402 



MOTION SICKNESS, II 



data available bearing on the present prob- 

 lem. 



Deacon frequency-analyzed his records 

 and thus obtained for any one experimental 

 condition the spectrum of pitch, roll, and 

 wave periods. This is the most useful repre- 

 sentation for data of this type and no doubt 

 should be adopted in all future work. When 

 coursing parallel and anti-parallel to the 

 wave direction, the spectrum of pitch- 

 periods was not coincident with the spec- 

 trum of wave periods (short periods being 

 more prominent in the former), but an over- 

 all Doppler effect was quite evident at 

 various speeds of the ship. On the other 

 hand the modal periods of roll were hardly 

 affected by course and speed .^ The spec- 



through 15° at a depth of 150 ft.). The 

 author has not, however, found any sys- 

 tematic, reliable data regarding roll at 

 various depths. Recently, in response to a 

 questionnaire issued to 15 fleet-type sub- 

 marines, the Bureau of Ships has collected 

 some data on motion near the surface under 

 various wave conditions (Table II).' It 

 would appear from the data, contrary to 

 common belief, that neither periods nor 

 amplitudes are of unusual character. There 

 was in the data a suggestion that longer wave 

 lengths led to longer periods, but the pre- 

 cision of these observations does not warrant 

 any conclusions other than the tentative 

 one that submarines near the surface do not 

 roll or pitch "abnormally" and hence esti- 



TABLE II 

 Some Characteristics op the Motion of U. S. Submarines 



trum of pitch periods was very broad (a 

 more or less flat distribution between 5 and 

 17 seconds). 



From the two studies just mentioned, 

 especially the last, it appears that extended 

 records of ship motion can be taken rather 

 easily, and that, as a rough approximation, 

 simple harmonic motion is acceptable. 

 Deacon's findings with respect to roll (and, 

 to some extent, pitch) suggest that whereas 

 the wave conditions are a modifying factor, 

 the motion of a vessel is governed chiefly by 

 its O"^"!! dynamic constants. 



All measurements mentioned thus far 

 have referred to surface vessels; data on sub- 

 marines are even more scarce. It has been 

 stated that roll periods of submarines are 

 considerably greater than those of surface 

 craft; also, that surface swell causes rolling 

 at depth as great as 100 ft. (an extreme case 

 has been reported in which a boat rolled 



* The range was only slightly shifted. 



mates of the sort deducible from Table I 

 can be supposed to apply. 



Summary 



The following conclusions may be sub- 

 mitted by way of summarizing features of 

 ship motion which bear on the motion sick- 

 ness problem. It should be emphasized, 

 however, that the paucity of accurate meas- 

 urements renders such conclusions very ten- 

 tative. 



1. Three to seven seconds probably in- 

 cludes all the important pitch periods en- 

 countered in naval vessels; 5 to 15 seconds 

 probably includes all the roll periods. 

 This appears to be true of submarines as 

 well as of surface craft. 



2. The simple harmonic assumption can- 

 not be considered a very good approximation 



' These data were made available to the author 

 through the courtesy of Mr. Charles Moore, Code 

 444, Bureau of Ships, Navy Department. 



