Phases  of  Steering, — In  the  preceding  discussion  the  helm
considered  to  bc^  put  over  suddenly;  even  with  powerful  steam
steering-engincs  some  time  is  taken  for  this  manoeuvre.  The  time
required  for  this  is  called  the  phase,  or  period  of  manwuvring^  Dur
ing  this  time  the  ship  acquires  considerable  motion,  both  of  drift  and
of  swinging;  the  consequence  is  that  the  water  does  not  strike  the
rudder  at  the  angle  of  the  tiller  «,  but  at  a  less  angle  a—^,  and  the
ship  also  loses  some  headway*

After  the  helm  is  put  over  to  a  certain  angle  the  ship  continui
to  turn  more  and  more  quickly,  until  it  finally  proceeds  at  unifo


2l\.
STEERING  AhID  MANCBU^^RING.  SSS

speed  in  a  circle.  There  are  then  two  other  phases  or  periods,  the
period  of  variable  motion  and  the  period  of  uniform  motion.
The  period  of  manoeuvring  for  ships  that  are  stepred  by  power
is  always  short.  It  is  interesting  since  the  greatest  moments  on  the
rudder-head  are  then  experienced.
The  period  of  variable  motion  is  really  of  the  greatest  practical
importance,  since  it  extends  over  a  time  covering  the  turning  of  the
ship  through  90°  to  180°  if  not  more.  A  ship  seldom  changes  course
by  an  angle  so  large  as  a  right  angle,  especially  when  at  high  speed.
The  period  of  uniform  motion  in  a  circle  is  most  easily  and  there-
fore most  often  studied;  its  study  is  important,  since  the  diameter
of  the  circle  of  uniform  motion  is  a  fair  indication  of  the  general
manoeuvring  power  of  the  ship.
