LOADS ACTING ON A SHIP AND THE ELASTIC RESPONSE OF A SHIP 



263 



Momen+ by Accelera+ion 



'"O iC 20 30 40 SO 

 Me+er 



Accelera+ion Forces by P 



10 20 30 40 SO 

 Meter 



Fig. 6 Acceleration forces by heaving and pitching. Hogging: 

 time 11. xii. 34, Uh. 40'. 40" (from Schnadel, 1937-1938) 



The real hogging moment in our 



case i.s therefore -|-4o..")()U metre-tons 



"The .static moment of oO,000 mt is nearly the greatest 

 influenced by sea. It corresponds to an effective wave 

 height of 5..') m or L/24, if the wave length is ecjual to 

 the ship's length. Considering the d.vnamic force 

 too, we get an effective wave height of 4.2 m for the 

 ship hove to, if we call 'effective' wave height the height 

 of wave to be taken for the normal strength calculation 

 to get the same moments and assume that the wave 

 length is ecjual to the ship's length." 



Fig. 8 shows the good agreement which was obtained 

 between bending moments calculated by the foregoing 

 procedure and those deduced from strain-gage measure- 

 ments. Fig. 9 shows a similarly good agreement be- 

 tween calculated and measured ship deflections. Schna- 

 del's filial conclusions were quoted in Section 3-5.12. 



The work of Schnadel on the MS San Francisco is 

 important in describing how the bending moment is 

 composed of contributions of liuoyancy and of a ship's 

 and ambient water's accelerations. He brought out the 

 significance of a ship's interference with wa^es. These 

 data, obser\-ed for the first time on a ship at sea, were 

 later confirmed by the model tests and by theoretical 

 computations. The computations were described in the 

 preceding .section and the model tests will be covered in 

 the following one. 



The word "confirmed" is u.sed in a broad and quali- 

 tative .sense. The data obtained on the MvS San Fran- 

 cisco scarcely can be considered accurate enough for a 

 detailed comparison with calculations and model tests. 

 The primary sources of possible inaccuracies are: 



1 Only one wave was analyzed. 



2 The determination of the wave profile from several 

 consecutive records introduces an uncertainty. The 

 uncertainty indicated by the spacing of two cur\'es in 

 Fig. 3 also can introduce an appreciable error. 



3 The total number of pressure gages is too small to 



Fig. 7 Shearing force and bending moment. Hogging: time 

 11. xii. 34, llh. 40'. 40" (from Schnadel, 1937-1938) 



give an accurate determination of the total pressure 

 foi'ce. 



4 The arrangement for time synchronization of data 

 did not permit suHicient precision. 



5 The relationships used in the analysis were based 

 on the properties of regular waves. Peculiarities of the 

 recorded wa\'e were not considered. 



An outstanding characteristic of the MS San Francisco 

 \'oyage is the fact that observations were made by 

 an outstanding group of qualified men. These men, 

 Schnadel, Horn, Weinbhnn, and Weiss, had developed the 

 neces.sary instrumentation and they used it to collect 

 aboard the ship. They had witnessed the conditions 

 which they subsequently analyzed. As a result, an un- 

 usually large amount of valuable iiifoi'mation was ob- 

 tained. 



The strain gages were of a mechanical type and did not 

 permit centralization. They had, however, the ad- 

 \'aiitage of a fi.xed zero point so that hogging and sagging 

 stresses were clearly defined. In the more advanced 

 type of electric strain gages, which ha\'e been used since 

 by other investigators, the zero calibration has been un- 

 certain. Only the total hogging-sagging stress ranges 

 were, therefore, reportetl. 



3.2 SS Ocean Vulcan. The general type of instru- 

 mentation used on the SS Ocean Vulcan^" was similar to 

 the MS San Francisco, but a much larger number of 

 instruments was u.sed. The general description of in- 

 strumentation was given in Section 3-5.13. The instru- 

 mentation was of a more advanced tj^pe than on the MS 

 San Francisco and all records were taken at a central 

 recording location. It had, however, two important 

 drawbacks. The electrical strain gages permitted the 

 hogging-sagging range to be measured, but were un- 

 certain in the distribution of stress between hogging and 

 sagging. The records were read from the photographs 

 of gages taken at about 0.4-sec intervals and no continu- 

 ous curves of data \\ere obtained. 



'" Admiralty Ship Welding Committee Reports Nos. 8 and 12 

 (see bibliography at end of chapter 3). 



