714 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.6 



TABLE 74. a Original and Increased Rudder-Area Data on U. S. Naval Auxiliaries of 



THE 1900-1920 Period 



All the vessels listed had single rudders. So far as known, the areas apply to the movable rudder blades only. 



in the future rudders will be made progressively- 

 larger, more effective, or more efficient. In an 

 era when combatant vessels are not the only 

 ones which have to confuse those who are drop- 

 ping or firing missiles from the air this is a logical 

 development. 



W. P. A. van Lammeren, L. Troost, and J. G. 

 Koning in 1948, and P. Mandel in 1953, gave 

 some rudder-area data which furnish a good first 

 approximation for the rudder-blade sizes on 

 modern vessels of a variety of types [RPSS, 1948, 

 Table 13, p. 343; SNAME, 1953, Table 3, p. 482]. 

 The ratios of Table 74. b have been adapted from 

 these tables, utilizing for guidance certain un- 

 published data from other vessels. Conforming 

 to the definitions of Sec. 37.2 the rudder area in 

 each case is taken to be that of the movable 

 blade only. 



There are practical and scientific difficulties in 

 working with tables of the kind represented by 

 74. a and 74. b, even when making a first approxi- 

 mation to the required rudder-blade area: 



(1) The published data imply, but do not state 

 that all the ships (for which the tabulated per- 

 centages are given) possessed adequate or satis- 

 factory steering and maneuvering qualities 



(2) Although they may indicate the length-depth 

 ratio of the rudder blade, they make no mention 

 whatever of its position with respect to the hull 

 or to fixed appendages in its vicinity. They 

 attempt no estimate of the probable proportion 



of transverse force exerted by the movable rudder 

 blade and by the adjacent hull or appendage. 



(3) They take httle or no account of the cutup or 

 cutaway at one or both ends of the hull. In 

 many cases this can be of small amount but can 

 have a relatively large effect. 



(4) If the tables mention other factors, or give 

 data on them, it is found difficult to correlate 

 these factors mth the reported steering or turning 

 performance. 



The projection of the propeller discs below the 

 main hull, mentioned by P. Mandel [SNAME, 

 1953, p. 481] and diagrammed by him in Table 3 

 on page 482 of that reference, is an important 

 feature in maneuvering. However, it could be 

 represented in a more logical and comprehensive 

 manner than that given in the reference. In 

 turning, the detrimental effect of the projecting 

 propeller blades is not related directly to the 

 outline of the maximum section but with respect 

 to the cross flows under the hull ai the propeller 

 positions. This cross flow has not been studied 

 systematically and it is not easy to establish a 

 suitable criterion for the propeller-blade pro- 

 jection. 



Until something better is developed, one may 

 use the area of the propeller disc(s) below a 

 horizontal Une drawn tangent to the under side 

 of the main hull or other large appendage at or 

 close to the fore-and-aft position of the propeller 

 disc(s). While the propeller blades on the inside 



