Davis and English 



installed in the sections and, because isolated two-dimensional hydrofoil theory 

 indicates that a positive incidence of the chord of the wetted face is necessary 

 with this type of section in order that cavitation shall not occur on the face, an 

 incidence or pitch increase was also incorporated. 



The model screw employing circular-arc cambered faces that came nearest 

 to satisfying all the design requirements is designated model No. W257, and Fig. 

 10 is a drawing of the 4-ft diameter ship screw. The features of this screw that 

 are worthy of comment are the relatively large root chord lengths and the 

 amount of leading edge sweepback. In addition, it will be seen that the large - 

 diameter hub has a significant amount of taper which is necessary to fair in with 

 the pod shape. These features are common to all the screws that have been con- 

 sidered for propelling the Bras d'Or, the particular chordal distribution and the 

 sweepback being incorporated for structural reasons in an attempt to keep the 

 leading edge stresses within reasonable limits in accordance with the results of 

 the De Havilland's structural testing and analysis. The amount of skew or 

 sweepback is specifically calculated to minimise torsional deflections of the 

 blades. 



The operating characteristics of this screw, as determined from the NPL 

 No. 1 water tunnel with the wake simulator, are given in Figs. 11, 12, and 13, 

 where it will be seen that the peak efficiency at the low cavitation numbers typi- 

 cal of high-speed operation is about 55 percent. This value of efficiency was 

 considered to be lower than might be achieved, and it was therefore decided to 

 continue the development programme in an attempt to improve the performance. 

 Before describing the remainder of this test programme, however, it is perti- 

 nent to digress and explain the reasons for the relatively low efficiency, and de- 

 scribe some relevant work that has been conducted at NPL on a screw not di- 

 rectly intended for the Bras d'Or. 



Johnson has shown in Ref. 11 that practical, fully cavitating, two-dimensional 

 circular-arc sections, in which an allowance for the structural thickness of the 

 foil is made, are, in terms of the lift-to-drag ratio, almost as efficient as the 

 best alternative section, viz., the 5-term section. This is sufficient justification 

 for using a circular-arc section, provided other factors such as structural 

 strength are not impaired. Clearly, in the propeller design process the simple 

 relationships between Cl and camber, and c^ and incidence, that hold in two di- 

 mensions require adjustment when propeller sections are being considered. This 

 adjustment is analogous to the lifting surface corrections required in noncavitat- 

 ing propellers, but it also includes the effect due to the presence of the cavities. 

 In the absence of any numerical information on the magnitude of the corrections 

 that should be applied to the two-dimensional data, it has been necessary to em- 

 ploy a purely empirical correction obtained from a simple analysis of previous 

 test results. This correction was then applied as a factor to both the basic two- 

 dimensional camber and to the incidence values. This led to propeller sections 

 with a moderate amount of circular-arc face camber, viz., about 2 percent of 

 the chord at 70 percent radius, together with a relatively large incidence, giving 

 a pitch -diameter ratio of about 1.26 at the same radius. Limited evidence from 

 the operating characteristics of a number of fully cavitating screws now sug- 

 gests that this method of applying the correction equally to the basic camber 

 and incidence has led to screw W257 having a deficiency of camber and possibly 

 an excess of incidence, and that it would have been more appropriate to apply 



974 



