208 LAUNCHING OF SHIPS IN RESTRICTED WATERS. 



shown on Plate 89 ; pistons on the weight carriers work in oil dashpots so as to pre- 

 vent vibrations of the brake recording pencil, and pistons may be changed so as 

 to give a sudden or gradual application of the brakes as desired. 



The cable pull exerted on the model by the brakes is recorded as follows : — The 

 aluminum idler pulley, as may be seen by referring again to Plate 87, carries the 

 bight of the cable which is attached to the model and being unwound from the 

 drum. Any retardation of the drum, as the cable is being paid out, exerts at once 

 a pull on the model, and the combined action of these two forces causes the swing- 

 ing arm to move. The angular motion of the latter is, however, controlled by the 

 action of two balanced springs at the lower end of the arm ; whatever movement 

 takes place is proportional to the resistance of the brakes and is recorded on the 

 paper by a pencil attached to the extreme upper end of the arm. This entire mecha- 

 nism is accurately calibrated in such a manner that all errors due to angularity, in- 

 equalities in the springs, etc., are entirely eliminated. 



A short account of the sequence of operations during a launch may serve to 

 explain more clearly the exact method of recording the desired information. As- 

 sume, first, that the model has been released and is moving down the ways; the 

 cable is paying out freely as the only resistance is that of the small brush on the 

 chronograph segments. 



1. At the designated moment, the controller operates the brake release and 

 allows the weight carrier and weights to drop. 



2. The weight carrier, acting through the cords, causes the brakes to grip the 

 revolving drum. 



3. The latter, although continuing to revolve and to act as a chronograph, is 

 retarded somewhat by the action of the brake. 



4. A pull is exerted on the cable, which, leading around the idler pulley so as 

 to effect a change of direction of 180 degrees, draws the latter toward the model 

 against the action of the double springs. 



5. The pencil attached to the arm records the cable pull on the moving paper 

 strip, while a stationary pencil traces at the same time a zero or reference line. 



From what has been said in the preceding paragraphs, it will be evident that 

 all elements of the launching conditions may be varied at will, using the apparatus 

 to record all the data for successive series of runs. The various unknown factors 

 are then determined as described below. With regard to the actual performance of 

 the model, it may be said that all parts of the mechanism functioned in a most satis- 

 factory manner and that the results for corresponding and similar runs were re- 

 markably consistent. 



To work up the results, it is of course necessary to apply the principles of 

 mechanical similitude as is done for all work in the model basin. 



The length ratio, L, being 96, the corresponding speed ratio is V -L = V 96 = 

 9.798. Inasmuch as the maximum launching speed of the model is about 3 feet per 

 second, corresponding to a ship speed of 29 feet per second, and as the vessel in 

 any circumstances would never attain such a high velocity, means are adopted to 



