188 



THEORY OF SEAKEEPING 



-2_ 



E 

 c 



+ 4 

 + G 



-6 



-4 



-2 



E 

 c 



w 



+ 2 

 + 4 

 + 6 



-6_ 

 -4 . 

 -2 



+ 4 

 + G 



20 



eschleunigungen b 



Geschwindigkei + en v 



Wege s 



Stampfwinkel 6 



30 



40 



Zeit in Sec - 



Fig. 28 Motion record of MS San Francisco in hove-to condi- 

 tion at 17 hr 34 min of December 1 1, 1934. Heaving motion is 

 computed from heaving accelerations. Dotted line at the bow, 

 light solid line at the stern, heavy solid line amidships. b — 

 accelerations in m sec"; ; — vertical velocity in m sec; 5 — heave 

 in meters; 6 — pitch angle in deg (from Horn, 1936) 



the storm during a hovc-to condition. A maximum 

 double amplitude of pitch of 22 deg was reached in this 

 condition. Fig. 2'J shows the ship's motions in a mod- 

 erate bow sea, and also when ho\'e-to in a se\'ere storm. 



In his paper Horn (193(3) ga\'e many curves showing 

 the distributions of amplitudes and t're(|uencies of pitch- 

 ing and rolling. In discussing the characteristics of 

 instruments he introduced the concept of irregular 

 summation of harmonic disturbances. However, all 

 analyses of wa\'e and ship motions were based on the 

 average harmonic wave. 



The forces acting on the ship and its responses were 

 ot)tained from se\'eral strain gages, a few water-pressure 

 gages at the ship's bottom, and from measurements of 

 hull deflections. The latter measurements were ob- 

 tained by a method similar to the one used on the SS 

 Hamburg, namely four targets forward and four aft 

 of the midship plane were photographed simultaneously 

 by a telescopic camera ("optograph"). The locations 

 of these instruments are shown in Fig. 27. The in- 

 struments were of two kinils; those recording during 

 short observational periods and those recording con- 

 tinuously, unattended. Table 4 shows sample data 

 from the first group and Table 5 shows data from the 

 second. 



Data from the first group were subjected to a detailed 

 analj'sis with reference to measured wave profiles, water 

 pressures, and accelerations. It was found that 

 measured stresses were considerably lower than those 

 computed by conventional methods using the recorded 

 wave profile. Schnadel introduced the concept of 

 "effective wave height" (wirksame Wellenhohen). This 

 is the lieight of an imaginary r[uasi-static wave which 

 would give the same stress or deflection as the actual 

 measured wave. It is designated in Talile 4 as H'^ 

 when computed from stresses (aus a) and H"„ when com- 

 puted from deflections (aus/). Other notations in the 

 tables are 



Stress — Spannung a 



Wave hollow (sagging condition) — Tal 



Wave crest (hogging condition) — Berg 



Tension — (-|-) Zug 



Compression — (— ) Druck 



Bending moment — Biegemoment J/ 



Deflection — ;Durchbiegung / 



Slam — Stoss 



Schnadel arrived at the following conclusions: 



(a) "The wa\-es met by a ship at sea exceed pre- 

 viously assumetl heights and steepnesses. To the ef- 

 fective wave height of .some 5.5 ni (18 feet) corresponds 

 an actual height of 9 to 10 m (29.5 to 32.8 feet) by 130 m 

 (426.5 feet) in length. 



(/)) "Foi' the hogging stress of a ship, the observed 

 wa\e heights mu.st be reduced. The reduction is .stronger 

 than is given by the so-called Smith effect. The reduc- 

 tion probably can lie! attril)uted to the disturbance of 



