ACTIVE TYPE OF STABILIZING GYRO. 221 



moments that are equal and opposite to those due to any wave impulse or any 

 outside disturbing cause. Although the mass moment of the ship is useful in 

 evening-up small differences between the acting moments, he proposes to deal in 

 principle with each impulse separately. 



Suppose a vessel is kept very close to its natural upright position, say within 

 a degree or so, it is evident that its momentary angular velocity, respecting its 

 variation, will be the result of the algebraic sum of all the moments that act upon 

 it, including the ship's small righting moment. Now if an active stabilizer, for 

 instance Mr. Sperry's gyro, is precessed with positive control in such a way that 

 this angular velocity of roll remains practically zero, the ship will be held within 

 very small angles; these angles will, in fact, be only such as are necessitated by 

 the particular type of control employed, and this condition is easily maintained 

 up to the maximum capacity of the stabilizer as against any wave impulse. 



To illustrate further the unique operation of Mr. Sperry's stabilizer, it would 

 be as though we were to effect stabilization by a non-pendulous gyro of the passive 

 type but possessed of no precession inertia, no suspension friction; in a word, 

 practically nascent yet with its full stabilizing and centering instinct, exerting 

 stabilizing couples precisely as required. One of the most interesting and significant 

 points in this connection is that under these conditions there is practically no 

 work done by the ship upon the gyro or by the gyro upon the ship because of there 

 being practically no angular velocity present. It is this fact, taken together with 

 its incessant operation, working, as Mr. Sperry states, much more often than the 

 half -period of the ship, that accounts for the smallness of the plant required. 



Much time has been devoted to arriving at the efficiency of these stabilizers. 

 Starting with a theorem laid down by Naval Constructor David W. Taylor, U. S. N., 

 it has been determined that the efficiency of the stabiUzer operating as described 

 above is very close to 100 per cent. 



Should some individual wave moment attack the ship which is of a magnitude 

 above the capacity of the stabilizing plant, its specific quenching efficiency still 

 remains very high, because: — 



1. Its roll-quenching impulses are always given when the ship's small angular 

 velocity is approximately at its maximum, thus avoiding the inefficient part of the 

 cycle of a pendulous stabilizer, even when in phase with both the ship and sea. 

 It is known that the pendulous stabilizer could never exceed about 70 per cent 

 efficiency and usually much less, because in practice it could never rise to — 



j sin^ ada 



2. Its moments are always applied with a maximum efficiency in the right 

 direction, and perfectly in phase with the ship in a mixed sea. A pendulous 

 stabilizer does not satisfy any part of this requirement. 



3. The cycle of wave impulses such as is shown in the first figures of this 

 paper affords really two points in each cycle at which the stabiUzer will always 

 bring the ship upright, so that each new set of wave impulses between these points 



