Page 284 - Introduction to Naval Architecture

MANOEUVRING 269

of wind and tide. Other vessels demanding good positional control are
offshore rigs. This leads to a desire to have the ability to produce lateral
thrusts at the bow as well as the stern. It has been seen that bow rudders
are likely to be ineffective because of their proximity to the neutral
point. The alternative is to put a thrust unit, usually a contra-rotating
propeller, in a transverse tube. Such devices are called lateral thrust units
or bow thrust units when fitted forward. Their efficiency is seriously
reduced by a ship's forward speed, the thrust being roughly halved at
about two knots. Some offshore rigs have dynamic positional control
provided by a number of computer controlled lateral thrust units.

SHIP HANDLING

Several aspects of the handling of a ship are not brought out by the
various manoeuvres discussed above.

Handling at low speed
At low speed any hydrodynamic forces on the hull and rudders are
small since they vary as the square of the speed. The master must use
other means to manoeuvre the ship, including:

(1) Using one shaft, in a twin shaft ship, to go ahead while the other
goes astern.
(2) When leaving, or arriving at, the dockside a stern or head rope
can be used as a pivot while going ahead or astern on the
propeller,
(3) Using the so-called paddle wheel effect which is a lateral force
arising from the non-axial flow through the propeller. The force
acts so as to cause the stern to swing in the direction it would
move had the propeller been a wheel running on a hard surface.
In twin screws the effects generally balance out when both shafts
are acting to provide ahead or astern thrust. In coming
alongside a jetty a short burst astern on one shaft can 'kick' the
stern in towards the jetty or away from it depending which shaft
is used.
(4) Using one of the special devices described above. For instance a
Kitchen rudder, a vertical axis propeller or a lateral thruster.

Interaction between ships
As discussed in Chapter 8 on resistance a ship creates a pressure field
as it moves through the water. The field shows a marked increase in
pressure near the bow and stern with a suction over the central
portion of the ship. This pressure field acts for quite an area around