SHIP CONSTRUCTION
Ship construction as we find it today is the result of centuries of experience. The main structure of a ship is composed of a multitude of parts that are for strength, watertightness and safety. No space can be wasted and weight must be kept as low as possible, the structural design and arrangement has developed in such way that most of the parts serve at least two purposes and sometimes more.
A ship may be considered as a huge box girder, the sides of which are composed of the shell plating and the decks. These parts are in turn strengthened by such members as the keel, frames, beams, keelsons, stringers, girders, and pillars. To appreciate the ship as a whole, it is good to understand the functions of each of the parts.
The keel is the backbone about which the ship is built.
It is of a rigid fabrication of plates and structural shapes which run fore and aft along the centerline of the ship. At the forward end is connected the stem, and at the after end the stern frame which supports both the rudder and the propeller.
The frames are the ribs of the ship. Their lower ends are attached at intervals along the keel, and their upper ends are attached through brackets to the beams which support the deck. Internal bracing is provided by keelsons and stringers which run fore and aft. The frames must determine the form of the ship and support and stiffen the shell plating.
The shell plating, although necessary for watertightness, is one of the principal strength members of the ship. Running continuously from the stem to the stern frame and from the keel
to the weather deck, it forms three of the sides of the box girder. The plating, along with frames, must be able to withstand the pressure of the water outside and the stresses which arise due to the waves or rubbing against a dock.
The main deck of the ship forms the fourth side of the girder and the reason is that it must be of strong construction. The plating is connected to beams which extend from side to side across the ship. The deck is specially strengthened by doubling plates in the regions where it is weakened by openings such as hatchways, and companionways, and also under all deck machinery, chocks, and bits. The deck is supported from below by girders and pillars.
The bottom, sides, and main deck of the ship, would not be strong enough to stand the stresses of an ocean voyage without some internal stiffening. This is provided by the lower decks and the main transverse bulkheads.
In addition to support for the shell and decks, the main transverse bulkheads are made watertight, and subdivide the vessel into watertight compartments, so that in the event of
damage, the water can be confined. All doors through these bulkheads must be fitted with gaskets so that they can be made watertight, and must be kept clear at all times so that they can be closed. The first bulkhead aft of the stem is known as the collision bulk- head as its purpose is to limit the flooding that might occur after a collision. No doors or other openings are permitted in this bulk head below the main deck.
Further protection against damage is provided by the double bottom tanks. These are formed by a second complete layer of watertight plating located a few feet above the outer bottom and extending from bilge to bilge. Any grounding or similar damage which merely pierces the bottom plating will flood one or more of these tanks instead of allowing water to enter one of the main holds. Under ordinary service conditions these tanks are used to carry fresh water, fuel oil or salt water ballast.
The engine and boiler rooms are usually located amidships. These necessary for the support of the engines and boilers. In order to provide sufficient headroom for the propelling machinery, it is usually necessary to omit one or more of the decks. To maintain the vessel's strength in the absence of these decks, several extra heavy web frames and transverse hold beams are fitted.
The propeller shaft extends through the after holds from the engine to the stern gland. As this must be accessible at all times for inspection and lubrication, it is enclosed in a narrow tunnel known as the shaft alley. The entrance to the shaft alley from the engine room is closed by a watertight door, and the sides are of watertight construction so that a fracture of the tail shaft or similar accident will cause only the tunnel to be flooded.
The necessity for good drainage requires special attention in the design and construction of a ship. Free water on the decks, in a hold, or in the bilges is detrimental to the stability of the vessel. The drainage system must be as efficient as possible.
The decks are cambered to permit drainage to the scuppers which lead the water either overboard or to the bilges. Sufficient scuppers and suction must be provided so that the drainage will be effective in any condition of list or trim of the ship. Solid bulwarks, where fitted around a deck, are pierced by large freeing ports to allow any water that is shipped to escape.
