ACTION OF RUDBEK AND PROPELLER 293
starboard than in the other direction. When necessary to turn short
round put the rudder to starboard, and the engines full speed ahead.
The screw race will press against the rudder, even if the vessel has no
headway, and she will cant to starboard. Eefore she gathers too much
headway the engines should be reversed to full speed astern, the helm
being shifted accordingly so as to obtain the benefit of the suction current.
Before she gathers too much sternway go full speed ahead again with
rudder to starboard and so on, alternately, until round.
It is advisable to have an anchor ready for dropping when turning
short round in narrow channels and to know that there is sufficient
depth of water when manoeuvring close to the banks.
TWIN SCREWS
The effects of twin screw propellers are not so complicated as those
of single screws. It is only necessary to take into account (1st) the
current caused by the screw, and to consider whether it is a discharge
current acting against the fore side of the rudder, or whether it is a
suction current drawn in against its after-side: (2nd) the transverse
thrust of the screws.
As the screws revolve in opposite directions* when both are going
ahead at the same speed, there should not be any turning effect from
the transverse thrust of the screws, and if the helm is ported or star-
boarded they will assist the action of the rudder Also when the screws
are going astern, the current drawn in by the propellers acts on the after-
side of the rudder, and if the ship is also moving astern they assist the
action of the rudder.
To turn a twin screw steamship short round.—This is done by going
ahead on one and astern on the other; the bow of the ship then turns
towards that side on which the screw is going astern. By regulating
the speeds of the propellers' so as to prevent the vessel getting headway,
the steamer can be made to turn round in her own length. When
turning short round in this way the helm should be kept amidships
If the starboard screw is right-handed and the port one left-handed,
the transverse thrust of both screws will assist in turning the ship short
round in either direction.
TURNING CIRCLES.
Very meagre information seems to be available regarding the
turning circles of merchant ships and the time taken by them to
*The starboard one is generally right-handed and the port one left-handed.