118 PERIOD OF VIBRATION OF STEAM VESSELS. 



dam to the after end of engine casing. The method of observing was to secure one 

 end of a steel tape to the heel of a rail stanchion and to hold the other end in the 

 hand with constant tension, watching the motion of the 90- foot mark past a scratch 

 on the deck plating. 



The observations took place in the Gulf of Mexico in July, 1914, with the 

 vessel loaded with about 11,000 tons, deadweight. The revolutions of the main 

 engine were increased from 68 to 74, when a slight vibration became noticeable. 

 It could not be measured by the battens, but at No. 3 station the motion amounted 

 to about ^ inch in 90 feet. When the revolutions were increased to 76, the 

 vibrations were quite appreciable. The motion on the batten amounted to about 

 ^ inch total. It was difficult to determine this with exactness, as it varied 

 slightly, but it never appeared to exceed % inch, and J^ inch seems near the true 

 reading. The motion in 90 feet as determined by the tape line was -^j inch at 

 No. I, -St inch at No. 2, yV inch at No. 3, A inch at No. 4, and ^ inch at No. 5. 

 The number of vibrations corresponded with the revolutions of the engine and 

 could be easily counted by the motion of the tape. The type of vibration was of 

 the two-nodal character, in which the bow and stern drop as the midship portion 

 of the vessel rises, and rise as the midship portion drops. The nodes seemed to lie 

 near the after end of the bridge deck-house and at the after cofferdam, but it was 

 difficult to decide where the nodes were. 



The variation in stress in the shelter-deck stringer corresponding to -^ inch 

 variation in length in 90 feet amounts to about 1,700 pounds per square inch. The 

 deck was already in compression, and the additional compressive stress therefore 

 amounted to about 850 pounds per square inch. 



During the tests the sea was smooth. When the engines were slowed down, 

 the vibrations ceased. When the revolutions were raised to 76 again, so that in- 

 dicator cards might be taken, the vibrations reappeared, and of the same intensity 

 as before. 



Here seemed to be a case of synchronism between the revolutions of the main 

 engine and the period of vibration of the hull structure occurring in the working 

 range of revolutions of the engine on a vessel of low speed. The literature on 

 the subject to which the author had access did not seem to be complete as regards 

 the period of vibration of the hull structure, although approximate formulae were 

 given by Herr Schlick in a paper read by him in 1894 before the Institution of 

 Naval Architects in England. 



From the considerations set forth in the Appendix, it would appear that the 

 general formula for small isochronous vibrations is 



in which T is the time in seconds for a double vibration ; g is the acceleration of 

 gravity; k^ is the square of the radius of gyration in feet, in the value of the 



