Sec. 7035 



SCREW-PROPELLER DESIGN 



G27 



the hub. A wake-adapted screw for the latter 

 type of wake variation has a pitch distribution 

 with the P/D ratio increasing toward the tip. 

 Just the opposite is obtained in the ABC design. 



70.34 Final Blade-Section Shapes for the 

 ABC Design by Lerbs' Method. The ABC ship 

 propeller, designed here, has hollow blade faces 

 in the outer sections and a mussel shape in the 

 outer radii, similar to many German designs of 

 World War II. Hollow-face sections are unusual 

 for merchant-ship propellers at the time of 

 writing (1955) but if they improve the cavitation 

 performance of a propeller they are worth while. 

 Taking advantage of modern production methods, 

 hollow sections are only slightly if any more 

 difficult to manufacture. By use of the a = 1.0 or 

 a = 0.8 meanlines, the hollow in this case could 

 probably be reduced or eliminated. Since the 

 charts used in this design, shown in Figs. 70. K 

 and 70.M, were not available for the a = 1.0 

 and a = 0.8 meanUnes in combination with 

 suitable thickness forms, the circular-arc meanline 

 was employed and the hollow sections accepted. 

 The design procedure is the same regardless of 

 what meanline or design chart is used. 



The propeller designer is cautioned, however, 

 that screw propellers with hollow-face sections do 

 not perform well when backing; some do not even 

 meet normal needs for routine ship maneuvering. 

 Whether they would in the case of the ABC ship 

 is not determined here. For this and other reasons 

 some propeller designers prefer to use airfoil 

 sections at the inner radii and circular-back 

 sections with straight faces (orthodox ogival 

 shapes) for the outer radii. 



In many cases the hollow can be removed by 

 reducing the camber ratio mxo/c until it is no 

 more than Q.5tx/c, where tx/c is the blade-thick- 

 ness ratio. The loss in lift due to reduction in 

 camber is then compensated for by the addition 

 of an angle of attack aj . The pitch angle 4> be- 

 comes /3/c -f «] -f ofj . 



The added angle of attack needed to compensate 

 the lift for any reduction in camber ratio depends 

 on the meanline. For a circular-arc meanline, the 

 correction is given by Eq. (70.xviii). This formula, 

 when used with a = 1.0 or a = 0.8 meanhnes, 

 introduces only a small error. 



^2 = 2(57. 3)/c 



[4^)] 



(70.xviii) 



where a^ is the added angle of attack in deg 



k is the curvature correction 



[A(?nxo/c)] is the reduction in camber ratio. 



This procedure was tried for the ABC design, 

 but it resulted in an unfair pitch distribution and 

 was considered unacceptable. It is moi'e or less a 

 trial-and-error method, i.e., the camber is reduced, 

 an angle of attack added, and the pitch distri- 

 bution checked. These three items are then 

 adjusted until satisfactory relationships are 

 obtained. Since the ABC ship propeller is an 

 unusual case, the hollow-face sections are accepted 

 rather than to adopt flat sections with an unfair 

 pitch distribution. In the normal merchant ship, 

 hollow sections can be avoided, if desired, by 

 using one of the other recommended meanlines, 

 and by adjustment of the angle of attack and 

 camber ratio as necessary. 



Hollow-face sections, as obtained in this design, 

 undoubtedly will have satisfactory cavitation 

 performance. However, when these sections have 

 thin leading edges, as they would with the 

 NACA 16 thickness forms, they are sensitive to 

 changes in angle of attack, which occur in any 

 wake field due to non-uniformity of flow into the 

 propeller. A blunter leading edge reduces this 

 sensitivity. For this reason, the NACA 65A 

 thickness form [NACA Rep. 903, 1948, pp. 6-7] 

 is used for the ABC ship propeller. It gives the 

 desired leading-edge thickness with only a sUght 

 loss in the cavitation characteristics along the 

 rest of the chord length. If the blade sections have 

 no hollow then the NACA 16 thickness forms are 

 better. 



The NACA 65A thickness form with the 

 circular-arc meanline gives airfoil sections which 

 are curved near the trailing edge. The back or 

 — Ap side is convex to the flow, and there is a 

 concavity on the face or +Ap side. Again this 

 can be avoided by using the a = 0.8 (modified) 

 meanline, in which case the trailing-edge surfaces 

 are straight lines. 



Undoubtedly more desirable blade sections will 

 be obtained with the new design charts, similar 

 to Fig. 70. K, in course of preparation when 

 the ABC project was underway [Eckhardt, M. K., 

 and Morgan, W. B., "A Propeller Design 

 Method," SNAME, 1955, pp. 334-338]. However, 

 the main purpose of this chapter is to outline a 

 method of propeller design. Availability of the 

 new charts will not change the method. 



70.35 Introducing Skew-Back in the ABC 

 Blade Profile. Sec. 70.16 states that a good 



