SHIP MOTIONS 



185 



MeSstellen 

 Dehnungsmesser (Zei&) 

 Blegungs- 



Verdrehungs- i-messer(Dr:Schafer) 

 Beschleunigungs 



Fig. 24 Location of instruments on SS Hamburg; 



strain gages; w accelerometers; u deflections (from Rempf and Hoppe 

 1926) 



E t 



U d) 



O "^ 

 o ^ 



si 



u o 



— Q- 



- 222mm= 1 Spannungsschwankung - 7.4 sek 



AuPenhaut des Doppelbodens 

 Spant 102-103, Gang A, Langsverband 



Spannungsdi-fferenz bei Resonanx 420kg/cm 



Me Slangs = 29Gmm, Li =360, "1 = 7.4 Stampfschwingung 



1 sek = 15 mm 





140 160 180 200 220 240 260 sek 

 Zeitmaps+ab I/GO nat. Grbt3e »- 



Fig. 2 5 Strain diagram for location 7 at keel (from Kempf and Hoppe, 1926) 



had all the characteristics of a superposition of two oscil- 

 lations — one at a regular wave fre(iueucy, the other at 

 the ship's own frecjuency, such as was demonstrated by 

 W. Froude (1861) for undamped rolling. Kempf 

 and Kent gave this explanation for the "beats" observed 

 in pitching. However, they considered the average 

 wa\'e as the regular one and neglected the heavy damp- 

 ing in pitching and heaving. In reality, as has been 

 shown since, these "beats" occur in an irregular wave 

 formation, and are explained by superposition of many 

 wave trains. 



In his wor.k J. L. Kent also gave data on rudder motions 

 and steering characteristics of ships under the influence 

 of wind and waves. This is particularly valuable be- 

 cause very little data on this subject can be found in 

 literature. 



Kempf and Hoppe (192(1) measured the SS Ham- 

 burg's speed by a log and visually estimated the average 

 wave height and length encountered. The log consisted 

 of a body t(jwed by a cable from the end of a pole ex- 

 tending a sufficient lateral distance from the side of the 



ship. The .speed was obtained from the body drag on 

 the basis of previous caliijration. The a\'erage speed 

 gi\'en l)y the log was compared with daily average speeds. 

 The instantaneous readings of the log were affected, 

 however, by wave impacts and liy the ship's pitching. 

 An apparatus for direct measurement of the ship's 

 motions was not available but the pitching and heaving 

 motions were defined by accelerometer readings at the 

 bow, amid.ship and stern. The angle of pitch was com- 

 puted fi-om the acceleration and the obser\'ed period. 

 Fig. 22 shows a sample accelerometer record, illustrat- 

 ing an apparent "beat" of 83.5 sec. Fig. 2o gi\-es the 

 extreme instantaneous positions of the ship in waves 

 which were taken in the analysis to l)e regular. These 

 conditions correspond appro.ximately to synchronism 

 i)etween a ship's natural ]:)it('hing freciiiency and the 

 frequency of wiwe encounter. 



Special efforts were made to observe ship deflections 

 and strains. The location of the strain gages is shown in 

 Fig. 24. Ship deflections were obtained by sighting 

 targets at the bow and stern. Fig. 25 shows the varia- 



