Page 335 - Introduction to Naval Architecture

SHIP DESIGN 319

temperatures according to requirements. The possible effect of the low
temperatures on surrounding structure must be considered. These
vessels are usually faster than general cargo ships with speeds up to 22
knots and up to 12 passengers are carried on some.

Container ships
Containers are re-usable boxes of 2435mm by 2435mm section, with
lengths of 6055, 9125 or 12190mm. They are used for most general
cargoes and liquid-carrying and refrigerated versions are in use. The
latter may have their own independent cooling plant or be supplied
with cooled air from the ship's refrigeration system.
The cargo-carrying section of the ship is divided into several holds
with the containers racked in special frameworks and stacked one upon
the other within the hold space. Cargo handling is by vertical
movement of the containers by a special quayside crane. Containers
may also be stacked on hatch covers and secured by special lashing
arrangements. Cargo holds are separated by a deep web-framed
structure to provide the ship with transverse strength. The structure
outboard of the container holds is a box-like arrangement of wing tanks
providing longitudinal strength. The wing tanks may be used for water
ballast and can be used to counter the heeling of the ship when
discharging containers. A double bottom is fitted which adds to the
longitudinal strength and provides additional ballast space.
Accommodation and machinery spaces are usually located aft leaving
the maximum length of full-bodied ship for container stowage. Cargo-
handling equipment is rarely fitted, since these ships travel between
specially equipped terminals to ensure rapid loading and discharge.
Container ships have carrying capacities from 1000 to 2500 TEUs or
more. The twenty foot equivalent unit (TEU) represents a 20ft (6055mm)
'standard' container. Container ships are faster than most general
cargo ships, with speeds up to 30 knots.
Barge carriers are a variant of the container ship. Standard barges
are carried into which the cargo has been previously loaded. The
barges, once unloaded, are towed away by tugs and return cargo barges
are loaded. Minimal or even no port facilities are required and the
system is particularly suited to countries with extensive inland
waterways.

Roll on/roll off ships
These vessels are designed for wheeled cargo, usually in the form of
trailers. The cargo can be rapidly loaded and unloaded through stern
or bow doors and sometimes sideports for smaller vehicles. Some have
been adapted to carry containers.