534 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.22 



peller axis. Were these relationships available it 

 would still be necessary to substitute in them 

 some data assumed for the propeller proposed 

 but not finally selected. It appears sufficient, 

 therefore, to state that the skeg ending slope ds 

 should vary with propeller I'adius at some rate 

 less than the geometric blade angle <j) of the 

 proposed propeller. Table 59. b lists these angles 

 for tenths of blade radii and a rather wide range 

 of P/D ratios. A good working rule, admittedly 

 an engineering compromise without theoretical 

 foundation until the analytic and experimental 

 development is carried further, is to vary the 

 offset termination with propeller radius as 

 sin" (t>. The 6s is then left to adjust itself by 

 proper fairing of the skeg ending into the offset 

 termination. A table listing the variation of </> 

 and sin" with R, for a P/D ratio of 0.98, is 

 given in Fig. 67. T, described later in this section. 



Practical considerations, both structural and 

 hydrodynamic, usually limit the maximum offset 

 of the trailing edge to somewhat less than the 

 half-diameter of the propeller-bearing boss. It 

 is not wise to work too large a hunk of metal into 

 a cast stern frame where the deflected portion 

 joins the boss. Too small a reentrant angle on the 

 inside of the deflected portion of the skeg or 

 stern frame encircling the propeller shaft bearing 

 is not conducive to good flow. 



The geometric blade angle still has an appre- 



ciable value at the tip of any propeller; it is about 

 10.8 deg for a P/D ratio of 0.6. Similarly, the 

 induced velocit}^ kiUr generated at the tip has a 

 small but appreciable axial component. Even 

 when the sin^ </> relationship is used, the skeg- 

 ending slope ds is a little more than zero opposite 

 the blade tips. The reason for using the sin^ <^ 

 function is to avoid offsets which are too large 

 opposite the tips; this is brought out more clearly 

 in Sec. 74.16, in connection with the design of a 

 contra-rudder. Further, if the propeller blades 

 are already heavily loaded at the tips, it is not 

 wise to load them further. On the other hand, 

 A. Betz makes the point ["Zur Theorie der 

 Leitapparate fiir Propeller (The Theory of 

 Contra- Vanes Applied to the Propeller)," Ing. 

 Archiv, 1938, Vol. 9, pp. 435-452; English 

 transl. in NACA Tech. Memo 909, Sep 1939, 

 p. 13] that efficiency is gained by carrying the 

 twist to distances far beyond the propeller radius. 

 This means that a skeg ending need not terminate 

 top and bottom with symmetrical waterlines. 



It is found, in most cases, that when a fair 

 median line is laid out, as in Fig. 67. T, and skeg 

 section lines are drawn on either side by the 

 equal-radii construction shown, there is a shght 

 hollow on the side of the skeg having the smaller 

 curvature. This is arbitrarily filled in to produce 

 a fair surface. 



By the requirements of Table 64. a, item (5), 



Stations 19 ia.75 18.5 



Fio. 67.T Design of Contra-Guide Skeg Ending for ABC Transom-Stern Ship 



