179 



CHAPTER XXL 



WATER PERFORMANCE. 



Example. The Condition for Getting Off the Water. Sup- 

 pose we are considering a flying boat whose hull lines and 

 angular setting have been copied from a certain tank test, and 

 whose performance curve is curve (4) plotted on page 157 and 

 whose propeller curves are given on page 170. 



Further suppose that the test made in the model tank has 

 been issued in the following form : 



"Full sized machine, total weight 10,000 pounds, getting- 

 off speed 46-875 knots: resistances in pounds for the full sized 

 machine at 10, 15, 20, 23, 26, and 29 knots are 900, 1350, 1720, 

 1800, 1550, and 1300 respectively." 



Our first care is to see if the necessary conditions of similarity 

 are all fulfilled. 



First, we must have geometrical similarity of the under water 

 lines : we have already supposed that this condition is met. 



Secondly, the hull must be set at the same angle relative 

 to the wings in our machine as in the prototype this condition 

 also we have met. 



Thirdly, we have to have 



w \ 



and we will suppose that in copying the old design we have been 

 careful, of course, to meet this condition. 



Fourthly, we have the condition to meet that our getting-off 

 speed divided by that of the prototype equals 



V? - 



^L \W 



Now the prototype's getting-off speed is given as 46-875 

 knots = 46-875 x 1-151 miles per hour. Also we have 

 W = 5700 and W = 10,000. Therefore our getting-off speed 

 ought to be 



46-875 x I "1 5 if 1Z25_) = 49-2 miles per hour. 

 \io,ooo/ 



