196 



THEORY OF SEAKEEPING 



Table 7 Behavior of Fishing Trawlers at Sea (from Mbckel 



1953) 



Shiii 



A 

 B 

 C 



I) 



Shi|i 



A 

 B 

 C 

 D 



Maxi- 

 mum 

 angle, 

 deg 



20 (i 

 17 

 29 . 

 28,0 



Maxi- 

 mum 

 angle, 

 deg 



18 

 20 

 Ifi 

 20 



- — Maximum acceleration — . 



Rolling 

 2 25 ft/sec= (i87 ni/see= 

 l.i»2ft/see= 0.586 m/sec* 

 4 81 ft/sec2 1 470 m/sec^ 

 2,83 ft /sec = 8(32 m /see = 



. Maximum acceleration — . 



Pitching 



12 (i ft./sec2 3,80 m/sec- 



14 4 ft/sec2 4.38m/sec= 



12,lft/see= 3.08m/sec2 



!l,Oft/sec= 2.92m/sec- 



Crew's views 



Good 



Too hard pitch 

 Too violent roll 

 Very good 



o t3 



en u) 



C 3 



■-;; o 



20 



4 5 G 



Time Sees 



Fig. 36 Curves showing agreement between computed bend- 

 ing moments (from Admiralty Ship Welding Committee, 1953) 



5.14 W. Mbckel's observations on fishing trawlers. 



Cajjlaiii Walter Alockcl (1 '.).");>) made several x-oyajjes 

 in fishing trawlers and reported on the comparative per- 

 formance of four ships engaged in the same service in 

 the same waters. A particularlj- valuable part of his 

 report can be found in the correlation of the opinions of 

 the crews with the data on accelerations. The latter 

 were either measured or deduced from amplitudes and 

 periods. This information is reproduced in some detail 

 in Talile 7. 



A model of the most seakindly of the four ships — the 

 S8 Stralsund — was tested later in a towing tank (Xumata 

 and Lewis, 1957; Korvin-Kroukovsky and Jacobs, 

 19.57). The speed for synchronism with waves in head 

 seas was found to be low which is typical for full ships 

 of a high displacement-length ratio. However, the 

 trawling operations were conducted at a speed well be- 

 low synchronism with waves ecjual to the ship's length. 

 The high sjieed used in travehng to and from fishing areas 

 was well abo\-e synchronism; the ship was in a "super 

 critical" condition, as defined by E. Y. Lewis (19.j5). 

 The ship was thus in favorable circumstances during all 

 phases of its .service. 



Another outstanding feature of Mockel's (195.3) re- 

 port is the vi\id description of a near-foundering of a 

 trawler in a following sea. While traveling at high speed, 

 the trawler was flooded by the breaking of a high wave 

 over the stern. The flooding of the deck well was so 

 severe that the ship lost its freeboard and stability and, 

 for half an hour, was on the verge of capsizing. After 

 drastic reduction of speed, it continued traveling without 

 further difficulty. 



In looking for explanations of the phenomenon, one 

 should recollect the statement of Kent (1924) that break- 

 ing waves in the open ocean are very rare. With Moc- 

 kel's description and Kent's statement in mind, the 

 present author paid particular attention to these phe- 

 nomena during a few sea vo^'ages he had the opportunity 

 to make. During these voj-ages onlj' relatively small 

 breaking waves were observed at a distance from the 

 .ship, thus confirming Kent's statement. On the 

 other hand, ocean wa\'es were obser\'ed to break violently 

 after interfering with a ship's own waves and particu- 

 larly after encountering a ship's wake. An occurrence 

 similar to Alockel's was observed when traveling at 

 high speed in a fallowing storm sea on a modified Liberty- 

 type ship. A large wave broke over the stern, reaching 

 the height of the stern's superstructure (gun-mount 

 deck). It had the appearance of a large surf breaker. 

 The author believes, therefore, that in Mockel's case 

 two causes of the trouble were present; namely, exist- 

 ence of a large stern wave and the .slow rate at which the 

 ocean waves overtook the ship. A large stern wave can 

 be assumed to exist in the wake of a ship of the full lines 

 of a trawler. Conversely, the fa\-orable conditions re- 

 sulting from reduced speed may be attributed to two 

 causes; that is, elimination of the stern wave and the 

 increased rate at which the ocean waves overtook 

 the .ship. 



5.15 Voyage of the SS Nissei Maru. An observa- 

 tional voyage by the cargo ship 8S Xissci Maru was de- 

 scribed by the Experiment Tank Committee of .Japan 

 (1954). The first half of the publication described the 



