PERIOD OF VIBRATION OF STEAM VESSELS. 



By William Gatewood, Esq., Member. 



[Read at the twenty-third general meeting of the Society of Naval Architects and Marine Engineers, held in 



New York, November 18 and 19, 1915.] 



The author was led to investigate this subject by the vibrations which oc- 

 curred in service on an oil tanker built by the shipyard with which he is con- 

 nected. This tanker is 460 feet long and is fitted with a quadruple expansion 

 engine installed in the stern, designed to indicate about 2,800 horse-power at 78 

 revolutions per minute. When running in ballast condition, no vibration was ex- 

 perienced. When loaded, however, longitudinal vibrations in a vertical plane were 

 experienced with the engines turning at revolutions between 72 and 76. These vi- 

 brations were of the two-nodal character, and their period corresponded with the 

 revolutions of the main engines. After several trips, the pitch of the propeller was 

 increased so that the revolutions in service might be less than 72, and under the 

 altered conditions no vibration was experienced. 



In order to investigate the matter more thoroughly, however, observations 

 were taken on the vessel after the pitch of the propeller had been increased. The 

 methods of taking observations were somewhat primitive and consisted of a de- 

 vice for measuring variations in the shape of the vessel in a vertical plane and an- 

 other device for measuring the change in length of the deck plating. 



A vertical batten marked off in inches was set up at mid length of the vessel, 

 in line with the inner edge of the shelter-deck stringer plate. A horizontal batten 

 was located at the height of the rail abreast the after end of the engine casing. 

 An observing station was established at the outboard end of the breakwater near 

 the bow. The distance from the observing station to the vertical batten was 192 

 feet 8 inches, and from the vertical batten to the horizontal batten was 189 feet 2 

 inches, a total distance of 381 feet 10 inches. This was the longest fore-and-aft 

 sight obtainable. The method of observing was to sit on the breakwater with head 

 against the rail and with binoculars to watch the motion of the horizontal batten 

 past the divisions on the vertical batten. 



Five stations were located for observing the stretching and compression of 

 the shelter-deck stringer plate. No. i station extended from the breakwater for 90 

 feet aft on the line of the inner edge of the shelter-deck stringer plate. No. 2 

 station overlapped No. i station, and extended 90 feet from just forward of the 

 bridge-house to nearly amidships. No. 3 station overlapped No. 2 station and 

 extended 90 feet over the mid length of the vessel. No. 4 station overlapped 

 No. 3 and extended 90 feet from forward of the mainmast to aft of the after cof- 

 ferdam. No. 5 station overlapped No. 4, and extended 90 feet from the after coffer- 



