250 



THEORY OF SEAKEEPING 



1.2 



ISO 200 



A/(L/I00)3 



Fig. 12 Trend of speed-length ratios for synchronous pitching 



of typical ships in regular head seas of length equal to ship length 



— defining zones of serious and moderate pitching in irregular 



storm seas (from E. V. Lewis, 195 54) 



As ail example of the practical use of Fig. 12, Lewis 

 (195()) investigated changes in proportions of a Victory 

 type .ship. A certain increase of the length and a de- 

 crease of the beam and draft were assumed, and the 

 resultant improvement in service was iii\'estigated. 

 The change is shown by the arrow in Fig. 13, and the 

 revised design is designated as "Proposed Trend." The 

 new data on the Mariner, with commonly u.sed reduced 

 draft (Allen and Sullivan, 1954) were added. The fre- 

 cjuency of occurrence of different types of weather was 

 considered and the final changes of revenue were e\'alu- 

 ated. The increase of revenue was shown to range from 

 12.8 per cent at a high cost of .ship construction with 

 cargo at S20 per ton, to 60.3 per cent at a low construction 

 cost and cargo at $30 per ton. 



Aertssen (1957, 1959) further extended E. V. Lewis' 

 work liy including in the chart .sea observation data, 

 which were obtained on several recently built Belgian 

 ships. On this basis he drew several demarcation lines 

 corresponding to different wind strengths on the Beaufort 

 .scale. The resultant chart is shown in Fig. 14. 



In a 1959 paper Aertssen presented a detailed analysis 

 of the power and speed of a passenger ship Jadolville 



ZONE OF SEVERE 

 PITCHING AKIDWET DECKS 



Legend 

 + Nom'inal Sea Speed 

 • Typical Heavy Wea+her Speed 



S.5 

 J l_ 



100 



150 



200 



250 



Displacement -Length Ratio, A/CL/lOO) 



Fig. 13 Sea speed of cargo ships in relation to zones of severe 

 and moderate pitching in irregular head seas (from Lewis, 1956) 



and a tanker Elisabeth. The particulars of these ships 

 are given in Table 5. The plot of the DHP versus ship 

 speed for SS Jadotville is presented in Fig. 15. This 

 figure shows the DHP required to maintain a given ship's 

 speed in different weather. The latter is defined by 

 Beaufort numbers and the relative wind direction (see 

 the inserted sketch). A no^^l and important feature of 

 this plot is the "limit of speed" line. This indicates the 

 maximum safe ship speed and engine power in any wind. 

 A higher speed is prevented by the necessity to reduce the 

 engine power in order to avoid dangerous ship motions. 

 It will be observed that in bow wind of Beaufort 7 only 

 about 7300 dhp can be used, although as much as 12,500 

 is a\'ailable. At Beaufort 6 a small reduction from the 

 service power of 9500 is indicated. The available excess 

 engine power over 9500 was used only in winds below 

 Beaufort (5. 



The foregoing discussion was limited to .ships at full 

 load. The permissible speed of ships in light condition 

 appears to be limited primarily by slamming. The 

 investigation of the Admiralty Ship Welding Committee 

 (3-1953) on the SS Ocean Vulcan showed that slamming 

 occurred on one out of each 3 da.ys of .-^hip at .sea and in 

 light condition. In the economic circumstances prevail- 

 ing over the past several years, many North Atlantic 

 westward \'o.yages are made by ships at small drafts, 

 and large reduction of speed is freciuently caused by 

 slamming. Therefore, it appears to be desirable to make 

 a separate im'estigation of the speed loss of .-^hips in 

 ballast condition, following the pattern set \\\) by Lewis 

 and Aertssen. 



In the author's experience, slamming appears to result 

 more often from encountering the swell than from the 

 wind .sea. In this case the wind .strength is not a .satis- 



