THE MOTION OP NAVAL VESSELS 



401 



well below .8 g. For certain craft— landing 

 craft and destroyers — the maximum angular 

 accelerations are considerably higher than 

 Sjoberg's limit, being in one extreme caes 

 (LCI, roll) 49° per sec.^ 



As already mentioned, the foregoing cal- 

 culations, although based on experimental 

 measurements of periods and amplitudes, 

 depend ultimately upon the assumption of 

 simple harmonic motion, and it is therefore 

 important to obtain continuous records of 

 displacement in order to check the assump- 

 tion. No doubt the difficulty of construct- 

 ing sensitive instruments which will resolve 



though the angular motion was not pre- 

 cisely simple harmonic, it was, on the other 

 hand, of such a type that in moderate seas 

 its coarse features could be described fairly 

 well by the simple harmonic assumption. 

 This was not true of records taken during a 

 storm; indeed a storm could be described 

 just as well by the erratic character of the 

 record as by the shift to higher average am- 

 plitudes. The observed periods of pitch 

 (moderate seas) — -4-5 seconds — were about 

 the same as those in sallying, but the periods 

 of roll — 8 seconds — were shorter; conse- 

 quently the average maximum linear acceler- 



TABLE I 

 Some Characteristics of the Motion of U. S. Naval Surface Craft 



Vessel Type 



Destroyer 



LCI 



LST 



Destroyer Escort. 

 Heavy Cruiser — 



Carrier 



Light Cruiser 



Escort Carrier — 



Battleship 



Transport 



Period of Roll 

 (Seconds) 



9.5 



4.5 



5.0 



8.0 



12.0 



16.0 



12.0 



13.0 



14.0 



14.0 



Amplitude of 



Roll 



(Degrees) 



25 

 25 

 15 

 25 

 20 

 15 

 20 

 20 

 12 

 25 



Period of 



Pitch 

 (Seconds) 



Amplitude of 



Pitch 



(Degrees) 



Average 

 Acceleration 



.26 g 

 •21 g 

 .19 g 

 .14 g 

 .14 g 

 .13 g 

 .13 g 

 .12 g 

 .12 g 

 .08 g 



This table is constructed entirely from unrestricted information. The periods and amplitudes are 

 average values kindly furnished the author by the Bureau of Ships, Navy Department. The "average 

 acceleration" is the arithmetic mean of the roll and pitch contributions at three points aboard the 

 vessel: at the bow on the water line, abeam of midships on the water line, and high on the foremast. 



the motion along two planes accounts in 

 part for the scarcity of data. Incidental 

 to other objectives, Morales and Birren 

 (24) attempted such measurements aboard 

 a battleship, using the level and cross-level 

 angles registered by an AA gun computer 

 trained on 000. Although their study was 

 purely preliminary, it indicated that the 

 technique was satisfactory, and that ex- 

 tensive routine observations could thus be 

 made on all classes of vessels carrying stable 

 elements (destroyers and up, and some 

 submarines; the newer submarines, un- 

 fortunately, do not carry these instruments). 

 It appeared from these records that,* al- 



ations were greater than those computed for 

 the battle ship in Table I. However, they 

 are still well within the .8 g maximum. 



A more detailed and complete study using 

 similar apparatus was undertaken at about 

 the same time by Deacon at the Admiralty 

 Research Laboratory (8). Deacon under- 

 took measurements aboard a small naval 

 auxiliary (about 150 ft. long, weighing 800 

 tons), along a coast where simultaneous 

 shore-based measurements of the waves 

 were possible. Although his investigations 

 were directed at a different problem, viz., 

 studying wave motion using ship motion as 

 an index, they probably contain the best 



