Page 267 - Introduction to Naval Architecture

10 Manoeuvring

All ships must be able to control their speed and follow an intended
course when in transit. Additionally, when entering congested water-
ways or harbours, they must be able to position themselves accurately.
Vessels used for oil drilling or extraction often need to hold a particular
position relative to the seabed with great precision.
To achieve this a ship must have the means of producing ahead and
astern thrust, turning moments and lateral thrust. The last two are
provided by rudders of various types assisted, in some cases, by lateral
thrust units at the bow and/or stern. Ahead and astern thrust is usually
provided by the main propulsion system as dealt with in Chapter 9 on
propulsion. Because rudders are usually sited close to the propulsors
there will exist an interaction between the two. Where more than one
shaft is fitted, a turning moment can be produced by going ahead on
one shaft and astern on the other.
The ease with which a vessel can maintain a straight course, or be
made to turn, will depend upon its directional stability. Sometimes this
characteristic is known as the ship's dynamic stability but should not be
confused with dynamical stability (see Chapter 5). A number of
measures are used to define the manoeuvring characteristics of a ship
and these are discussed. They are defined and measured in still water
conditions. The influence of wind, waves and current must be allowed
for in applying the data to practical sea-going conditions. Wind effects
can be very important especially in ships with large superstructures
such as cruise liners and ferries. Indeed strong winds may prevent a
ship turning into the wind if it has large windage areas aft. When
operating close aboard another ship, close to a bank, or in shoaling
water, the ship experiences additional forces that may throw it off the
intended course.
A submarine is a special case as it operates in three dimensions which
brings with it a need to control its position and attitude in depth as well
as azimuth. Submarines are dealt with in one section and the rest of the
chapter is devoted to surface vessels.
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