If  the  approximate  development  like  that  of  Fig.  184  is  preferred,
the  construction  just  given  may  be  made,  producing  the  triangle  BIJ,
The  half-breadth  IJ  may  then  be  laid  off  on  a  horizontal  line  through


fnaie  the


"•^"•*crr-''^^.'.ru„,


"'^'-"%^'ii'^^-o,


446


N/lk'/IL  ARCHITECTURE.
makes  an  angle  such  as  GCK  with  the  vertical  line  KC.     The  pro-
jection  on  a  transverse  plane  is  not  affected,  but  is  still  represented


Fig.  191.
hv  FIr.  188.  The  projection  of  the  same  blade  on  a  plane  parallel
t\>  the  shaft  is  obtained  by  laying  off  the  half- widths,  like  EF,  Fig.  189,
{www  the  inclined  line  GC.    To  get  the  end  projection  of  the  other


PROPUlSfON  OF  SHIPS,


4471


blade,  draw  RS  in  Fig.  190  parallel  to  GC^  and  in  place  of  O  take  0\
the  projection  of  F  on  OS;  draw  the  line  O'/i;  since  both  J?  and  O'
are  shifted  to  the  left  the  same  distance  the  line  O'R  makes  the  same
angle  with  OS  as  does  OR  of  Fig.  189.  The  half-breadth  07  is
now  made  equal  to  DF  of  Fig.  188.
Fig.  191  gives  a  section  of  a  blade  by  a  plane  through  the  shaft
and  the  middle  of  the  blade,  which  here  has  a  rake.  Fig,  192  gives
the  sections  of  the  blade  by  planes  at  DF  and  corresponding  places.
The  four  outer  sections  are  flat  on  the  acting  face  and  are  rounded
on  the  back;  the  inner  section  is  slightly  rounded  on  the  face  also.
