Chapter 2-SHIP DESIGN AND CONSTRUCTION 



BASIC SHIP STRUCTURE 



In considering the structure of a ship, it 

 is common practice to liken the ship to a box 

 girder. Like a box girder, a ship may be sub- 

 jected to tremendous stresses. The magnitude 

 of stress is usually expressed in pounds per 

 square inch (psi). 



When a pull is exerted on each end of a bar, 

 as in part A of figure 2-2, the bar is under the 

 type of stress called tension . When a pressure 

 is exerted on each end of a bar, as in part 

 B of figure 2-2, the bar is under the type of 

 stress called compression. If an equal but 

 opposite pull is exerted on the upper and lower 

 bars, as shown in part C of figure 2-2, the 

 pins connecting these bars are subjected to a 

 stress at right angles to their length. This 

 stress is called shear. When a shaft, bar, or 

 other material is subjected to a twisting motion, 

 the resulting stress is known as torsional 

 stress . Torsional stress is not illustrated in 

 figure 2-2. 



When a material is compressed, it is short- 

 ened. When it is subjected to tension, it is 

 lengthened. This change in shape is called strain . 

 The change of shape (strain) may be regarded 

 as an effect of stress. 



If a simple beam is supported at its two 

 ends and various vertical loads are applied 

 over the center of the span, the beam will 



bend (fig. 2-3). As the beam bends, the upper 

 section of the beam compresses and the lower 

 part stretches. Somewhere between the top and 

 bottom of the beam, there is a section which 

 is neither in compression nor in tension; this 

 is known as the neutral axis . The greatest 

 stresses in tension and compression occur 

 near the middle of the length of the beam, 

 where the loads are applied. 



LONGITUDINAL BENDING AND STRESSES 



In an I-beam, the greater mass of struc- 

 tural material is placed in the upper and lower 

 flanges to resist compression and tension. Rel- 

 atively little material is placed in the web 

 which holds the two flanges so that they can 

 work together; the web, being near the neutral 

 axis, is less subject to tension and compres- 

 sion stresses than are the flanges. The web 

 does take care of shearing stresses, which 

 are sizeable near the supports. 



A ship in a seaway can be considered similar 

 to this I-beam (or, more correctly, it can be 

 likened to a box girder) with supports and dis- 

 tributed loads. The supports are the buoyant 

 forces of the waves; the loads are the weight 

 of the ship's structure and the weight of every- 

 thing contained within the ship. 



The ship shown in figure 2-4 is supported 

 by waves, with the bow and stern each riding 

 a crest and the midship region in the trough. 

 This ship will bend with compression at the 

 top and tension at the bottom. A ship in this 

 condition is said to be sagging . In a sagging 

 ship, the weather deck tends to buckle under 

 compressive stress and the bottom plating tends 

 to stretch under tensile stress. A sagging ship 

 is undergoing longitudinal bending — that is, it 

 is bending in a fore-and-aft direction. 



When the ship advances half a wave length, 

 so that the crest is amidships and the bow and 

 stern are over troughs, as shown in figure 2-5, 



OAO LOAD LOAD NEUTRAL ^^^^^ 



If ^ f ''r^^ I-BEAM I 



147.8 



Figure 2-2.— Stresses in metal: (A) tension; 

 (B) compression; (C) shear. 



147.9 



Figure 2-3.— I-beam with load placed 

 over center. 



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