178 



HYDRAULICS AND ITS APPLICATIONS 



The fact that the resistance per square foot over the aft part of the 

 surface is less than at a point nearer the prow may be explained as 

 follows. v The first portion of the surface, in passing through the water, 

 experiences resistance, and communicates motion, in its own direction, to 

 the water. The succeeding portion of the surface is then in contact with 

 a. bodj <jt w&ter having a smaller relative velocity, and hence producing a 

 smaller resistance per unit area, while it would appear that the velocity 

 of the accompanying current increases until at some point in the surface 

 a balance is obtained between the amount of energy given to the 



Length in Feet 

 FIG. 92. 



accompanying stream per second, and the energy dissipated by eddy 

 formation in the surrounding fluid and in producing motion of a greater 

 volume of this water against viscous resistances. After this point is 

 reached, the velocity of the accompanying current and the resistance per 

 square foot of surface remain approximately constant. It will be noted 

 that the mean resistance per snuare foot of area diminishes very slightly 

 for lengths above 50 feet. 



A short resume of Mr. Froude*s results is given on p. 176, these; parti- 

 cular experiments being carried out at a velocity of 10 feet per second. 1 



Curves (Figs. 90 and 91) have been prepared from the results of these 

 experiments, and show respectively the resistance with length at constant 

 speed, and with speed for a given length, for a varnished or painted iron 

 surfaca In Fig. 92 values of / in the formula, Resistance = f S v n , have 



1 From the British Association Report, 1874. 



