CHAP. xill. STEAM  PROPULSION. 555

experience  conjSrms  these  experiments  and  shows  that  a  partial
emersion  of  a  screw  results  in  serious  diminution  of  thrust,
frequently  accompanied  by  racing  of  the  engines  imless  they
are  fitted  with  "  governors."

These  remarks  on  the  comparative  efficiency  of  single  and
twin  screws  have  been  carried  to  some  length,  because  of  their
important  bearing  upon  the  future  of  steam  navigation. The
constant  additions  which  are  being  made  to  the  sizes  and  speeds
of  ships  make  it  necessary  to  use. greater  and  greater  powers ;
and  the  question  arises  whether  it  is  desirable  to  pui  all  this
power  upon  a  single  shaft  and  a  single  screw,  or  to  duplicate
the. machinery  and  the    propellers. The  risks  run  through
accidents  to  the  shafts  or  propellers  of  single-screw  steamers
receive  too  frequent  illustration  in  practice  to  need  comment
here ;  and  the  proportionate  decrease  in  sail-power,  now  generally
accepted  in  the  larger  steamships,  makes  the  consequences  of
disablement  of  the  propelling  apparatus  more  serious  than  they,
were  formerly. Even  if  it  could  be  shown  that  twin-screws  were
less  efficient  propellers  tl^an  single-screws,  their  advantages  in
other  respects  would  recommend  them  for  adoption  at  least  in
the  larger  classes  of  ocean-going  merchant  steamers. There  is
far  less  risk  of  total  disablement  of  the  propelling  apparatus  and
with  either  screw  at  work  a  twin-screw  ship  is  not  merely  under
control,  but  able  to  make  good  headway. Twin-screws  give
greater  handiness  to  a  ship,  and  enable  her  to  be  manoeuvred
in  case  of  serious  damage  to  the  rudder  or  steering  gear. The
duplication  of  engines  and  propellers  also  enables  the  water-
tight   subdivision    of   the    engine-rooms    to    be    increased,  as
explained  on  page  26. Against  these  undoubted  advantages
are  to  be  set  the  following  considerations: — That  twin-screws
would  be  more  liable  to  damage  than  single-screws  when  ships
are  going  into  or  out  of  docks,  coming  alongside  wharves,  or
taking  the  ground. That  more  space  might  be  required  for  the
machinery  and  shaft-passages,  and  a  larger  engine-room  staff
found  necessary  because  of  the  duplication  of  the  machinery ;
while  the  weight  and  cost  of  the  twin-screw  engines  might  be
greater  than  those  for  single-screw  engines. 