Sec. 74.4 



MOVABLE-APPENDAGE DESIGN 



711 



be taken for granted. A separate calculation or 

 model test is called for to cover tliis condition. 



An L-shaped or J-shaped rudder hung behind 

 a thin hull or skeg should not have an excessively 

 long forward balance portion, indicated in dia- 

 grams 1 and 2 of Fig. 74. Ca. When such a rudder is 

 swung hard-over, say to the right, the forward 

 end of the balance portion savings to the left or 

 to port. The region of 4-Ap on the ahead or 

 starboard side of the balance then lies under the 

 port side of the skeg. In normal circumstances a 

 large — Ap is being developed here to augment 

 the transverse force to port. Although, so far as 

 known, no specific studies have been made on 

 this point it appears that the presence of the 

 -|-Ap and — Ap regions so near each other is 

 detrimental to maneuvering. 



R. E. Barry reports a situation similar to this 

 on a French cruiser, as indicating a rudder shape 

 and position to be avoided in practice ["Random 

 Notes by an Old Seaman," Mar. Eng.'g., Feb 

 1921, p. 137 and Figs. 8, 9]. This vessel had a 



Negative Differential Pressure -6p\ 

 on This 5idc of Leodino Edqe 

 of Balance Portion 



+Ap an Rudder, 



Elevation of Toil of 



Blade, When Swung 



j^\.o Near Side 



in This Region, Especially 

 When 6alonce Portion is 

 Excessively Lono 



+Ap Field on For Side of Skcq, 

 Opposing the Desired Turn 



Leading Edge/ ot Zero Angli 



+Ap Field Extended from Balance Por- 

 tion to Sl^eg Ahead, of Such Intensity 



and Extent as to Overpower the Rudder; 

 Causing 5htp to Turn 

 in Wrong 

 Direction^ 



Situation 

 Remedied by 

 Cutting Off 

 Balance 

 Portion and 

 Plating Up 

 Opening as 

 Shown 

 Broken 

 Lines 



Mp Field Extended from Balance Por- 

 tion to Skeg Ahead, of Such Intensitvj 



and Extent as to Overpower the Rudder; 

 Causing Ship to Turn 

 in Wrong ^,-^<S^C'^ 

 Direction,^'-''^;^^ \ \ 



Profile of Stern and Rudder for Ship as Originall\; Built 



Profile of Ship 

 and Rudder after 

 Alterations to 

 Correct Reversed 

 Rudder Effects 



+Ap's Developed on 

 Starboard Side 

 of Balance PortI 

 Swung to Port 



+Ap's Acting to 

 Starboard on Port 

 Side of Skeg 



z 



Fig. 74.Ca Effect of Pressure Field Exerted by 

 THE Balance Portion of a Rudder 



Fig. 74.Cb Example op Reversed Rudder Action 



Due to Pressure Field Exerted by the Balance 



Portion of a Rudder 



balanced rudder swinging in a close-fitting aper- 

 ture, sketched at 1 and 2 in Fig. 74.Cb. When 

 steermg it was found that the ship swung iia the 

 opposite direction to that in which the rudder was 

 angled. According to Barry this showed ". . . that 

 the banked up pressure on the side (indicated by 

 the four right-hand vectors in diagram 2 of the 

 figure) was greater than on the rudder. When the 

 balance was cut off and the aperture plated up, 

 the vessel steered satisfactorily." 



A situation such as that described in the pre- 

 ceding paragraphs is avoided if there is an aperture 

 of appreciable area forward of the rudder blade. 

 A rudder and stern embodying this arrangement 

 are diagrammed at 4 in Fig. 74.D. Actually, to 

 obtain the rapid response to rudder action dis- 

 cussed in Sec. 74.18, the aperture ahead of the 

 rudder is made rather large compared to the 

 blade area, mdicated by diagram 3 of Fig. 74.D. 



A small aperture, fitted on some older vessels 

 abreast the tips of wing propellers and sketched 

 in diagram 2 of the figure, is too small to afford a 



