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



261 



Feet 



Fig. 3 Complete wave measurement (from Schnadel, 1937-1938) 



The wave profile on the ship's side was recorded con- 

 tinuously on a running strip of photographic paper. 

 For a wave longer than the ship, this record gave at any 

 instant only a part of the complete wave profile. Since 

 ship motions had been recorded at the same time and 

 had been interconnected by time signals, it was possible 

 to piece together several sections of the record and to 

 obtain the entire wave profile. The results of this process 

 are shown in Fig. 3. This diagram covers the period 

 from the 36th to the 46th second following 11 hr 40 niin 

 AM on December 11, 1934. 



The wa\e profile, defined by several successive ob- 

 servations, may have contained errors caused by the 

 change of wave form with time. Schnadel assumed 

 this error to be small as far as the main wave profile 

 was concerned. However, the main wave profile was 

 covered by smaller waves. These changed their forms 

 rapidly and affected the readings of individual gages. 

 The range of the uncertainty, caused by this action, was 

 shown by the spacing of two profiles drawn in Fig. 3. 

 The wave length was found to be 186m (610 ft), mean 

 wave height from trough to crest, 14m (46 ft) and the 

 ratio of length to height was 13.3. This is exceptional 

 steepness for a wave of such a size. 



Figs. 4 and 5 show ship attitudes, the wave profiles, .4, 

 and the pressure head profiles B and C at hogging and 

 sagging conditions. The vertical scale is 2.5 times the 

 horizontal one. The pressure head profiles D and E 

 resulted from the correction of pressure heads to the 

 condition of a free wave. This process can be best ex- 

 plained by the following ([uotation from the original 

 paper : 



"The acceleration forces of heaving and pitching cause 

 an acceleration of the added mass of water which is in- 



cluded in the measurement of the diaphragms.^ If we 

 intend to compare the measurement of the water pressure 

 with the normal theory of waves, we have to take into 

 account the acceleration pressure of the water. For the 

 acceleration of the added mass the formulas of Lewis^ 

 and Lockwood Taylor* are used. A simple investiga- 

 tion shows that the water pressure is increased by the 

 added mass in the case of hogging and reduced in 

 the case of sagging. Therefore, we have to subtract the 

 acceleration forces on the crest and to sum up in 

 the trough. 



"The pressure foimd by this operation contains the in- 

 fluence of the Smith effect and the influence of the 

 ship's hull in waves. If we calculate the influence of 

 the Smith effect, we are able to determine the influence 

 of the hull. For a wave 186m length and 14m height, 

 we get the influence of the Smith effect on the crest with 

 75% of the height of water column. Our measurements 

 show that the pressure in the wave crest is reduced to 

 60%." 



The following passage is also useful in clarifying the 

 ciuestions as to water pressures: "The investigations of 

 Gerstner and Hagen show that the water pressure in an 

 iuidisturl)ed wave does not agree with the height of the 

 corresponding water column. Smith' has drawn the 

 conclusion that the calculated wave height should be 

 taken much less than the observed wave height. The 

 investigations of Smith are only partially correct, as 

 they neglect the disturbance of the wave by the ship 



* I.e., of the bottom pressure gages. 



' F. M. Lewis (3-1929). 



' Lockwood Taylor, 1930. 



9 W. E. Smith, Trans. INA, 1883, p. 135 



