Sec. 70 Jl 



SCREW-PROPELLER DESIGN 



621 



Substituting in Eq. (70.x) 

 (908) (3 ,3 12 .5) 



(4.123)(97.2)(20)'(/o/^) 



+ 



= 6,000 + 



(20)\97.2)' 

 12,788 

 whence ta/D = 0.053. 



The radial distribution of the maximum thick- 

 ness of the blade elements is obtained from the 

 following equation, given by J. D. van Manen 

 and L. Troost ["The Design of Ship Screws," 

 SNAME, 1952, Fig. 11, p. 453]: 



D^ 



+ /I 



(h 'tjp ] 

 D D J 



(70.xi) 



where tx/D is the ratio of the maximum blade- 

 section thickness to the diameter at any 0-diml 

 radius a;' and t-nJD is assumed as 0.003, a typical 

 value. 

 Values of / for the various radii are: 



x' = R/Rm„ 0.2 0.3 0.4 



/ 0.788 0.665 0.551 



0.5 

 0.443 



0.6 

 0.344 



0.7 

 0.251 



0.8 

 0.162 



0.9 

 0.079 



1.00 

 



The calculations for tx/D values are made in 

 accordance with Eq. (70. xi) and the values are 

 set down in Col. B of Table 70.h. 



Any method of strength calculation may be 

 used in this phase, as long as the required tx/D 

 ratio at each radius is obtained. A designer is by 

 no means restricted to using the formula shown in 

 this section. The strength of the propeller is 

 related definitely to the design problem, but is 

 independent of the fundamental propeller theory. 



70.31 Blade-Section Shaping by Cavitation 

 Criteria. It is now possible, by using cavitation 

 criteria, to determine the camber mx of the mean- 

 line, the chord length c, and the lift coefficient of 

 each blade element. This is done by the use of 

 charts; Fig. 70. K is one of them. 



To use these charts it is first necessary to 

 decide on the blade-section shape. Modern airfoil 

 shapes are found as satisfactory as any. They 

 are obtained by superposing a given set of thick- 

 ness ordinates on a given meanline. What happens, 

 in effect, is that a selected airfoil sectipn, with a 

 straight base chord through its midwidth, is 

 bent until this base chord becomes the selected 

 curved meanline. In this way, all cambered airfoil 

 sections are transformed from symmetrical sec- 

 tions. 



The same shape is used for all blade sections 

 from the root to the tip. Good blade sections are 



obtained with the NACA 16, 66, 64A, and 65A 

 thickness forms. These, in combination with one 

 of the meanlines recommended in the following 

 paragraph, give airfoil sections which have low 

 drag-lift ratios and uniform distribution of 

 pressure along the chord length. For the latter 

 reason they have favorable cavitation charac- 

 teristics. The four recommended thickness forms 

 are listed in the order of preference, although 

 there is little difference among them. The NACA 

 66 form has zero thickness at the trailing edge; 

 therefore, for use on marine screw propellers it 

 must be modified slightly to give finite thickness 

 at that edge. The other thickness distributions can 

 be used directly without modification. The NACA 

 16 thickness form has performance characteristics 

 similar to those of the TMB EPH section. The 

 latter is a combination of an ellipse at the nose, 

 two parabolas along the two sides, and a hyperbola 

 at the tail; it derives its name from the first 

 letters of these three types of curve. It is an 

 efficient thickness form but unfortunately there 

 is no EPH design chart available, corresponding 

 to those for the NACA sections. A propeller 

 designer should use it with caution because the 

 eddying abaft its trailing edge might cause the 

 blade to flutter or to sing. Additional comments 

 on thickness forms are made in Sec. 70.34. 



Meanlines commonly used with the thickness 

 forms listed are the circular-arc and the so-called 

 a = 1.0, a = 0.8, and a = 0.8 (modified) meanline. 

 The "a" meanlines have uniform chordwise 

 pressure distribution from the leading edge to the 

 point designated by a = x/c, where a; is a distance 

 from the leading edge. From this point to the 

 trailing edge the load decreases linearly. The 

 a = 0.8 (modified) meanline has slight curvature 

 in the decreasing portion of the load curve. 

 Because of this pressure distribution the "a" 

 meanlines have good cavitation characteristics. 

 Also when combined with a given thickness form 

 they give blade sections with less hollow on the 

 face than if the same thickness form were used 

 with the circular-arc meanline. The a = 0.8 

 (modified) meanline is usually associated with the 

 NACA 6A-series airfoils. Complete data for the 

 NACA 16 and 66 thickness forms and the a = 1.0 

 and a = 0.8 meanlines are given in NACA Report 

 824, 1945. The data for the NACA 64A and 65A 

 thickness forms and the a = 0.8 (modified) mean- 

 line are given in NACA Report 903, 1948. The 

 exact process for combining a meanline and a 

 thickness distribution to obtain an airfoil section 



