Sec. 74.7 



MOVABLE-APPENDAGE DESIGN 



719 



to, but will usually differ from the total movable 

 area tentatively assumed when the blade was 

 first sketched. Adjustments are made to the area 

 or shape or aspect ratio, one or all, and a second 

 hydrofoil calculation is made, similar to the first. 



For the case of the compound or flap-type 

 rudder the exact value of the ratio of the movable- 

 blade lateral force F^ to the hull lateral force F,f , 

 and the exact value of the lift coefficients used to 

 calculate the movable-blade force, depend upon 

 the tightness assumed for the hinge closure. Un- 

 fortunately the effect of actual hinge gaps and 

 hinge leakage is not yet assessed in the full scale. 

 Data from aeronautical tests require some analysis 

 before they are applicable to the ship problem. 



Similar comment applies to the assessment of 

 the effects of horizontal gaps between parts of the 

 rudder and the hull, and a determination of the 

 proper effective aspect ratio for each part of the 

 rudder. Certain useful data derived from tests of 

 low-aspect-ratio hydrofoils are included in Sec. 

 44.3. 



In the event that the maximum turning moment 

 is required in a steady turn, to meet certain 

 operational needs, the physical action on both 

 rudder and ship is no longer simple or well known. 

 The design procedure is not yet outlined and may 

 not be for some time to come. 



As an indication of the absolute magnitude of 

 the moments required to maneuver a ship there 

 are available the results of model tests in which 

 the rudder was angled by various amounts while 

 the model was constrained to travel in steady, 

 straight-ahead motion. This corresponds, as pre- 

 viously described, to the few moments after a 

 signal to turn is given but before a ship has time 

 to change direction from its straight approach 

 path. Among these tests are some conducted by 

 G. H. Bottomley at the NPL, Teddington 

 ["Maneuvering of Single-Screw Ships: The Effect 

 of Rudder Proportions on Maneuvering and Pro- 

 pulsive Efficiency," Inst. Civ. Engrs., London, 

 1935, No. 175]. The model represented a ship 

 having the following characteristics: 



Lpp , 400 ft Cp , 0.70 



B, 52 ft CG assumed at midlength 



H, 23 ft between perpendiculars 



A, 9,400 t CG to rudder stock, about 



V, 14 kt 200 ft 



Pe , 2,200 horses Area A « of standard rudder, 



Ps , 3,160 horses 144 ft^ 



An/[L{H)] = 0.016. 



The model was run with the usual fixed rudd('r 

 post of rectangular section ahead of the rudder, 

 then with four fixed fins of uniform thickness 

 added abaft the post (and ahead of the rudder), 

 representing full-scale lengths of 0, 2.08, 4.16, 

 and 6.25 ft. The combined full-scale fin-and- 

 rudder area was always 144 ft^ indicated in 

 Fig. 1 of the reference. The area of a fairing of 

 varied width added ahead of the rudder post, as 

 well as the area of the post itself, was not included 

 in the nominal fin-and-rudder area. 



On the basis of these test data Bottomley then 

 calculated the fore-and-aft length of four hy- 

 pothetical rudders which, when placed behind the 

 four fins mentioned, would give the same initial 

 ship-turning moment at small rudder angles 

 (5 deg) as the original rudder (called R4) when 

 mounted abaft the rectangular-section rudder 

 post. If four rudders of these lengths had been 

 mounted abaft the four fins mentioned, horizontal 

 sections through the assemblies would have 

 appeared as sketched in Fig. 74. F. Assuming that 



\rn R ^ Behind Rudder 

 ^^ Post Only 



Fig. 74.F Ruddeb-Fin Assemblies of G. H. 



Bottomley 



By calculation, all five of these assemblies give the same 



initial ship-turning moment for small rudder angles 



of the order of 5 deg. 



Bottomley's calculations were correct, and that 

 the scale effects in the model tests were insig- 

 nificant, each of the five assemblies appearing in 

 the figure would give an initial turning moment 

 of 2,400 ft-tons on the ship used as the basis of 

 the tests, reckoned about the CG. 



The values of this moment at rudder angles of 

 15 and 35 deg are given in Table 74. c, copied from 

 the Bottomley reference, together with the torques 

 that would have been required to hold the rudders 

 in those positions. Considered as a design refer- 

 ence, the tabulated ship-turning moments are 



