Srr. 7031 



SCREW-PROPELLER DE.STGN 



62.^ 



is explained iu detail in NACA Report 824, 1945, 

 pages 3-4. 



The chart in Fig. 70. K was constructed for 

 Karman-Trefftz blade sections. This type of sec- 

 tion, obtained by a conformal transformation 

 from a circle, has a circular-arc face, a circular-arc 

 back, and a circular-arc meanline. Fig. 70. K can 

 be used, with only a .slight sacrifice in accuracy, 

 for a circular-arc meanline in combination with 

 various thickness forms, such as the NACA 16 or 

 the 60-69 series. It can not be used, however, for 

 any other meanlines. In the design of the propeller 

 for the ABC ship the circular-arc meanline is 

 adopted because, at the time of writing (early 

 1955), the chart shown in Fig. 70. K was the only 

 one available. Similar charts for the following 

 combinations of thickness forms and meanlines 

 are under construction at the David Taylor 

 Model Basin: 



The fifth form listed embodies an NACA 66 

 nose and a parabolic tail, developed by the David 

 Taylor Model Basin to eliminate the objectionable 

 thin trailing edge mentioned earlier in this section. 



The design chart corresponding to the blade 

 section selected, in this case the chart reproduced 

 in Fig. 70. K for Karman-Trefftz sections, is 

 entered with the cavitation number iT(sigma) and 

 the product Cl(c/^x). The latter product is 

 obtained at each radius by dividing C l{c/D) by 

 tx/D] the value of each of these terms has already 

 been determined for the ABC propeller. The 

 cavitation number a is calculated by the formulas: 



(70.xii) 

 (70.xiii'l 



o.5pr 



Y, 



.Yn) 



(70.xiv) 



Cavitation Number (J - ( pc^- e)/0.5/oV ^ 



Fig. 70.K Diagram of Cavitation Critebia fob Cibcular-Arc Camber Lines and Blade Sections 



