682 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 7B.S 



73.8 Layout of Contra-Struts Abaft Pro- 

 pellers. If the propeller bearing and the barrel 

 carrying it are both mounted abaft the propeller, 

 as in the ABC arch-stern design, the procedure for 

 laying out contra-struts to hold the bearing barrel 

 or strut hub follows in general that described in 

 Sec. 74.16 for a contra-rudder. There is, however, 

 a much smaller background of experience for 

 struts as to acceptable limits for median-line 

 angles, twist offsets, and the like. 



In the design of the ABC arch-stern struts, it 

 was evident at an early stage that, because of the 

 large barrel radius, 2.25 ft, a reference value of 

 0.85 times this radius at O.liZMai of the propeller 

 was much too large, in that the median-line slopes 

 at the leading edges of the struts became excessive. 

 Falling back upon the designer's judgment, as 

 must often be done, this ratio was halved. The 

 basic offset is therefore (0.85/2) 2.25 ft or 0.9562 

 ft, listed at the bottom of the table on Fig. 73. F. 



Even so, the median-line slope of the leading 

 edge at 0.2Ru„^ is 32.5 deg, although this could 

 be reduced somewhat by changing the shape of 

 the median line. The remaining offsets are deter- 

 mined by using the sin' relationship described 

 in Sec. 07.22. Details of the resulting design of 

 contra-strut and hub assembly, with a heavy belt 

 plate in the shell abreast the propeller and strut 

 arms, are drawn in Fig. 73. F. 



A considerable number of references relating to 

 contra-propellers and guide vanes, both forward 

 of and abaft a screw propeller, are listed by 

 W. P. A. van Lammeren [RPSS, 1948, References 

 136 and 137 on pp. 299-300]. 



73.9 The Design of Bossings Around Pro- 

 peller Shafts. The first step in the design of 

 any bossing, whether long or short, or of the 

 straight (fairing) or deflection type, is to position 

 the screw propellers to be carried by it. This 

 matter is covered in Sees. 67.23 and 69.3. When 



Arch Section at Midlength of Strut Arms, S'.'ib /^boft Propeller Ceni 



Rodlol PositI 



Double Extra-Heavy Shell Plate- 

 in WQv of strut Arms ond 



Pr 



opeller 



eg Slope I _ 



- ,/o:3V3/7^/X^EIIipt''i col' Contou?^— Typicol Strut-Arn 

 /l,n/^///////uyy,,,^,^/ . /, ^J!^ 5,rtlon Before 



5.'0 Plus A llowonce for Bendi ng Bending 



Fig. 73. F Layout of Quadruple Strut Arms and Hub for ABC Arch-Stern Ship 



