitch- 

 ing amplitudes of 3^2 to 4 degrees was observed on tests 

 of these destroyers at 17 knots in head and bow .seas. 

 A table given by Warnsinck and St. Denis (1957) shows 

 that, in a condition II head sea at 17 knots, slamming 

 occurred at an average rate of 6 per 10 min of run, or 

 roughlj' per 100 dominant waves. There was no slam- 

 ming in sea condition I. 



On the basis of the foregoing informati(jn, limiting 



