194 



THEORY OF SEAKEEPING 



/Deck 



Deckv 



Wave Profile Indica+ors and Pressure Gages-Sec+ions D, E,F, 6 



Fig. 33 Locations of wave-profile indicators ( — ) and of pressure gages (®) along contours of ship sections (from Admiralty Ship 



Welding Committee, 1953) 



Press Gage L<t is in Well on Frame No.l€2^ 



Fig. 34 Locations of wave-profile indicators ( — ) and of pres- 

 sure gages (0) along contours of ship sections (from Admiralty 

 Ship Welding Committee, 1953) 



and rolling angles were not obtained. Of particular 

 interest are the charts reproduced here in Figs. 39 and 

 40. The first shows the relationship between the pitch- 

 ing period and the period of encounter. At the periods 

 of encounter up to the natural period (or possibly 

 slightly higher) a ship pitches with a frequency ap- 

 proximately ecjual to the natural one. At longer periods 

 of encounter it pitches with the freciuency of encounter. 

 Fig. 40 shows that rolling periods of a ship at sea are 

 concentrated around the natural rolling period of the 

 ship. 



Fig. 41 shows the relative ship and wave position as 

 reconstructed from records. The upper diagram shows 

 the ship in a following sea. The ship log entry recorded 

 the wind as having a strength of 7 to 8 on the Beaufort 

 scale. The observers (and the Admiralty forecast) 

 declared it to be 8 to 9. Waves were visuallj' observed 

 to be 35 ft high and 300 to 600 ft long. The record dem- 

 onstrates an apparent absence of phase lag and this is 

 typical at a low frequency of encounter. 



The lower diagram in Fig. 41 shows the ship in head 



