I^OGARITHMIC SPEED-POWER DIAGRAM. 341 



It will be noted that this does not eliminate the difference between wake 

 gain and thrust deduction, for which, apparently, in this case allowance 

 must be made as high as four per cent at the higher speeds. 



This question of the interaction between hull and propeller is as much 

 a matter of doubt in other methods of procedure as in the present case, and 

 is dependent upon so many factors that it cannot be predetermined with 

 certainty. 



To assist in determining under what conditions cavitation will take place 

 contours are shown on Fig. 8, Plate 136, for a coefficient which, multiplied by 

 diameter in feet, gives the propulsive thrust per square inch of projected area. 

 As has been shown by Mr. Taylor and others, this is not a perfectly safe 

 criterion for judging as to cavitation, and must be taken in conjunction with 

 tip velocity, as well as many other things. It is, however, of assistance to 

 know the value of this thrust per square inch for comparative purposes. 



