Page 322 - Introduction to Naval Architecture

SHIP DESIGN 307

consequence. For instance, as long as enough oil is arriving in port
each day it does not matter to the customer how long it has been on
passage. It is important to the ship owner and a balance is needed
between speed, size and number of ships required to achieve the
desired flow rate economically.
For high speed ships wavemaking resistance is a major factor and the
design will have a finer form. At low speeds frictional resistance will
dominate and fuller, bluffer forms can be used with greater cargo
carrying ability on a given length. When considering speed, allowance
must be made for the average voyage conditions expected. Two ships
capable of the same still water maximum speed may differ significantly
in their ability to maintain speed in rough weather. The conditions that
might restrict a ship's speed were discussed under seakeeping.

Manoeuvrability
Manoeuvrability is not too important for a ship in the open ocean. In
restricted waters it can be critical. Stopping distances of the huge
supertankers are very large. Astern power must be adequate to give the
desired deceleration. A balance must be struck between giving a ship
very good manoeuvrability and relying upon tugs for assistance in port.
What is meant by good manoeuvrability and means of providing it in a
ship were discussed in Chapter 10 on manoeuvrability. Twin shafts,
azimuthing propellers and lateral thrust units are some of the means,
These cost money and the cost must be set against the cost and delays
of using tugs, remembering they might not always be available when
needed. Ferries which frequendy berth and unberth will normally be
designed to operate without the assistance of tugs except in exceptional
weather conditions. For long haul ships providing a high degree of
manoeuvrability could be uneconomic.
Floating oil drilling rigs require exceptionally good abilities in
maintaining their position relative to the ground and for that reason
they are provided with dynamic positioning systems. A series of
thrusters under computer control are constantly correcting the
position against the effects of wind, current and waves. In such vessels
fin stabilizers are of little use for reducing roll and some form of tank
system would be fitted if needed. Mine countermeasure ships also need
to be able to maintain an accurate path over the ground if a suspected
minefield is to be swept with the minimum number of passes and in
maximum safety.

Machinery
The power installed depends upon the speed required under typical
operating conditions and the hull form, as discussed in Chapter 9 on
propulsion. The type of machinery chosen will reflect a balance