MOTION OF SUBMERGED BODIES. 



393 



ART. 113. RESISTANCE TO MOTION OF SUBMERGED BODIES. 



If the plane already considered forms the end of a solid prism whose 

 axis is parallel to the direction of motion and whose length is more than 

 about three times the diameter of the plane, the directions of the stream 

 lines are modified as shown in Fig. 178. Eddies are still formed at C, 

 after which the motion becomes parallel to the axis of the prism, until at 

 B a second formation of eddies takes place. 



The total eddy formation at B and C is now less than at B alone with 

 a plane surface, and this results in a higher pressure on the rear of the 

 prism than on the plane and a consequent smaller resistance to motion. 

 Even including skin friction, with a moderate ratio of length to diameter 

 the resistance of the prism 



than that of the 

 Putting the resist- 



, Wa 

 ance equal to k 



is 

 plane. 



we 



FIG. 178. 



have k = '55 with a prism 



having plane ends and a 



length of about three times 



its diameter. If fitted with 



a tapering cutwater the production of eddies at C is obviated and the value 



of k becomes about *40. Fitted also with a tapering stern eddy formation 



is largely prevented at B, and k is reduced to about "125. The resistance 



is now largely due to skin friction and should be calculated on that 



assumption (p. 175). 



Experiments by Dubuat, on a square prism, section a X a, length /, 

 gave resistances in the following ratios : 



ART. 114. RESISTANCE OF SHIPS. 



The resistance to the motion of a ship is due mainly to skin friction, 

 but also to the formation of surface waves and of eddies (chiefly at the 

 stern). Mr. Froude found that although the velocities of gliding vary 

 largely at different points of the -hull of a ship, no sensible error is 



