One other comparison made in the course of these tests is of general interest. A 

 number of modern single-screw ships, including the MARINER class, have been fitted with 

 a semi-balanced rudder and no rudder shoe. This arrangement enables a larger diameter 

 propeller to be fitted without sacrifice of clearances or, conversely, larger clearances on 

 the same diameter, both very desirable features in view of the large powers now being trans- 

 mitted through a single shaft and the accompanying risk of propeller-excited vibration. In 

 the endeavor to obtain such clearances with the normal single-screw aperture and rudder, the 

 rudder shoe has become longer and more vulnerable, and a number of fractures have occurred, 

 so that on this score also the new arrangement, generally known as a "clearwater stern," ' 

 has something to offer. 



The stern arrangements of the Series 60 parent of 0.60 C g with the normal and clear- 

 water sterns are shown in Figure 58, and the results of the model propulsion tests in Figure 

 59 and Table 41. 



At a service speed of 22 knots, there is no noticeable difference in propeller performai 



The shp is higher for the conventional stern only because of its higher ehp, and this 

 persists throughout the speed range. 



TABLE 41 

 Comparison of Clearwater and Conventional Sterns 



on 0.60 Cg Model 4210 of Series 60 



(Principal dimensions: 600 ft X 80 ft X 32 ft X 26,349 tons) 



Stern Arrangement 



Normal 



Clearwater 



Propeller Number 



2422 



1967 



D, ft 



25.62 



25.33 



P, ft 



25.62 



26.67 



BAR 



0.456 



0.464 



Rake, deg 



6.5 



6.0 



Number of blades 



4 



4 



V, knots 



22.04 



22.04 



shp 



23,250 



22,530 



ehp/shp 



0.77 



0.77 



H 



1.073 



1.089 



% 



0.709 



0J09 



^rr 



1.012 



0.997 



X-20 



