34 DISCUSSION ON MAY MEETING PAPERS. 



In a vessel that is stabilized against roll, it is quite evident that the distortions in the 

 ship's structure, and also in the gear casing, are considerably reduced, which is conducive 

 to decreased vi'ear. 



Automatic records taken upon several vessels in a heavy sea indicate that an increase 

 in roll occasionally increases the angle of pitch. This peculiar phenomenon is self explana- 

 tory if careful consideration is given to the action of a vessel vi^hen pitching without rolling 

 or when pitching with a marked angle due to heavy roll. 



Another point of very important magnitude is to be considered when diagnosing the 

 trouble in reduction gears, particularly in the high-speed double reduction type, due to the 

 effect of pitching, and especially in vessels with a quick period and large angles of longitu- 

 dinal oscillations. The gyroscopic effect of large gears, revolving at a high angular velocity, 

 when possessing considerable inertia, is of great importance. The resultant pressure on the 

 bearings, supporting the pinions and gears, no doubt changes under the influence of gyro- 

 scopic effect. 



Bearings naturally must and do have considerable clearance between the babbitted sur- 

 face and the revolving journals. This clearance varies with the diameter of the journal, 

 method of lubrication, etc. The direction of the maximum pressure upon the bearing will 

 be disturbed by the gyroscopic effect and will have the tendency of changing the parallelism 

 that existed between the teeth of the gears in contact. 



Gyroscopic effect on many occasions, under the influence of unfavorable sea conditions, 

 will express itself in additional distorting strain in the large gears and also in the shaft 

 or spindle carrying them. No matter how minute this distortion is, it will effect the mis- 

 alignment of the two faces of the gear and pinion, and possibly this misalignment in some 

 instances will pass the region of prescribed safety. The gyroscopic effect is co-periodic and 

 proportional to the angular velocity of the pitch. 



I may recall a case. Two years ago I passed on a transport to France, studying the 

 conditions of the sea, and found that the vessel was pitching 7 degrees on either side of the 

 horizontal — that is, about 14 degrees total, with a period of pitch of about 8}4 seconds. Of 

 course there are vessels that have a very much slower period, and naturally the gyroscopic 

 effect with a slower period would not be as large, because the angle of velocity is a function 

 of the amplitude of pitch and the period of oscillation. 



In conclusion I will refer to the well-known fact that the torsional pulsations in a heavy 

 sea are transmitted from the propellers laboring under constantly varying conditions, espe- 

 cially in twin-screw ships. The torsional pulsations are due, among other things, to the 

 constant change of hydrostatic pressure in the vicinity of the propeller, due to roll, pitch and 

 the rotation of blades. Efficiency of the propeller is very much effected when air cavities 

 are encountered, this, in turn, increasing the torsional unbalance. 



Extreme cases of a combination of a heavy roll and pitch will force the propeller par- 

 tially out of the water, even on a properly loaded vessel. 



It is quite evident that the elimination of roll will reduce the major element of distor- 

 tion in any vessel and will also provide the best possible conditions for the propeller to work 

 in, thus insuring the highest efficiency and proper performance of not only the gear reduc- 

 tion unit but also all the other apparatus aboard ship. (Applause.) 



Mr. J. T. Dalcher, Member: — As previously pointed out by Mr. Smith, the Federal 

 Shipbuilding Company has quite extensively investigated the question of comparison between 



