26 LAUNCHING OF SHIPS IN RESTRICTED WATERS. 
A direct comparison of the brake-resistance curves of the model and the ship, 
and the plotting of the probable brake-resistance curve of the California is a 
somewhat difficult undertaking, in view of the fact that the average travel of the 
friction cables through the brakes was only about 137 feet. Any part of the travel 
during which a fixed number of the brakes were in operation was so short as to 
preclude an accurate determination of the effect of brake resistance on the velocity 
of the vessel. A reasonably accurate estimate has been arrived at in the following 
manner, with a second computation which is in close agreement. 
Referring to Plate 10, upon which all the curves obtained from launching 
data are shown, it will be found that at 710 feet travel, when 78 feet clear of the 
ways, the ship was traveling at the rate of 17.1 feet per second. The deceleration 
at this point was 0.6 foot per second, indicating a total retarding force of 609,000 
pounds. Of this the water resistance, computed by the formula R, =87.2V?, was 
435,800 pounds, leaving the brake resistance as the remainder, 173,200 pounds. 
With two brakes in operation at 830 pounds pressure and four at 600 pounds 
pressure, this gives a coefficient of friction of about 0.129. As an extension and 
check of this computation, consider the behavior of the ship from 660 to gro feet 
travel, at which point all the brakes had ceased to have further effect. It is found 
that of the total reduction in kinetic energy for this portion of the travel, 
98,730,000 foot-pounds, the water resistance had absorbed by far the greater part, 
namely, 80,782,500 foot-pounds, leaving 17,947,500 foot-pounds as the energy 
absorbed by the brakes in action. Summing up the brake effect, in terms of 
hydraulic pressure in the cylinders and travel of the respective friction cables, 
it is found that the average coefficient of friction is about 0.123. This figure, 
which is but slightly different from that above, gives an average coefficient of 
friction of 0.126, as compared with the coefficient of 0.24 as found by the pre- 
liminary experiments of a year and a half ago. 
It must be remembered, however, that these tests were carried out with the 
limited means then at hand, which permitted only a comparatively short pull on 
one friction wire at a rate not exceeding about one-half foot per second. It must be 
remembered also that the tests were conducted with only two cables, which pre- 
sented clean, bright surfaces to the brake blocks on most of the runs. The friction 
cables in place at the time of the launch, however, were covered with a partly 
oxidized coating of heavy oil, just as they had been delivered by the contractor, 
and the presence of this oil coating may possibly have reduced the friction by an 
appreciable extent. It appears quite likely, therefore, that at the speeds and 
temperatures at which these brakes run during a launching operation there is a 
considerable reduction in the assumed coefficient of friction, although the data 
given above are unfortunately neither sufficiently accurate nor extensive to fix 
this value within very close limits. 
In connection with the launching of the battleship Montana the model is now 
being rebuilt and the apparatus slightly modified. The tank will be increased in 
width from 40 inches to about 72 inches and the new model will be provided with 
all the minute appendages in addition to the launching cradle. The subject of 
