200 



THEORY OF SEAKEEPING 



Direction of Wave 

 relative +o Ship 



Position of Film Frame No.302 



Positions of Ship are shown at Approximately 4 Seconds Intervals 

 C Vertical Scale is 3.4 Times Horizontal Scale) 



Analysis of Wave Profiles fnom Large S+orm Waves recorded in 6.E.7 



Direction of Wave 

 relative to Ship 



Position 4 Seconds before Sla 



Position of Ship ^ Second before Slam (Film Frame No.550) 

 on of Bows 2 Second after Slam 

 Position 4 Seconds after Slam 



k 340'0'-^ 4* 350'0" >\ 



Period of Encounter Period of Encounter 



8.2 sees 8.2 sees 



(Vertical Scale is 3.4 Times Horizontal Scale) 



Fig. 41 Analyses of wave profiles (from Admiralty Ship Welding Committee, 1953) 



feature in that large speed and torque variation are 

 observed with oscillation periods of 1 5 to 20 sec and RPM 

 cycles as long as 60 sec. A similar periodicity is ob- 

 served in the helm angle. This results from the fact 

 that the celerity of 100-ft-long waves is very close to a 

 ship speed of 13.47 knots. This is a good illustration of 

 large variations in speed and propeller torcjue which can 

 be cau.sed by mild disturbances of long periods. 



Observations 69 to 82 in Table 9 are taken from the 

 rough-weather passage made iu predominantly head seas 

 during the ^^ancouver to Honolulu voyage. Observa- 

 tion No. 77 corresponds to Fig. 46. The report also 

 contained a photograph showing water being shipped 

 during this observation period. Other photographs 

 showed that slamming occurred and water was shipped 

 1 hr after observational period No. 74, and also before 

 No. 79. Data for observation No. 82 are included in 

 Table 9 because they correspond to the maximum re- 

 duction of engine power under storm conditions. 



Fig. 47 shows the obser\ed relationship between the 



ship's period of pitching oscillation and frequency of 

 encounter with waves. It confirms the data given in 

 Fig. 39 for the SS Ocean Vulcan in that, at short 

 periods of encounter, a ship pitches in its natural period 

 and, at long periods of encounter, it pitches with the 

 period of encounter. A ship rolls predominantly with 

 its natural period, regardless of the period of encounter. 

 This observation in irregular sea waves agrees with the 

 theoretical expectation for rolling of a ship with low 

 damping and a sharply peaked foi'm of the response 

 curve. 



In regular waves a ship model rolls most in beam seas 

 and does not roll at all in head or following seas. This 

 is shown by the curves in Fig. 48 which depict towing- 

 tank tests. An actual ship at sea rolls in response to 

 wave components of varying directions. Fig. 48 shows 

 that in this case a large amount of rolling was observed 

 in head and following seas. 



Almost all obseivers on ships at sea, whose work has 

 been reviewed in this monograph, have commented on 



