198 



THEORY OF SEAKEEPING 



Table 8 Items Measured and Instruments Used on SS Nissei 

 Moru (from Experiment Tank Committee of Japan, 1954) 



C. 13 



waves observed were far more irregular as to both time 

 and locality than was previously expected, and no wave 

 trains were observed which encountered the ship in 

 regular periods and amplitudes. The condition of the 

 sea surface around the ship seemed to be very confused, 

 and the size and profile of the waves seemed to be i-e- 

 markably different as to time and locality. 



"Therefore, it was thought difficult to secure a stand- 

 ard sea condition affecting the sea performance of the 

 ship, by measuring the waves in a certain location and at 

 a certain instant, even though they were measured as 

 accurately as possible. Measuring wanes by a stereo- 

 camera has a weak point in connection with said features.-' 



"As to the observations of waves, though high ac- 

 curacy could not be expected, observers made efforts 

 to get an average value considering the irregularities of 

 the waves as to both time and locality." 



At the date of publication the analysis of the stereo- 

 photographic data had not been completed, and only a 

 few samjjles of these were given. As in the case of 

 Weinblum and Block (1!J.S6) and the Admiralty Ship 

 Welding Committee Report (1953), the data suffered 

 from the smallness of the ocean area covered. Hardly 

 more than one long wave length is included. It appears 

 that the value of the stereo-photographs may be limited 

 to showing the nature of small waves overlaying large 

 ones and to demonstrating the non-symmetry of waves. 

 For instance, they confirmed the visually observed fact 



100 



200 300 400 500 GOO 700 800 

 Wovelength- Feet 



Fig. 38 Steepness of waves observed during trials (from Ad- 

 miralty Ship Welding Committee, 1953) 



that the slope of a wave is steeper on the lee-side than on 

 the weather side. 



The observed relationships between wa\e heights and 

 lengths are shown in Fig. 43. It appears that an en- 

 velop curve could be fitted similar in form to the dotted 

 line in Fig. 37. The wave heights obser\'ed on the SS 

 Nissei ]\Iaru are, however, about 60 per cent as high as 

 those observed on the SS Ocean Vulcan. 



Fig. 44 shows a comparison of observed wave periods 

 with the periods expected on the basis of the trochoidal- 

 wa\'e theory, For any observed period the sea wave is 

 shown to be shorter than expected from the simple 

 theory. 2^ 



The results of each 3-min observation were summarized 

 on charts, two of which were gi\'en in the report. These 

 are reproduced here in Figs. 45 and 46. The headings of 

 the figures, E.xp. No. 18 and Exp. No. 77, designate ob- 

 servation numbers. The data of 154 observation periods 



'^ Italics bv the author. 



^* This is in agrcenii'iit with theoretical deductions of \V. I'ierson 

 given in Section 1-8.6. 



