1. How can the bulkheads be classified according to their arrangement in the hull and their tightness? 2. What are the functions of bulkheads? 3. In what way do the bulkheads assist the ship's unsinkability? 4. Where is the collision bulkhead placed?
Steel and iron bulkheads are walls of plating extending transversely from side to side or longitudinally, throughout the whole or the part of the length of a vessel. They are valuable as a means of dividing the large volume contained in the hull of a ship into a number or separate compartments, and of entirely shutting off the engine and boiler spaces from the hold spaces - a feature of very great importance where inflammable cargo is carried in the adjoining compartments. But their chief function is to give structural strength and safety in the event of the shell being perforated and water finding access. Transverse bulkheads afford great support to both the longitudinal and the transverse framing of a vessel and contribute to general strength. When transverse bulkheads are well constructed and stiffened, and made perfectly watertight, they may afford protection in case of collision or perforation of the shell plating by confining the water to one of the compartments into which they divide the hull. They must, therefore, be securely connected to the shell plating, deck, and bottom of the vessel, and in addition, the plating must be of sufficient thickness and stiffened and supported in such a manner as to withstand the pressure of water which would come upon it in the event of such a compartment being perforated and flooded. Longitudinal bulkheads provide valuable and necessary strength to the hull. Bulkheads are classified into watertight, oiltight, and ordinary bulkheads serving to divide a vessel's volume into compartments. The first of these are the most important, except in the case of tankers, where, of course, the oiltight bulkheads are of the greatest significance.
Watertight bulkheads have three functions: 1) to divide a vessel into watertight compartments; 2) to increase her transverse strength; 3) to prevent the spreading of fire that might break out. The forepeak or collision bulkhead prevents a ship from going down in case of her colliding with another vessel. It has consequently to be made strong. Special bulkhead construction has come into use which is characterized by lightness and omission of stiffeners. Such bulkheads are called corrugated ones.
DOUBLE BOTTOM
1. Where is the double bottom arranged in the hull? 2. What is the double bottom used for? 3. What is the difference between inner bottom and double bottom? 4. What liquids are stored in the double bottom tanks?
When in former times a cargo vessel was at sea unloaded her draught was often so shallow that only a small portion of her propeller was immersed and she herself caught much wind. The result was that she was difficult to steer. To minimize this deficiency the vessel was ballasted either with sand or stones. Later on vessels were ballasted with liquid matter. Water ballast was stored in the lower part of the vessel, in the so-called "double bottom". By means of filling the double bottom with water ballast the vessel's immersion and seaworthiness, particularly her stability, were increased. The construction has afterwards been improved and simplified by the adoption of the so-called cellular system, in which the depth of the floors is increased, so that they reach the inner bottom, some of these floors being made watertight. Apart from ship's seaworthiness, the strength of the vessel is considerably increased when she is provided with a double bottom. The construction of two bottoms (the outer and the inner bottom) enhances the safety of human life at sea, which is especially important in the case of passenger vessels. Cargo vessels are built either with or without double bottoms. Nowadays the double bottom is no longer exclusively used for carrying water ballast. Feed water, fresh water, fuel oil for the engines and oil fuel for the boilers and lubricating oil are also stored in the double bottom.
KEELS
1. What is a flat plate keel? 2. What bottom longitudinal member is provided in a single-bottomed vessel? 3. What is the function of a bilge keel? 4. What types of keels can be found in the ship's hull and what are their duties?
In former times all vessels were provided with projecting keels; nowadays most ships have flat plate keels. The function of a projecting keel was to add considerably to ship's longitudinal strength. In modern vessels, having flat keels, such a strengthening member is still by all means an essential requirement and is consequently built within the vessel. It forms the center keelson of a single-bottomed vessel, and the center girder of double-bottomed ship. Keelsons are only found in vessels having single bottoms. The continuous keelsons are to be regarded as longitudinal scantlings or bottom stringers. The center girder is a very important longitudinal scantling which extends the whole length of the double bottom. The center girder is made continuous fore-and-aft; its required depth and thickness are regulated by the classification societies. The center girder is mostly constructed watertight all along. It is limited at its bottom by the flat keel plate and at its top by the center strake of the inner bottom. Flat (plate) keel is the heavy central bottom strake of shell plating to which the center girder or vertical keel is attached.
Bilge keels. Many vessels are provided with bilge keels, whose function is to reduce the rolling angle of a rolling vessel in stormy weather, and which are therefore often termed rolling chocks. Bilge keels have no influence on the rolling period, but owing to their damping effect sometimes considerably reduce the rolling angle, in consequence of which traveling by sea becomes less unpleasant to the passengers. There is also a bar keel which projects below the bottom in the center line plane and nowadays is used in small ships. Some ships have a tunnel keel formed by two vertical watertight plates nearly to the center line plane with different pipelines inside.
FRAMES
1. What is the duty of frames? 2. Why do frames act as stiffeners? 3. What types of frames can be installed in the ship's hull? 4. What is a frame ring?
One of the transverse girders forming the ribs of the hull and extending from the keel to the highest continuous deck is a frame. The duty of the frames is to strengthen the ship's hull. The frames act as stiffeners holding the shell plating in shape and maintaining the transverse form of the ship. But for the frames, the shell plating would soon be bulged in or buckled. The term "frame" when used in a general sense includes a combination of three parts: frame bar, reverse bar, and floor plate. When the structure consists of beams, side frames and a floor it is termed as a frame ring. While determining the dimensions of the frames, water pressure and the vessel's draught are taken into consideration. In order to indicate the position of the frames they are numbered from stern to stem. Besides the ordinary frames there are special types of frames: intermediate frames, deep frames, web frames, open frames, and intercostal frames. Intermediate frames are found where the shell requires additional strengthening, e.g. in way of the ice doubling. Web frames are found in the engine and boiler spaces and in the vessels built on the longitudinal framing system.
FLOORS
1. In what part of the hull are floors arranged? 2. What is floor's function? 3. Why should floors be more solid with the increase of draught and in way of engine room? 4. What types of floors are there on board a ship?
Floor is one of the most important elements of vessel's transverse strength. The duty of the floors is to strengthen the ship's bottom. They are met with both in single and in double-bottomed vessels. Sea going vessels have a floor to every frame, river craft often one floor to every second frame. As the water pressure in ship's bottom increases with the draught, the bottom should be made stronger, and, consequently, the floor more solid in proportion as the draught increases. Moreover, the ship's bottom must be made stronger with an increase of its width and length. Besides the water pressure, the ships bottom has to take up stresses caused by the rolling and pitching of the vessel and the vibration of the engines. In view of the latter the floors in the engine room have to be made more solid than elsewhere. Floors are the most important elements of vessel's transverse strength. In double-bottomed vessels three types of floors are distinguished: 1) the plate or solid floor with openings for structural lightness and man access; 2) watertight or oiltight as continuation of transverse bulkheads; 3) bracketed floors of rectangular plates and stiffeners.
