Passing  from  these  general  considerations  respecting  pro-
pulsion by  sails  to  the  practical  problems  which  the  naval
architect  has  to  solve  in  determining  the  sail-spread  appropriate


494


NAVAL  ARCHITECTURE.
CHAP.  XII.
to  any  new  design,  it  becomes  necessary  to  note  an  important
distinction.  In  all  his  calculations  the  naval  architect  is
accustomed  to  deal  only  with  j^in  sail  or  working  sail,  and  not
to  include  all  the  sails  with  which  a  ship  may  be  famished.
Plain  sail  may  be  defined  as  that  which  would  be  commonly  set
in  a  fresh  breeze  (Force  5  to  6),  which  is  usually  assumed  to
correspond  to  a  pressure  of  about  1  lb.  per  square  foot  of  canvas.
The  following  tabular  statement  shows  concisely  what  sails
would  generally  be  included  in  the  plain  saU  of  various  classes
of  ships;  and  although  the  sails  not  included  are  of  value,
especially  in  light  winds,  yet  it  will  be  obvious  that  those  named
in  the  table  are  very  much  more  important.
style  of  Rig.
Plain  sail.
bhip      ..
Jib,  fore  and  main  courses,  driver,  three  topsails,
three  top-gallant  sails.
and

Barque..
As  ship,  except  galf-topsail  on  mizen-mast

Brig     ..
As  ship,  exclusive  of  mizen-mast.
Schooner.
Jib,  fore  stay-sail,  fore-sail,  and  main-sail.
Cutter ..
Jib,  fore-sail,  and  main-sail.
NoUs  to  Table,

In  brigs,  one-half  the  main  course  and  the  driver  are  sometimes  taken  instead
of  the  whole  of  the  main  course.
In  schooners,  the  fore  topsail  is  sometimes  included.
In  yawls,  besides  the  sails  named  for  cutters,  the  gaff-sail  on  the  misen  is
included.
It  will  be  understood  in  what  follows  that,  except  in  any  cases
specially  mentioned,  we  are  dealing  only  with  plain  sail,  and  not
with  total  sail  area.
