SHIP MOTIONS 



195 



-Mai'n Cable run 



-Icln Coble run - 



Positions of Electric Resistance Strain Gages 

 Fig. 35 Positions of electric resistance strain gages (from Admiralty Ship Welding Committee, 1953) 



seas. A wind of strength 6 is indicated. Waves were 

 observed to be 10 ft high and 200 ft long. The average 

 daily ship .speed was reduced to 5.5 knots. The ship's 

 motions appeared to correspond to synchronism, the 

 phase lag being about 90 deg and the highest angle of 

 pitch occurring when the wave crest was amidship. 

 Fig. 41 is particularly interesting in that it shows the 

 ship's position } 9 sec before and after a slam. 



The first digits in the designations in Fig. 41 are the 

 voyage numbers. The letter following designates an 

 eastward or westward crossing and the numl)ers after 

 the letters are the consecutive obserx-ation numl)ers 

 during each crossing. 



The SS Ocean Vulcan trials were not designed to in- 

 clude detailed measurements of slamming phenomena. 

 Certain general observations were made, however, on 

 the conditions tending to produce slamming. Slam- 

 ming was encountered only on the west-bound crossings 

 during which the ship was in a ballast condition with the 

 forward drafts ranging from 8-ft-'2 to 10-ft-l. Slam- 

 ming was experienced on ;51 days, or nearly one day in 

 every three in which the ship was in a ballast condition 

 on the open ocean. It is estimated that the ship ex- 



perienced between 2000 and 4000 slams during the 17- 

 month period of the trial.s. 



Quoting from Report Xo. 8: "The period of the hull 

 vibration set up by slamming was measured I)}' a stop 

 watch on several occasions. It was found to be constant 

 at 100 cycles per minute, which corresponds to the fre- 

 (luency of the two-node vertical viliration for the ballast 

 condition. The vibration following a heax-y slam could 

 still be clearly identihed l^y those on board 30 seconds 

 after impact." 



Report No. 8 stated that slamming caused a greater 

 increase in the sagging stresses amidship than in the 

 hogging stresses. This confirms the findings of Schnadel 

 (1936, 1937/38). Further discu.ssion of stresses will be 

 deferred to Chapter 5. It should be mentioned, how- 

 ever, that the relatively small increa,se in bending stress 

 as a result of slamming on the MS San Francisco and 

 SS Ocean Vulcan can be attributed to the low power and 

 low speed of these ships. Larger stress increases can be 

 expected in faster ships. -^ 



" See Section 5.Ui — VVarnsinck and St. Denis (1057 

 Jasper and Birmingham (1958). 



and also 



