734 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.17 



Elevotion, Looking Aft 

 £6' DWb^ 



Stations 20 19.5 19.33 



:.N Arrangement and Details of a Contha-Hokn and Rudder for ABC Transom-Stern 



Ship 



ft. However, the leading-edge offsets of the horn 

 above the shaft axis and the balance portion of 

 the rudder foil below it were left unchanged. This 

 necessitated a short length of reverse curvature 

 (inward) just above the hub fairing, shown in the 

 end elevation at the left of Fig. 74. N. 



The maximum median-lme slope at the leading 

 edge of the contra-fairing on the horn, just above 

 the hub fairing, is 22.5 deg. This is considered 

 acceptable because of the large angle djt Avith 

 which the water leaves the propeller-blade 

 elements in this region. Separation is not a problem 

 here provided the mcident flow from the blade 

 elements at the root strikes the leading edge of 

 the horn at about this angle. 



The fixed horn and the tail of the rudder (when 

 at zero angle) are considered as a smgle assembly 

 when laying out the section outlines for the 

 horn. The thickness ratio at each level is governed 

 by (1) the profiles of the rudder and horn, which 

 establish the chord length c, and (2) by the 

 necessary thickness for the horn, to give it 

 structural stiffness. The latter determines the 

 value of tx . No section designations or sets of 

 coordinates are specified or recommended here 



because there are a considerable number of good 

 ones from which the designer may choose [Mandel, 

 P., SNAME, 1953, pp. 486-488]. 



An important feature of section outlines for 

 compound rudders, similar to the one shown in 

 Fig. 74.N, is the shape of the extreme nose. This 

 statement holds even though contra-guide sec- 

 tions are worked into the forward or fixed portion. 

 A shape too nearly circular leads to cavitation or 

 separation along the sides of the section, a little 

 abaft the nose, especially if the radius is large. 

 A nose that is too pointed may also be subject to 

 cavitation or separation on the "lee" side, when 

 there is no contra-shape and when the angled 

 flow from a propeller ahead strikes the leading 

 edge of the rudder at a large angle with the 

 meanline plane through the rudder. A practical 

 example of this is described in the last paragraph 

 of Sec. 74.16. 



M. Kinoshita and S. Okada show the results of 

 measurements on models made by M. Yamagata, 

 in which the flow just below the propeller hub 

 makes an angle of over 50 deg with the meanline 

 plane of the rudder [Int. Shipbldg. Prog., 1955, 

 Vol. 2, No. 9, Fig. 8, p. 238]. It is clear that, as 



