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Ships and Shipbuilding, the types and construction of any large buoyant type vessel in which people travel or transport goods over the surface of the water. The term boat usually denotes smaller vessels, but no criterion of differentiation is generally accepted. The term shipbuilding is applied to the construction of large vessels. See also Boats and Boatbuilding.
Structurally, a ship is a hollow beam subjected to severe bending and twisting as it moves through waves and as its own load changes. The principles of ship structure and the nomenclature of its parts have long been established and are essentially the same whether the ship is a wooden sailing ship or a large modern tanker. The backbone of all ships is the keel, a longitudinal beam or girder located at the very bottom of the vessel and extending from bow to stern. At the forward end of the keel is attached the upright or nearly upright stem that forms the front of the vessel. A similar sternpost is usually set at the after end of the keel. The ship is given its shape by a series of symmetrically curved ribs or frames that run transversely and that are fastened to the keel at their centers. At and near their centers on the bottom of the ship the frames are made considerably larger than at the sides and are known as floors. The ribs are held in position by longitudinal stringers, or clamps, that run the full length of the ship and that are curved to conform with the shape of the hull. Additional bracing is provided by beams extending across the width of the ship and fastened at either end to the opposite sides of the ribs. In very small vessels only one set of beams, set at the ends of the ribs, is used, and these beams serve as supports for the deck. In larger vessels, several series of beams are employed, the number corresponding to the number of decks with which the ship is provided.
The skin of the vessel is mounted outside the frame. In steel vessels the skin consists of a number of metal plates riveted or welded to the frame, and in wooden vessels it consists of a number of horizontal planks called strakes. The term strakes is also sometimes applied to individual rows of steel plate in a metal hull. Transverse walls of wood or metal plate, depending upon the type of vessel, run from one side of the ship to the other at several places in the length of the hull. These walls, called bulkheads, stiffen the frame and often are used to break the hull up into watertight compartments as a safety measure, so that a leak in the hull will flood only part of the ship and sufficient buoyancy will be maintained by the other compartments to keep the ship afloat.
A number of modifications of the standard method of ship framing have been devised, particularly for use in tankers. Many tankers use a system of longitudinal framing in which a comparatively small number of large transverse frames is employed and the chief framing members are longitudinal and run the full length of the ship. The interior of the tankers built on this system is divided into a number of compartments by a longitudinal bulkhead running the length of the ship at the centerline and by a number of transverse bulkheads. The longitudinal system of construction has been applied to other forms of cargo vessels as well as tankers.
The actual construction of any type of ship, whether made of wood or metal, is complicated by the various curves of the hull, the compound angles formed by the joints between the various structural members, and the necessity of producing a hull that is absolutely symmetrical and "fair" (having regular curves and surfaces that are smooth). Because of these requirements, it is almost impossible to build any form of ship directly from small-scale plans, as is done in the case of other kinds of structures. Instead the shipbuilder, before beginning work, must resort to the practice of lofting, or laying down, the lines of the ship.
Lofting consists essentially of preparing in full size an exact skeleton plan of the hull to be built. From this plan the builder determines the dimensions and forms of the frames and plates, which, when assembled, make up the ship structure. The entire plan of the ship structure is laid down on the flat floor of a special room or building known as the mold loft. Paper or wooden patterns, called templates, of the pieces of the hull are then constructed from this plan and are used in the building shops to cut and form the plates and frames to the required configuration. The more complex the ship, the more detailed are the loft templates. Although the principles of lofting have remained unchanged for centuries, specific techniques have been improved in recent decades. The skeleton plan, instead of being laid down full size, is prepared with great precision to small scale, typically one-tenth of full size. Templates are taken from this plan and photographed, resulting in transparencies less than one-hundredth of full size. The transparencies are either projected onto an uncut piece of construction material and the piece marked off, or the transparencies are used directly by automatic cutting machines. For many parts of the hull, computers may be used to describe the space of the part and the description may be used to control the cutting machine directly.
Contributed by: Howard I. Chapelle
"Ships and Shipbuilding," Microsoftฎ Encartaฎ 97 Encyclopedia.
ฉ 1993-1996 Microsoft Corporation. All rights reserved.
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