Page 332 - Introduction to Naval Architecture

SHIP DESIGN 317

MERCHANT SHIPS

It has been seen that the development of merchant ship types has been
dictated largely by the nature of the cargo. The range of ships are listed
15
in a number of publications. An annual publication of the RINA
reviews the more interesting and significant ships of the previous year.
Designs can be divided broadly into general cargo, bulk cargo and
passenger vessels.
The general cargo carrier is a flexible design of vessel which will go
anywhere and carry anything. Specialist forms include container ships,
roll on/roll off ships and barge carriers. Bulk cargoes may be liquid,
solid or liquefied gas. Particular designs of vessel exist for the carriage
of each. Passenger-carrying vessels include cruise liners and ferries.

General cargo ships
The general cargo ship has several large clear open cargo-carrying
spaces or holds. One or more decks may be present within the holds.
They are known as 'tween decks and provide increased flexibility in
loading and unloading, permit cargo segregation and improved
stability. Access to the holds is by openings in the deck known as
hatches.
Hatches are made as large as strength considerations permit in order
to reduce the amount of horizontal movement of cargo within the ship.
Hatch covers are, nowadays, made of steel although old vessels used
wood. They must be watertight and rest upon coamings around the
hatch. The coamings of the upper or weather deck hatches are raised
above the deck to reduce the risk of flooding in heavy seas.
A double bottom is fitted along the ship's length, divided into various
tanks. These may be used for fuel, lubricating oils, fresh water or ballast
sea water. Fore and aft peak tanks are fitted and may be used to carry
ballast and to trim the ship. Deep tanks are often fitted and used to
carry liquid cargoes or water ballast. Water ballast tanks can be filled
when the ship is only partially loaded in order to provide a sufficient
draught for stability, better weight distribution for longitudinal
strength and better propeller immersion. There is usually one hold aft
of the accommodation and machinery space to improve the trim of the
vessel when partially loaded. General cargo ships are typically from
2000 to 15000 tonne displacement with speeds from 12 to 18 knots.

Refrigerated cargo ships
A refrigeration system provides low temperature holds for carrying
perishable cargoes. The holds are insulated to reduce heat transfer.
The cargo may be carried frozen or chilled and holds are at different